Date: Mon, 8 Apr 2013 18:41:23 +0000 Subject: Vendor import of llvm trunk r178860: http://llvm.org/svn/llvm-project/llvm/trunk@178860 --- docs/tutorial/LangImpl7.html | 2164 ------------------------------------------ 1 file changed, 2164 deletions(-) delete mode 100644 docs/tutorial/LangImpl7.html (limited to 'docs/tutorial/LangImpl7.html') diff --git a/docs/tutorial/LangImpl7.html b/docs/tutorial/LangImpl7.html deleted file mode 100644 index 4d5a4aa..0000000 --- a/docs/tutorial/LangImpl7.html +++ /dev/null @@ -1,2164 +0,0 @@ - - - - - Kaleidoscope: Extending the Language: Mutable Variables / SSA - construction - - - - - - - -

Kaleidoscope: Extending the Language: Mutable Variables

- -

Up to Tutorial Index
Chapter 7 -
-
Chapter 8: Conclusion and other useful LLVM - tidbits

- -

Written by Chris Lattner

- - -

Chapter 7 Introduction

- - -

- -

Welcome to Chapter 7 of the "Implementing a language -with LLVM" tutorial. In chapters 1 through 6, we've built a very -respectable, albeit simple, functional -programming language. In our journey, we learned some parsing techniques, -how to build and represent an AST, how to build LLVM IR, and how to optimize -the resultant code as well as JIT compile it.

- -

While Kaleidoscope is interesting as a functional language, the fact that it -is functional makes it "too easy" to generate LLVM IR for it. In particular, a -functional language makes it very easy to build LLVM IR directly in SSA form. -Since LLVM requires that the input code be in SSA form, this is a very nice -property and it is often unclear to newcomers how to generate code for an -imperative language with mutable variables.

- -

The short (and happy) summary of this chapter is that there is no need for -your front-end to build SSA form: LLVM provides highly tuned and well tested -support for this, though the way it works is a bit unexpected for some.

- -

- - -

Why is this a hard problem?

- - -

- -

-To understand why mutable variables cause complexities in SSA construction, -consider this extremely simple C example: -

- -

-int G, H;
-int test(_Bool Condition) {
-  int X;
-  if (Condition)
-    X = G;
-  else
-    X = H;
-  return X;
-}
-

- -

In this case, we have the variable "X", whose value depends on the path -executed in the program. Because there are two different possible values for X -before the return instruction, a PHI node is inserted to merge the two values. -The LLVM IR that we want for this example looks like this:

- -

-@G = weak global i32 0   ; type of @G is i32*
-@H = weak global i32 0   ; type of @H is i32*
-
-define i32 @test(i1 %Condition) {
-entry:
-  br i1 %Condition, label %cond_true, label %cond_false
-
-cond_true:
-  %X.0 = load i32* @G
-  br label %cond_next
-
-cond_false:
-  %X.1 = load i32* @H
-  br label %cond_next
-
-cond_next:
-  %X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
-  ret i32 %X.2
-}
-

- -

In this example, the loads from the G and H global variables are explicit in -the LLVM IR, and they live in the then/else branches of the if statement -(cond_true/cond_false). In order to merge the incoming values, the X.2 phi node -in the cond_next block selects the right value to use based on where control -flow is coming from: if control flow comes from the cond_false block, X.2 gets -the value of X.1. Alternatively, if control flow comes from cond_true, it gets -the value of X.0. The intent of this chapter is not to explain the details of -SSA form. For more information, see one of the many online -references.

- -

The question for this article is "who places the phi nodes when lowering -assignments to mutable variables?". The issue here is that LLVM -requires that its IR be in SSA form: there is no "non-ssa" mode for it. -However, SSA construction requires non-trivial algorithms and data structures, -so it is inconvenient and wasteful for every front-end to have to reproduce this -logic.

- -

- - -

Memory in LLVM

- - -

- -

The 'trick' here is that while LLVM does require all register values to be -in SSA form, it does not require (or permit) memory objects to be in SSA form. -In the example above, note that the loads from G and H are direct accesses to -G and H: they are not renamed or versioned. This differs from some other -compiler systems, which do try to version memory objects. In LLVM, instead of -encoding dataflow analysis of memory into the LLVM IR, it is handled with Analysis Passes which are computed on -demand.

- -

-With this in mind, the high-level idea is that we want to make a stack variable -(which lives in memory, because it is on the stack) for each mutable object in -a function. To take advantage of this trick, we need to talk about how LLVM -represents stack variables. -

- -

In LLVM, all memory accesses are explicit with load/store instructions, and -it is carefully designed not to have (or need) an "address-of" operator. Notice -how the type of the @G/@H global variables is actually "i32*" even though the -variable is defined as "i32". What this means is that @G defines space -for an i32 in the global data area, but its name actually refers to the -address for that space. Stack variables work the same way, except that instead of -being declared with global variable definitions, they are declared with the -LLVM alloca instruction:

- -

-define i32 @example() {
-entry:
-  %X = alloca i32           ; type of %X is i32*.
-  ...
-  %tmp = load i32* %X       ; load the stack value %X from the stack.
-  %tmp2 = add i32 %tmp, 1   ; increment it
-  store i32 %tmp2, i32* %X  ; store it back
-  ...
-

- -

This code shows an example of how you can declare and manipulate a stack -variable in the LLVM IR. Stack memory allocated with the alloca instruction is -fully general: you can pass the address of the stack slot to functions, you can -store it in other variables, etc. In our example above, we could rewrite the -example to use the alloca technique to avoid using a PHI node:

- -

-@G = weak global i32 0   ; type of @G is i32*
-@H = weak global i32 0   ; type of @H is i32*
-
-define i32 @test(i1 %Condition) {
-entry:
-  %X = alloca i32           ; type of %X is i32*.
-  br i1 %Condition, label %cond_true, label %cond_false
-
-cond_true:
-  %X.0 = load i32* @G
-  store i32 %X.0, i32* %X   ; Update X
-  br label %cond_next
-
-cond_false:
-  %X.1 = load i32* @H
-  store i32 %X.1, i32* %X   ; Update X
-  br label %cond_next
-
-cond_next:
-  %X.2 = load i32* %X       ; Read X
-  ret i32 %X.2
-}
-

- -

With this, we have discovered a way to handle arbitrary mutable variables -without the need to create Phi nodes at all:

- -

Each mutable variable becomes a stack allocation.
Each read of the variable becomes a load from the stack.
Each update of the variable becomes a store to the stack.
Taking the address of a variable just uses the stack address directly.

- -

While this solution has solved our immediate problem, it introduced another -one: we have now apparently introduced a lot of stack traffic for very simple -and common operations, a major performance problem. Fortunately for us, the -LLVM optimizer has a highly-tuned optimization pass named "mem2reg" that handles -this case, promoting allocas like this into SSA registers, inserting Phi nodes -as appropriate. If you run this example through the pass, for example, you'll -get:

- -

-$ llvm-as < example.ll | opt -mem2reg | llvm-dis
-@G = weak global i32 0
-@H = weak global i32 0
-
-define i32 @test(i1 %Condition) {
-entry:
-  br i1 %Condition, label %cond_true, label %cond_false
-
-cond_true:
-  %X.0 = load i32* @G
-  br label %cond_next
-
-cond_false:
-  %X.1 = load i32* @H
-  br label %cond_next
-
-cond_next:
-  %X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
-  ret i32 %X.01
-}
-

- -

The mem2reg pass implements the standard "iterated dominance frontier" -algorithm for constructing SSA form and has a number of optimizations that speed -up (very common) degenerate cases. The mem2reg optimization pass is the answer to dealing -with mutable variables, and we highly recommend that you depend on it. Note that -mem2reg only works on variables in certain circumstances:

- -

mem2reg is alloca-driven: it looks for allocas and if it can handle them, it -promotes them. It does not apply to global variables or heap allocations.
mem2reg only looks for alloca instructions in the entry block of the -function. Being in the entry block guarantees that the alloca is only executed -once, which makes analysis simpler.
mem2reg only promotes allocas whose uses are direct loads and stores. If -the address of the stack object is passed to a function, or if any funny pointer -arithmetic is involved, the alloca will not be promoted.
mem2reg only works on allocas of first class -values (such as pointers, scalars and vectors), and only if the array size -of the allocation is 1 (or missing in the .ll file). mem2reg is not capable of -promoting structs or arrays to registers. Note that the "scalarrepl" pass is -more powerful and can promote structs, "unions", and arrays in many cases.

- -

-All of these properties are easy to satisfy for most imperative languages, and -we'll illustrate it below with Kaleidoscope. The final question you may be -asking is: should I bother with this nonsense for my front-end? Wouldn't it be -better if I just did SSA construction directly, avoiding use of the mem2reg -optimization pass? In short, we strongly recommend that you use this technique -for building SSA form, unless there is an extremely good reason not to. Using -this technique is:

- -

Proven and well tested: llvm-gcc and clang both use this technique for local -mutable variables. As such, the most common clients of LLVM are using this to -handle a bulk of their variables. You can be sure that bugs are found fast and -fixed early.
Extremely Fast: mem2reg has a number of special cases that make it fast in -common cases as well as fully general. For example, it has fast-paths for -variables that are only used in a single block, variables that only have one -assignment point, good heuristics to avoid insertion of unneeded phi nodes, etc. -
Needed for debug info generation: -Debug information in LLVM relies on having the address of the variable -exposed so that debug info can be attached to it. This technique dovetails -very naturally with this style of debug info.

- -

If nothing else, this makes it much easier to get your front-end up and -running, and is very simple to implement. Lets extend Kaleidoscope with mutable -variables now! -

- -

- - -

Mutable Variables in Kaleidoscope

- - -

- -

Now that we know the sort of problem we want to tackle, lets see what this -looks like in the context of our little Kaleidoscope language. We're going to -add two features:

- -

The ability to mutate variables with the '=' operator.
The ability to define new variables.

- -

While the first item is really what this is about, we only have variables -for incoming arguments as well as for induction variables, and redefining those only -goes so far :). Also, the ability to define new variables is a -useful thing regardless of whether you will be mutating them. Here's a -motivating example that shows how we could use these:

- -

-# Define ':' for sequencing: as a low-precedence operator that ignores operands
-# and just returns the RHS.
-def binary : 1 (x y) y;
-
-# Recursive fib, we could do this before.
-def fib(x)
-  if (x < 3) then
-    1
-  else
-    fib(x-1)+fib(x-2);
-
-# Iterative fib.
-def fibi(x)
-  var a = 1, b = 1, c in
-  (for i = 3, i < x in 
-     c = a + b :
-     a = b :
-     b = c) :
-  b;
-
-# Call it. 
-fibi(10);
-

- -

-In order to mutate variables, we have to change our existing variables to use -the "alloca trick". Once we have that, we'll add our new operator, then extend -Kaleidoscope to support new variable definitions. -

- -

- - -

Adjusting Existing Variables for Mutation

- - -

- -

-The symbol table in Kaleidoscope is managed at code generation time by the -'NamedValues' map. This map currently keeps track of the LLVM "Value*" -that holds the double value for the named variable. In order to support -mutation, we need to change this slightly, so that it NamedValues holds -the memory location of the variable in question. Note that this -change is a refactoring: it changes the structure of the code, but does not -(by itself) change the behavior of the compiler. All of these changes are -isolated in the Kaleidoscope code generator.

- -

-At this point in Kaleidoscope's development, it only supports variables for two -things: incoming arguments to functions and the induction variable of 'for' -loops. For consistency, we'll allow mutation of these variables in addition to -other user-defined variables. This means that these will both need memory -locations. -

- -

To start our transformation of Kaleidoscope, we'll change the NamedValues -map so that it maps to AllocaInst* instead of Value*. Once we do this, the C++ -compiler will tell us what parts of the code we need to update:

- -

-static std::map<std::string, AllocaInst*> NamedValues;
-

- -

Also, since we will need to create these alloca's, we'll use a helper -function that ensures that the allocas are created in the entry block of the -function:

- -

-/// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of
-/// the function.  This is used for mutable variables etc.
-static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
-                                          const std::string &VarName) {
-  IRBuilder<> TmpB(&TheFunction->getEntryBlock(),
-                 TheFunction->getEntryBlock().begin());
-  return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0,
-                           VarName.c_str());
-}
-

- -

This funny looking code creates an IRBuilder object that is pointing at -the first instruction (.begin()) of the entry block. It then creates an alloca -with the expected name and returns it. Because all values in Kaleidoscope are -doubles, there is no need to pass in a type to use.

- -

With this in place, the first functionality change we want to make is to -variable references. In our new scheme, variables live on the stack, so code -generating a reference to them actually needs to produce a load from the stack -slot:

- -

-Value *VariableExprAST::Codegen() {
-  // Look this variable up in the function.
-  Value *V = NamedValues[Name];
-  if (V == 0) return ErrorV("Unknown variable name");
-
-  // Load the value.
-  return Builder.CreateLoad(V, Name.c_str());
-}
-

- -

As you can see, this is pretty straightforward. Now we need to update the -things that define the variables to set up the alloca. We'll start with -ForExprAST::Codegen (see the full code listing for -the unabridged code):

- -

-  Function *TheFunction = Builder.GetInsertBlock()->getParent();
-
-  // Create an alloca for the variable in the entry block.
-  AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
-  
-    // Emit the start code first, without 'variable' in scope.
-  Value *StartVal = Start->Codegen();
-  if (StartVal == 0) return 0;
-  
-  // Store the value into the alloca.
-  Builder.CreateStore(StartVal, Alloca);
-  ...
-
-  // Compute the end condition.
-  Value *EndCond = End->Codegen();
-  if (EndCond == 0) return EndCond;
-  
-  // Reload, increment, and restore the alloca.  This handles the case where
-  // the body of the loop mutates the variable.
-  Value *CurVar = Builder.CreateLoad(Alloca);
-  Value *NextVar = Builder.CreateFAdd(CurVar, StepVal, "nextvar");
-  Builder.CreateStore(NextVar, Alloca);
-  ...
-

- -

This code is virtually identical to the code before we allowed mutable variables. The -big difference is that we no longer have to construct a PHI node, and we use -load/store to access the variable as needed.

- -

To support mutable argument variables, we need to also make allocas for them. -The code for this is also pretty simple:

- -

-/// CreateArgumentAllocas - Create an alloca for each argument and register the
-/// argument in the symbol table so that references to it will succeed.
-void PrototypeAST::CreateArgumentAllocas(Function *F) {
-  Function::arg_iterator AI = F->arg_begin();
-  for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) {
-    // Create an alloca for this variable.
-    AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]);
-
-    // Store the initial value into the alloca.
-    Builder.CreateStore(AI, Alloca);
-
-    // Add arguments to variable symbol table.
-    NamedValues[Args[Idx]] = Alloca;
-  }
-}
-

- -

For each argument, we make an alloca, store the input value to the function -into the alloca, and register the alloca as the memory location for the -argument. This method gets invoked by FunctionAST::Codegen right after -it sets up the entry block for the function.

- -

The final missing piece is adding the mem2reg pass, which allows us to get -good codegen once again:

- -

-    // Set up the optimizer pipeline.  Start with registering info about how the
-    // target lays out data structures.
-    OurFPM.add(new DataLayout(*TheExecutionEngine->getDataLayout()));
-    // Promote allocas to registers.
-    OurFPM.add(createPromoteMemoryToRegisterPass());
-    // Do simple "peephole" optimizations and bit-twiddling optzns.
-    OurFPM.add(createInstructionCombiningPass());
-    // Reassociate expressions.
-    OurFPM.add(createReassociatePass());
-

- -

It is interesting to see what the code looks like before and after the -mem2reg optimization runs. For example, this is the before/after code for our -recursive fib function. Before the optimization:

- -

-define double @fib(double %x) {
-entry:
-  %x1 = alloca double
-  store double %x, double* %x1
-  %x2 = load double* %x1
-  %cmptmp = fcmp ult double %x2, 3.000000e+00
-  %booltmp = uitofp i1 %cmptmp to double
-  %ifcond = fcmp one double %booltmp, 0.000000e+00
-  br i1 %ifcond, label %then, label %else
-
-then:		; preds = %entry
-  br label %ifcont
-
-else:		; preds = %entry
-  %x3 = load double* %x1
-  %subtmp = fsub double %x3, 1.000000e+00
-  %calltmp = call double @fib(double %subtmp)
-  %x4 = load double* %x1
-  %subtmp5 = fsub double %x4, 2.000000e+00
-  %calltmp6 = call double @fib(double %subtmp5)
-  %addtmp = fadd double %calltmp, %calltmp6
-  br label %ifcont
-
-ifcont:		; preds = %else, %then
-  %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
-  ret double %iftmp
-}
-

- -

Here there is only one variable (x, the input argument) but you can still -see the extremely simple-minded code generation strategy we are using. In the -entry block, an alloca is created, and the initial input value is stored into -it. Each reference to the variable does a reload from the stack. Also, note -that we didn't modify the if/then/else expression, so it still inserts a PHI -node. While we could make an alloca for it, it is actually easier to create a -PHI node for it, so we still just make the PHI.

- -

Here is the code after the mem2reg pass runs:

- -

-define double @fib(double %x) {
-entry:
-  %cmptmp = fcmp ult double %x, 3.000000e+00
-  %booltmp = uitofp i1 %cmptmp to double
-  %ifcond = fcmp one double %booltmp, 0.000000e+00
-  br i1 %ifcond, label %then, label %else
-
-then:
-  br label %ifcont
-
-else:
-  %subtmp = fsub double %x, 1.000000e+00
-  %calltmp = call double @fib(double %subtmp)
-  %subtmp5 = fsub double %x, 2.000000e+00
-  %calltmp6 = call double @fib(double %subtmp5)
-  %addtmp = fadd double %calltmp, %calltmp6
-  br label %ifcont
-
-ifcont:		; preds = %else, %then
-  %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
-  ret double %iftmp
-}
-

- -

This is a trivial case for mem2reg, since there are no redefinitions of the -variable. The point of showing this is to calm your tension about inserting -such blatent inefficiencies :).

- -

After the rest of the optimizers run, we get:

- -

-define double @fib(double %x) {
-entry:
-  %cmptmp = fcmp ult double %x, 3.000000e+00
-  %booltmp = uitofp i1 %cmptmp to double
-  %ifcond = fcmp ueq double %booltmp, 0.000000e+00
-  br i1 %ifcond, label %else, label %ifcont
-
-else:
-  %subtmp = fsub double %x, 1.000000e+00
-  %calltmp = call double @fib(double %subtmp)
-  %subtmp5 = fsub double %x, 2.000000e+00
-  %calltmp6 = call double @fib(double %subtmp5)
-  %addtmp = fadd double %calltmp, %calltmp6
-  ret double %addtmp
-
-ifcont:
-  ret double 1.000000e+00
-}
-

- -

Here we see that the simplifycfg pass decided to clone the return instruction -into the end of the 'else' block. This allowed it to eliminate some branches -and the PHI node.

- -

Now that all symbol table references are updated to use stack variables, -we'll add the assignment operator.

- -

- - -

New Assignment Operator

- - -

- -

With our current framework, adding a new assignment operator is really -simple. We will parse it just like any other binary operator, but handle it -internally (instead of allowing the user to define it). The first step is to -set a precedence:

- -

- int main() {
-   // Install standard binary operators.
-   // 1 is lowest precedence.
-   BinopPrecedence['='] = 2;
-   BinopPrecedence['<'] = 10;
-   BinopPrecedence['+'] = 20;
-   BinopPrecedence['-'] = 20;
-

- -

Now that the parser knows the precedence of the binary operator, it takes -care of all the parsing and AST generation. We just need to implement codegen -for the assignment operator. This looks like:

- -

-Value *BinaryExprAST::Codegen() {
-  // Special case '=' because we don't want to emit the LHS as an expression.
-  if (Op == '=') {
-    // Assignment requires the LHS to be an identifier.
-    VariableExprAST *LHSE = dynamic_cast<VariableExprAST*>(LHS);
-    if (!LHSE)
-      return ErrorV("destination of '=' must be a variable");
-

- -

Unlike the rest of the binary operators, our assignment operator doesn't -follow the "emit LHS, emit RHS, do computation" model. As such, it is handled -as a special case before the other binary operators are handled. The other -strange thing is that it requires the LHS to be a variable. It is invalid to -have "(x+1) = expr" - only things like "x = expr" are allowed. -

- -

-    // Codegen the RHS.
-    Value *Val = RHS->Codegen();
-    if (Val == 0) return 0;
-
-    // Look up the name.
-    Value *Variable = NamedValues[LHSE->getName()];
-    if (Variable == 0) return ErrorV("Unknown variable name");
-
-    Builder.CreateStore(Val, Variable);
-    return Val;
-  }
-  ...  
-

- -

Once we have the variable, codegen'ing the assignment is straightforward: -we emit the RHS of the assignment, create a store, and return the computed -value. Returning a value allows for chained assignments like "X = (Y = Z)".

- -

Now that we have an assignment operator, we can mutate loop variables and -arguments. For example, we can now run code like this:

- -

-# Function to print a double.
-extern printd(x);
-
-# Define ':' for sequencing: as a low-precedence operator that ignores operands
-# and just returns the RHS.
-def binary : 1 (x y) y;
-
-def test(x)
-  printd(x) :
-  x = 4 :
-  printd(x);
-
-test(123);
-

- -

When run, this example prints "123" and then "4", showing that we did -actually mutate the value! Okay, we have now officially implemented our goal: -getting this to work requires SSA construction in the general case. However, -to be really useful, we want the ability to define our own local variables, lets -add this next! -

- -

- - -

User-defined Local Variables

- - -

- -

Adding var/in is just like any other other extensions we made to -Kaleidoscope: we extend the lexer, the parser, the AST and the code generator. -The first step for adding our new 'var/in' construct is to extend the lexer. -As before, this is pretty trivial, the code looks like this:

- -

-enum Token {
-  ...
-  // var definition
-  tok_var = -13
-...
-}
-...
-static int gettok() {
-...
-    if (IdentifierStr == "in") return tok_in;
-    if (IdentifierStr == "binary") return tok_binary;
-    if (IdentifierStr == "unary") return tok_unary;
-    if (IdentifierStr == "var") return tok_var;
-    return tok_identifier;
-...
-

- -

The next step is to define the AST node that we will construct. For var/in, -it looks like this:

- -

-/// VarExprAST - Expression class for var/in
-class VarExprAST : public ExprAST {
-  std::vector<std::pair<std::string, ExprAST*> > VarNames;
-  ExprAST *Body;
-public:
-  VarExprAST(const std::vector<std::pair<std::string, ExprAST*> > &varnames,
-             ExprAST *body)
-  : VarNames(varnames), Body(body) {}
-  
-  virtual Value *Codegen();
-};
-

- -

var/in allows a list of names to be defined all at once, and each name can -optionally have an initializer value. As such, we capture this information in -the VarNames vector. Also, var/in has a body, this body is allowed to access -the variables defined by the var/in.

- -

With this in place, we can define the parser pieces. The first thing we do is add -it as a primary expression:

- -

-/// primary
-///   ::= identifierexpr
-///   ::= numberexpr
-///   ::= parenexpr
-///   ::= ifexpr
-///   ::= forexpr
-///   ::= varexpr
-static ExprAST *ParsePrimary() {
-  switch (CurTok) {
-  default: return Error("unknown token when expecting an expression");
-  case tok_identifier: return ParseIdentifierExpr();
-  case tok_number:     return ParseNumberExpr();
-  case '(':            return ParseParenExpr();
-  case tok_if:         return ParseIfExpr();
-  case tok_for:        return ParseForExpr();
-  case tok_var:        return ParseVarExpr();
-  }
-}
-

- -

Next we define ParseVarExpr:

- -

-/// varexpr ::= 'var' identifier ('=' expression)? 
-//                    (',' identifier ('=' expression)?)* 'in' expression
-static ExprAST *ParseVarExpr() {
-  getNextToken();  // eat the var.
-
-  std::vector<std::pair<std::string, ExprAST*> > VarNames;
-
-  // At least one variable name is required.
-  if (CurTok != tok_identifier)
-    return Error("expected identifier after var");
-

- -

The first part of this code parses the list of identifier/expr pairs into the -local VarNames vector. - -

-  while (1) {
-    std::string Name = IdentifierStr;
-    getNextToken();  // eat identifier.
-
-    // Read the optional initializer.
-    ExprAST *Init = 0;
-    if (CurTok == '=') {
-      getNextToken(); // eat the '='.
-      
-      Init = ParseExpression();
-      if (Init == 0) return 0;
-    }
-    
-    VarNames.push_back(std::make_pair(Name, Init));
-    
-    // End of var list, exit loop.
-    if (CurTok != ',') break;
-    getNextToken(); // eat the ','.
-    
-    if (CurTok != tok_identifier)
-      return Error("expected identifier list after var");
-  }
-

- -

Once all the variables are parsed, we then parse the body and create the -AST node:

- -

-  // At this point, we have to have 'in'.
-  if (CurTok != tok_in)
-    return Error("expected 'in' keyword after 'var'");
-  getNextToken();  // eat 'in'.
-  
-  ExprAST *Body = ParseExpression();
-  if (Body == 0) return 0;
-  
-  return new VarExprAST(VarNames, Body);
-}
-

- -

Now that we can parse and represent the code, we need to support emission of -LLVM IR for it. This code starts out with:

- -

-Value *VarExprAST::Codegen() {
-  std::vector<AllocaInst *> OldBindings;
-  
-  Function *TheFunction = Builder.GetInsertBlock()->getParent();
-
-  // Register all variables and emit their initializer.
-  for (unsigned i = 0, e = VarNames.size(); i != e; ++i) {
-    const std::string &VarName = VarNames[i].first;
-    ExprAST *Init = VarNames[i].second;
-

- -

Basically it loops over all the variables, installing them one at a time. -For each variable we put into the symbol table, we remember the previous value -that we replace in OldBindings.

- -

-    // Emit the initializer before adding the variable to scope, this prevents
-    // the initializer from referencing the variable itself, and permits stuff
-    // like this:
-    //  var a = 1 in
-    //    var a = a in ...   # refers to outer 'a'.
-    Value *InitVal;
-    if (Init) {
-      InitVal = Init->Codegen();
-      if (InitVal == 0) return 0;
-    } else { // If not specified, use 0.0.
-      InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0));
-    }
-    
-    AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
-    Builder.CreateStore(InitVal, Alloca);
-
-    // Remember the old variable binding so that we can restore the binding when
-    // we unrecurse.
-    OldBindings.push_back(NamedValues[VarName]);
-    
-    // Remember this binding.
-    NamedValues[VarName] = Alloca;
-  }
-

- -

There are more comments here than code. The basic idea is that we emit the -initializer, create the alloca, then update the symbol table to point to it. -Once all the variables are installed in the symbol table, we evaluate the body -of the var/in expression:

- -

-  // Codegen the body, now that all vars are in scope.
-  Value *BodyVal = Body->Codegen();
-  if (BodyVal == 0) return 0;
-

- -

Finally, before returning, we restore the previous variable bindings:

- -

-  // Pop all our variables from scope.
-  for (unsigned i = 0, e = VarNames.size(); i != e; ++i)
-    NamedValues[VarNames[i].first] = OldBindings[i];
-
-  // Return the body computation.
-  return BodyVal;
-}
-

- -

The end result of all of this is that we get properly scoped variable -definitions, and we even (trivially) allow mutation of them :).

- -

With this, we completed what we set out to do. Our nice iterative fib -example from the intro compiles and runs just fine. The mem2reg pass optimizes -all of our stack variables into SSA registers, inserting PHI nodes where needed, -and our front-end remains simple: no "iterated dominance frontier" computation -anywhere in sight.

- -

- - -

Full Code Listing

- - -

- -

-Here is the complete code listing for our running example, enhanced with mutable -variables and var/in support. To build this example, use: -

- -

-# Compile
-clang++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
-# Run
-./toy
-

- -

Here is the code:

- -

-#include "llvm/DerivedTypes.h"
-#include "llvm/ExecutionEngine/ExecutionEngine.h"
-#include "llvm/ExecutionEngine/JIT.h"
-#include "llvm/IRBuilder.h"
-#include "llvm/LLVMContext.h"
-#include "llvm/Module.h"
-#include "llvm/PassManager.h"
-#include "llvm/Analysis/Verifier.h"
-#include "llvm/Analysis/Passes.h"
-#include "llvm/DataLayout.h"
-#include "llvm/Transforms/Scalar.h"
-#include "llvm/Support/TargetSelect.h"
-#include <cstdio>
-#include <string>
-#include <map>
-#include <vector>
-using namespace llvm;
-
-//===----------------------------------------------------------------------===//
-// Lexer
-//===----------------------------------------------------------------------===//
-
-// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
-// of these for known things.
-enum Token {
-  tok_eof = -1,
-
-  // commands
-  tok_def = -2, tok_extern = -3,
-
-  // primary
-  tok_identifier = -4, tok_number = -5,
-  
-  // control
-  tok_if = -6, tok_then = -7, tok_else = -8,
-  tok_for = -9, tok_in = -10,
-  
-  // operators
-  tok_binary = -11, tok_unary = -12,
-  
-  // var definition
-  tok_var = -13
-};
-
-static std::string IdentifierStr;  // Filled in if tok_identifier
-static double NumVal;              // Filled in if tok_number
-
-/// gettok - Return the next token from standard input.
-static int gettok() {
-  static int LastChar = ' ';
-
-  // Skip any whitespace.
-  while (isspace(LastChar))
-    LastChar = getchar();
-
-  if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
-    IdentifierStr = LastChar;
-    while (isalnum((LastChar = getchar())))
-      IdentifierStr += LastChar;
-
-    if (IdentifierStr == "def") return tok_def;
-    if (IdentifierStr == "extern") return tok_extern;
-    if (IdentifierStr == "if") return tok_if;
-    if (IdentifierStr == "then") return tok_then;
-    if (IdentifierStr == "else") return tok_else;
-    if (IdentifierStr == "for") return tok_for;
-    if (IdentifierStr == "in") return tok_in;
-    if (IdentifierStr == "binary") return tok_binary;
-    if (IdentifierStr == "unary") return tok_unary;
-    if (IdentifierStr == "var") return tok_var;
-    return tok_identifier;
-  }
-
-  if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
-    std::string NumStr;
-    do {
-      NumStr += LastChar;
-      LastChar = getchar();
-    } while (isdigit(LastChar) || LastChar == '.');
-
-    NumVal = strtod(NumStr.c_str(), 0);
-    return tok_number;
-  }
-
-  if (LastChar == '#') {
-    // Comment until end of line.
-    do LastChar = getchar();
-    while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
-    
-    if (LastChar != EOF)
-      return gettok();
-  }
-  
-  // Check for end of file.  Don't eat the EOF.
-  if (LastChar == EOF)
-    return tok_eof;
-
-  // Otherwise, just return the character as its ascii value.
-  int ThisChar = LastChar;
-  LastChar = getchar();
-  return ThisChar;
-}
-
-//===----------------------------------------------------------------------===//
-// Abstract Syntax Tree (aka Parse Tree)
-//===----------------------------------------------------------------------===//
-
-/// ExprAST - Base class for all expression nodes.
-class ExprAST {
-public:
-  virtual ~ExprAST() {}
-  virtual Value *Codegen() = 0;
-};
-
-/// NumberExprAST - Expression class for numeric literals like "1.0".
-class NumberExprAST : public ExprAST {
-  double Val;
-public:
-  NumberExprAST(double val) : Val(val) {}
-  virtual Value *Codegen();
-};
-
-/// VariableExprAST - Expression class for referencing a variable, like "a".
-class VariableExprAST : public ExprAST {
-  std::string Name;
-public:
-  VariableExprAST(const std::string &name) : Name(name) {}
-  const std::string &getName() const { return Name; }
-  virtual Value *Codegen();
-};
-
-/// UnaryExprAST - Expression class for a unary operator.
-class UnaryExprAST : public ExprAST {
-  char Opcode;
-  ExprAST *Operand;
-public:
-  UnaryExprAST(char opcode, ExprAST *operand) 
-    : Opcode(opcode), Operand(operand) {}
-  virtual Value *Codegen();
-};
-
-/// BinaryExprAST - Expression class for a binary operator.
-class BinaryExprAST : public ExprAST {
-  char Op;
-  ExprAST *LHS, *RHS;
-public:
-  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
-    : Op(op), LHS(lhs), RHS(rhs) {}
-  virtual Value *Codegen();
-};
-
-/// CallExprAST - Expression class for function calls.
-class CallExprAST : public ExprAST {
-  std::string Callee;
-  std::vector<ExprAST*> Args;
-public:
-  CallExprAST(const std::string &callee, std::vector<ExprAST*> &args)
-    : Callee(callee), Args(args) {}
-  virtual Value *Codegen();
-};
-
-/// IfExprAST - Expression class for if/then/else.
-class IfExprAST : public ExprAST {
-  ExprAST *Cond, *Then, *Else;
-public:
-  IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else)
-  : Cond(cond), Then(then), Else(_else) {}
-  virtual Value *Codegen();
-};
-
-/// ForExprAST - Expression class for for/in.
-class ForExprAST : public ExprAST {
-  std::string VarName;
-  ExprAST *Start, *End, *Step, *Body;
-public:
-  ForExprAST(const std::string &varname, ExprAST *start, ExprAST *end,
-             ExprAST *step, ExprAST *body)
-    : VarName(varname), Start(start), End(end), Step(step), Body(body) {}
-  virtual Value *Codegen();
-};
-
-/// VarExprAST - Expression class for var/in
-class VarExprAST : public ExprAST {
-  std::vector<std::pair<std::string, ExprAST*> > VarNames;
-  ExprAST *Body;
-public:
-  VarExprAST(const std::vector<std::pair<std::string, ExprAST*> > &varnames,
-             ExprAST *body)
-  : VarNames(varnames), Body(body) {}
-  
-  virtual Value *Codegen();
-};
-
-/// PrototypeAST - This class represents the "prototype" for a function,
-/// which captures its name, and its argument names (thus implicitly the number
-/// of arguments the function takes), as well as if it is an operator.
-class PrototypeAST {
-  std::string Name;
-  std::vector<std::string> Args;
-  bool isOperator;
-  unsigned Precedence;  // Precedence if a binary op.
-public:
-  PrototypeAST(const std::string &name, const std::vector<std::string> &args,
-               bool isoperator = false, unsigned prec = 0)
-  : Name(name), Args(args), isOperator(isoperator), Precedence(prec) {}
-  
-  bool isUnaryOp() const { return isOperator && Args.size() == 1; }
-  bool isBinaryOp() const { return isOperator && Args.size() == 2; }
-  
-  char getOperatorName() const {
-    assert(isUnaryOp() || isBinaryOp());
-    return Name[Name.size()-1];
-  }
-  
-  unsigned getBinaryPrecedence() const { return Precedence; }
-  
-  Function *Codegen();
-  
-  void CreateArgumentAllocas(Function *F);
-};
-
-/// FunctionAST - This class represents a function definition itself.
-class FunctionAST {
-  PrototypeAST *Proto;
-  ExprAST *Body;
-public:
-  FunctionAST(PrototypeAST *proto, ExprAST *body)
-    : Proto(proto), Body(body) {}
-  
-  Function *Codegen();
-};
-
-//===----------------------------------------------------------------------===//
-// Parser
-//===----------------------------------------------------------------------===//
-
-/// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
-/// token the parser is looking at.  getNextToken reads another token from the
-/// lexer and updates CurTok with its results.
-static int CurTok;
-static int getNextToken() {
-  return CurTok = gettok();
-}
-
-/// BinopPrecedence - This holds the precedence for each binary operator that is
-/// defined.
-static std::map<char, int> BinopPrecedence;
-
-/// GetTokPrecedence - Get the precedence of the pending binary operator token.
-static int GetTokPrecedence() {
-  if (!isascii(CurTok))
-    return -1;
-  
-  // Make sure it's a declared binop.
-  int TokPrec = BinopPrecedence[CurTok];
-  if (TokPrec <= 0) return -1;
-  return TokPrec;
-}
-
-/// Error* - These are little helper functions for error handling.
-ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
-PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
-FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
-
-static ExprAST *ParseExpression();
-
-/// identifierexpr
-///   ::= identifier
-///   ::= identifier '(' expression* ')'
-static ExprAST *ParseIdentifierExpr() {
-  std::string IdName = IdentifierStr;
-  
-  getNextToken();  // eat identifier.
-  
-  if (CurTok != '(') // Simple variable ref.
-    return new VariableExprAST(IdName);
-  
-  // Call.
-  getNextToken();  // eat (
-  std::vector<ExprAST*> Args;
-  if (CurTok != ')') {
-    while (1) {
-      ExprAST *Arg = ParseExpression();
-      if (!Arg) return 0;
-      Args.push_back(Arg);
-
-      if (CurTok == ')') break;
-
-      if (CurTok != ',')
-        return Error("Expected ')' or ',' in argument list");
-      getNextToken();
-    }
-  }
-
-  // Eat the ')'.
-  getNextToken();
-  
-  return new CallExprAST(IdName, Args);
-}
-
-/// numberexpr ::= number
-static ExprAST *ParseNumberExpr() {
-  ExprAST *Result = new NumberExprAST(NumVal);
-  getNextToken(); // consume the number
-  return Result;
-}
-
-/// parenexpr ::= '(' expression ')'
-static ExprAST *ParseParenExpr() {
-  getNextToken();  // eat (.
-  ExprAST *V = ParseExpression();
-  if (!V) return 0;
-  
-  if (CurTok != ')')
-    return Error("expected ')'");
-  getNextToken();  // eat ).
-  return V;
-}
-
-/// ifexpr ::= 'if' expression 'then' expression 'else' expression
-static ExprAST *ParseIfExpr() {
-  getNextToken();  // eat the if.
-  
-  // condition.
-  ExprAST *Cond = ParseExpression();
-  if (!Cond) return 0;
-  
-  if (CurTok != tok_then)
-    return Error("expected then");
-  getNextToken();  // eat the then
-  
-  ExprAST *Then = ParseExpression();
-  if (Then == 0) return 0;
-  
-  if (CurTok != tok_else)
-    return Error("expected else");
-  
-  getNextToken();
-  
-  ExprAST *Else = ParseExpression();
-  if (!Else) return 0;
-  
-  return new IfExprAST(Cond, Then, Else);
-}
-
-/// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
-static ExprAST *ParseForExpr() {
-  getNextToken();  // eat the for.
-
-  if (CurTok != tok_identifier)
-    return Error("expected identifier after for");
-  
-  std::string IdName = IdentifierStr;
-  getNextToken();  // eat identifier.
-  
-  if (CurTok != '=')
-    return Error("expected '=' after for");
-  getNextToken();  // eat '='.
-  
-  
-  ExprAST *Start = ParseExpression();
-  if (Start == 0) return 0;
-  if (CurTok != ',')
-    return Error("expected ',' after for start value");
-  getNextToken();
-  
-  ExprAST *End = ParseExpression();
-  if (End == 0) return 0;
-  
-  // The step value is optional.
-  ExprAST *Step = 0;
-  if (CurTok == ',') {
-    getNextToken();
-    Step = ParseExpression();
-    if (Step == 0) return 0;
-  }
-  
-  if (CurTok != tok_in)
-    return Error("expected 'in' after for");
-  getNextToken();  // eat 'in'.
-  
-  ExprAST *Body = ParseExpression();
-  if (Body == 0) return 0;
-
-  return new ForExprAST(IdName, Start, End, Step, Body);
-}
-
-/// varexpr ::= 'var' identifier ('=' expression)? 
-//                    (',' identifier ('=' expression)?)* 'in' expression
-static ExprAST *ParseVarExpr() {
-  getNextToken();  // eat the var.
-
-  std::vector<std::pair<std::string, ExprAST*> > VarNames;
-
-  // At least one variable name is required.
-  if (CurTok != tok_identifier)
-    return Error("expected identifier after var");
-  
-  while (1) {
-    std::string Name = IdentifierStr;
-    getNextToken();  // eat identifier.
-
-    // Read the optional initializer.
-    ExprAST *Init = 0;
-    if (CurTok == '=') {
-      getNextToken(); // eat the '='.
-      
-      Init = ParseExpression();
-      if (Init == 0) return 0;
-    }
-    
-    VarNames.push_back(std::make_pair(Name, Init));
-    
-    // End of var list, exit loop.
-    if (CurTok != ',') break;
-    getNextToken(); // eat the ','.
-    
-    if (CurTok != tok_identifier)
-      return Error("expected identifier list after var");
-  }
-  
-  // At this point, we have to have 'in'.
-  if (CurTok != tok_in)
-    return Error("expected 'in' keyword after 'var'");
-  getNextToken();  // eat 'in'.
-  
-  ExprAST *Body = ParseExpression();
-  if (Body == 0) return 0;
-  
-  return new VarExprAST(VarNames, Body);
-}
-
-/// primary
-///   ::= identifierexpr
-///   ::= numberexpr
-///   ::= parenexpr
-///   ::= ifexpr
-///   ::= forexpr
-///   ::= varexpr
-static ExprAST *ParsePrimary() {
-  switch (CurTok) {
-  default: return Error("unknown token when expecting an expression");
-  case tok_identifier: return ParseIdentifierExpr();
-  case tok_number:     return ParseNumberExpr();
-  case '(':            return ParseParenExpr();
-  case tok_if:         return ParseIfExpr();
-  case tok_for:        return ParseForExpr();
-  case tok_var:        return ParseVarExpr();
-  }
-}
-
-/// unary
-///   ::= primary
-///   ::= '!' unary
-static ExprAST *ParseUnary() {
-  // If the current token is not an operator, it must be a primary expr.
-  if (!isascii(CurTok) || CurTok == '(' || CurTok == ',')
-    return ParsePrimary();
-  
-  // If this is a unary operator, read it.
-  int Opc = CurTok;
-  getNextToken();
-  if (ExprAST *Operand = ParseUnary())
-    return new UnaryExprAST(Opc, Operand);
-  return 0;
-}
-
-/// binoprhs
-///   ::= ('+' unary)*
-static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
-  // If this is a binop, find its precedence.
-  while (1) {
-    int TokPrec = GetTokPrecedence();
-    
-    // If this is a binop that binds at least as tightly as the current binop,
-    // consume it, otherwise we are done.
-    if (TokPrec < ExprPrec)
-      return LHS;
-    
-    // Okay, we know this is a binop.
-    int BinOp = CurTok;
-    getNextToken();  // eat binop
-    
-    // Parse the unary expression after the binary operator.
-    ExprAST *RHS = ParseUnary();
-    if (!RHS) return 0;
-    
-    // If BinOp binds less tightly with RHS than the operator after RHS, let
-    // the pending operator take RHS as its LHS.
-    int NextPrec = GetTokPrecedence();
-    if (TokPrec < NextPrec) {
-      RHS = ParseBinOpRHS(TokPrec+1, RHS);
-      if (RHS == 0) return 0;
-    }
-    
-    // Merge LHS/RHS.
-    LHS = new BinaryExprAST(BinOp, LHS, RHS);
-  }
-}
-
-/// expression
-///   ::= unary binoprhs
-///
-static ExprAST *ParseExpression() {
-  ExprAST *LHS = ParseUnary();
-  if (!LHS) return 0;
-  
-  return ParseBinOpRHS(0, LHS);
-}
-
-/// prototype
-///   ::= id '(' id* ')'
-///   ::= binary LETTER number? (id, id)
-///   ::= unary LETTER (id)
-static PrototypeAST *ParsePrototype() {
-  std::string FnName;
-  
-  unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary.
-  unsigned BinaryPrecedence = 30;
-  
-  switch (CurTok) {
-  default:
-    return ErrorP("Expected function name in prototype");
-  case tok_identifier:
-    FnName = IdentifierStr;
-    Kind = 0;
-    getNextToken();
-    break;
-  case tok_unary:
-    getNextToken();
-    if (!isascii(CurTok))
-      return ErrorP("Expected unary operator");
-    FnName = "unary";
-    FnName += (char)CurTok;
-    Kind = 1;
-    getNextToken();
-    break;
-  case tok_binary:
-    getNextToken();
-    if (!isascii(CurTok))
-      return ErrorP("Expected binary operator");
-    FnName = "binary";
-    FnName += (char)CurTok;
-    Kind = 2;
-    getNextToken();
-    
-    // Read the precedence if present.
-    if (CurTok == tok_number) {
-      if (NumVal < 1 || NumVal > 100)
-        return ErrorP("Invalid precedecnce: must be 1..100");
-      BinaryPrecedence = (unsigned)NumVal;
-      getNextToken();
-    }
-    break;
-  }
-  
-  if (CurTok != '(')
-    return ErrorP("Expected '(' in prototype");
-  
-  std::vector<std::string> ArgNames;
-  while (getNextToken() == tok_identifier)
-    ArgNames.push_back(IdentifierStr);
-  if (CurTok != ')')
-    return ErrorP("Expected ')' in prototype");
-  
-  // success.
-  getNextToken();  // eat ')'.
-  
-  // Verify right number of names for operator.
-  if (Kind && ArgNames.size() != Kind)
-    return ErrorP("Invalid number of operands for operator");
-  
-  return new PrototypeAST(FnName, ArgNames, Kind != 0, BinaryPrecedence);
-}
-
-/// definition ::= 'def' prototype expression
-static FunctionAST *ParseDefinition() {
-  getNextToken();  // eat def.
-  PrototypeAST *Proto = ParsePrototype();
-  if (Proto == 0) return 0;
-
-  if (ExprAST *E = ParseExpression())
-    return new FunctionAST(Proto, E);
-  return 0;
-}
-
-/// toplevelexpr ::= expression
-static FunctionAST *ParseTopLevelExpr() {
-  if (ExprAST *E = ParseExpression()) {
-    // Make an anonymous proto.
-    PrototypeAST *Proto = new PrototypeAST("", std::vector<std::string>());
-    return new FunctionAST(Proto, E);
-  }
-  return 0;
-}
-
-/// external ::= 'extern' prototype
-static PrototypeAST *ParseExtern() {
-  getNextToken();  // eat extern.
-  return ParsePrototype();
-}
-
-//===----------------------------------------------------------------------===//
-// Code Generation
-//===----------------------------------------------------------------------===//
-
-static Module *TheModule;
-static IRBuilder<> Builder(getGlobalContext());
-static std::map<std::string, AllocaInst*> NamedValues;
-static FunctionPassManager *TheFPM;
-
-Value *ErrorV(const char *Str) { Error(Str); return 0; }
-
-/// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of
-/// the function.  This is used for mutable variables etc.
-static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
-                                          const std::string &VarName) {
-  IRBuilder<> TmpB(&TheFunction->getEntryBlock(),
-                 TheFunction->getEntryBlock().begin());
-  return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0,
-                           VarName.c_str());
-}
-
-Value *NumberExprAST::Codegen() {
-  return ConstantFP::get(getGlobalContext(), APFloat(Val));
-}
-
-Value *VariableExprAST::Codegen() {
-  // Look this variable up in the function.
-  Value *V = NamedValues[Name];
-  if (V == 0) return ErrorV("Unknown variable name");
-
-  // Load the value.
-  return Builder.CreateLoad(V, Name.c_str());
-}
-
-Value *UnaryExprAST::Codegen() {
-  Value *OperandV = Operand->Codegen();
-  if (OperandV == 0) return 0;
-  
-  Function *F = TheModule->getFunction(std::string("unary")+Opcode);
-  if (F == 0)
-    return ErrorV("Unknown unary operator");
-  
-  return Builder.CreateCall(F, OperandV, "unop");
-}
-
-Value *BinaryExprAST::Codegen() {
-  // Special case '=' because we don't want to emit the LHS as an expression.
-  if (Op == '=') {
-    // Assignment requires the LHS to be an identifier.
-    VariableExprAST *LHSE = dynamic_cast<VariableExprAST*>(LHS);
-    if (!LHSE)
-      return ErrorV("destination of '=' must be a variable");
-    // Codegen the RHS.
-    Value *Val = RHS->Codegen();
-    if (Val == 0) return 0;
-
-    // Look up the name.
-    Value *Variable = NamedValues[LHSE->getName()];
-    if (Variable == 0) return ErrorV("Unknown variable name");
-
-    Builder.CreateStore(Val, Variable);
-    return Val;
-  }
-  
-  Value *L = LHS->Codegen();
-  Value *R = RHS->Codegen();
-  if (L == 0 || R == 0) return 0;
-  
-  switch (Op) {
-  case '+': return Builder.CreateFAdd(L, R, "addtmp");
-  case '-': return Builder.CreateFSub(L, R, "subtmp");
-  case '*': return Builder.CreateFMul(L, R, "multmp");
-  case '<':
-    L = Builder.CreateFCmpULT(L, R, "cmptmp");
-    // Convert bool 0/1 to double 0.0 or 1.0
-    return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
-                                "booltmp");
-  default: break;
-  }
-  
-  // If it wasn't a builtin binary operator, it must be a user defined one. Emit
-  // a call to it.
-  Function *F = TheModule->getFunction(std::string("binary")+Op);
-  assert(F && "binary operator not found!");
-  
-  Value *Ops[2] = { L, R };
-  return Builder.CreateCall(F, Ops, "binop");
-}
-
-Value *CallExprAST::Codegen() {
-  // Look up the name in the global module table.
-  Function *CalleeF = TheModule->getFunction(Callee);
-  if (CalleeF == 0)
-    return ErrorV("Unknown function referenced");
-  
-  // If argument mismatch error.
-  if (CalleeF->arg_size() != Args.size())
-    return ErrorV("Incorrect # arguments passed");
-
-  std::vector<Value*> ArgsV;
-  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
-    ArgsV.push_back(Args[i]->Codegen());
-    if (ArgsV.back() == 0) return 0;
-  }
-  
-  return Builder.CreateCall(CalleeF, ArgsV, "calltmp");
-}
-
-Value *IfExprAST::Codegen() {
-  Value *CondV = Cond->Codegen();
-  if (CondV == 0) return 0;
-  
-  // Convert condition to a bool by comparing equal to 0.0.
-  CondV = Builder.CreateFCmpONE(CondV, 
-                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
-                                "ifcond");
-  
-  Function *TheFunction = Builder.GetInsertBlock()->getParent();
-  
-  // Create blocks for the then and else cases.  Insert the 'then' block at the
-  // end of the function.
-  BasicBlock *ThenBB = BasicBlock::Create(getGlobalContext(), "then", TheFunction);
-  BasicBlock *ElseBB = BasicBlock::Create(getGlobalContext(), "else");
-  BasicBlock *MergeBB = BasicBlock::Create(getGlobalContext(), "ifcont");
-  
-  Builder.CreateCondBr(CondV, ThenBB, ElseBB);
-  
-  // Emit then value.
-  Builder.SetInsertPoint(ThenBB);
-  
-  Value *ThenV = Then->Codegen();
-  if (ThenV == 0) return 0;
-  
-  Builder.CreateBr(MergeBB);
-  // Codegen of 'Then' can change the current block, update ThenBB for the PHI.
-  ThenBB = Builder.GetInsertBlock();
-  
-  // Emit else block.
-  TheFunction->getBasicBlockList().push_back(ElseBB);
-  Builder.SetInsertPoint(ElseBB);
-  
-  Value *ElseV = Else->Codegen();
-  if (ElseV == 0) return 0;
-  
-  Builder.CreateBr(MergeBB);
-  // Codegen of 'Else' can change the current block, update ElseBB for the PHI.
-  ElseBB = Builder.GetInsertBlock();
-  
-  // Emit merge block.
-  TheFunction->getBasicBlockList().push_back(MergeBB);
-  Builder.SetInsertPoint(MergeBB);
-  PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), 2,
-                                  "iftmp");
-  
-  PN->addIncoming(ThenV, ThenBB);
-  PN->addIncoming(ElseV, ElseBB);
-  return PN;
-}
-
-Value *ForExprAST::Codegen() {
-  // Output this as:
-  //   var = alloca double
-  //   ...
-  //   start = startexpr
-  //   store start -> var
-  //   goto loop
-  // loop: 
-  //   ...
-  //   bodyexpr
-  //   ...
-  // loopend:
-  //   step = stepexpr
-  //   endcond = endexpr
-  //
-  //   curvar = load var
-  //   nextvar = curvar + step
-  //   store nextvar -> var
-  //   br endcond, loop, endloop
-  // outloop:
-  
-  Function *TheFunction = Builder.GetInsertBlock()->getParent();
-
-  // Create an alloca for the variable in the entry block.
-  AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
-  
-  // Emit the start code first, without 'variable' in scope.
-  Value *StartVal = Start->Codegen();
-  if (StartVal == 0) return 0;
-  
-  // Store the value into the alloca.
-  Builder.CreateStore(StartVal, Alloca);
-  
-  // Make the new basic block for the loop header, inserting after current
-  // block.
-  BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction);
-  
-  // Insert an explicit fall through from the current block to the LoopBB.
-  Builder.CreateBr(LoopBB);
-
-  // Start insertion in LoopBB.
-  Builder.SetInsertPoint(LoopBB);
-  
-  // Within the loop, the variable is defined equal to the PHI node.  If it
-  // shadows an existing variable, we have to restore it, so save it now.
-  AllocaInst *OldVal = NamedValues[VarName];
-  NamedValues[VarName] = Alloca;
-  
-  // Emit the body of the loop.  This, like any other expr, can change the
-  // current BB.  Note that we ignore the value computed by the body, but don't
-  // allow an error.
-  if (Body->Codegen() == 0)
-    return 0;
-  
-  // Emit the step value.
-  Value *StepVal;
-  if (Step) {
-    StepVal = Step->Codegen();
-    if (StepVal == 0) return 0;
-  } else {
-    // If not specified, use 1.0.
-    StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0));
-  }
-  
-  // Compute the end condition.
-  Value *EndCond = End->Codegen();
-  if (EndCond == 0) return EndCond;
-  
-  // Reload, increment, and restore the alloca.  This handles the case where
-  // the body of the loop mutates the variable.
-  Value *CurVar = Builder.CreateLoad(Alloca, VarName.c_str());
-  Value *NextVar = Builder.CreateFAdd(CurVar, StepVal, "nextvar");
-  Builder.CreateStore(NextVar, Alloca);
-  
-  // Convert condition to a bool by comparing equal to 0.0.
-  EndCond = Builder.CreateFCmpONE(EndCond, 
-                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
-                                  "loopcond");
-  
-  // Create the "after loop" block and insert it.
-  BasicBlock *AfterBB = BasicBlock::Create(getGlobalContext(), "afterloop", TheFunction);
-  
-  // Insert the conditional branch into the end of LoopEndBB.
-  Builder.CreateCondBr(EndCond, LoopBB, AfterBB);
-  
-  // Any new code will be inserted in AfterBB.
-  Builder.SetInsertPoint(AfterBB);
-  
-  // Restore the unshadowed variable.
-  if (OldVal)
-    NamedValues[VarName] = OldVal;
-  else
-    NamedValues.erase(VarName);
-
-  
-  // for expr always returns 0.0.
-  return Constant::getNullValue(Type::getDoubleTy(getGlobalContext()));
-}
-
-Value *VarExprAST::Codegen() {
-  std::vector<AllocaInst *> OldBindings;
-  
-  Function *TheFunction = Builder.GetInsertBlock()->getParent();
-
-  // Register all variables and emit their initializer.
-  for (unsigned i = 0, e = VarNames.size(); i != e; ++i) {
-    const std::string &VarName = VarNames[i].first;
-    ExprAST *Init = VarNames[i].second;
-    
-    // Emit the initializer before adding the variable to scope, this prevents
-    // the initializer from referencing the variable itself, and permits stuff
-    // like this:
-    //  var a = 1 in
-    //    var a = a in ...   # refers to outer 'a'.
-    Value *InitVal;
-    if (Init) {
-      InitVal = Init->Codegen();
-      if (InitVal == 0) return 0;
-    } else { // If not specified, use 0.0.
-      InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0));
-    }
-    
-    AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
-    Builder.CreateStore(InitVal, Alloca);
-
-    // Remember the old variable binding so that we can restore the binding when
-    // we unrecurse.
-    OldBindings.push_back(NamedValues[VarName]);
-    
-    // Remember this binding.
-    NamedValues[VarName] = Alloca;
-  }
-  
-  // Codegen the body, now that all vars are in scope.
-  Value *BodyVal = Body->Codegen();
-  if (BodyVal == 0) return 0;
-  
-  // Pop all our variables from scope.
-  for (unsigned i = 0, e = VarNames.size(); i != e; ++i)
-    NamedValues[VarNames[i].first] = OldBindings[i];
-
-  // Return the body computation.
-  return BodyVal;
-}
-
-Function *PrototypeAST::Codegen() {
-  // Make the function type:  double(double,double) etc.
-  std::vector<Type*> Doubles(Args.size(),
-                             Type::getDoubleTy(getGlobalContext()));
-  FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
-                                       Doubles, false);
-  
-  Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
-  
-  // If F conflicted, there was already something named 'Name'.  If it has a
-  // body, don't allow redefinition or reextern.
-  if (F->getName() != Name) {
-    // Delete the one we just made and get the existing one.
-    F->eraseFromParent();
-    F = TheModule->getFunction(Name);
-    
-    // If F already has a body, reject this.
-    if (!F->empty()) {
-      ErrorF("redefinition of function");
-      return 0;
-    }
-    
-    // If F took a different number of args, reject.
-    if (F->arg_size() != Args.size()) {
-      ErrorF("redefinition of function with different # args");
-      return 0;
-    }
-  }
-  
-  // Set names for all arguments.
-  unsigned Idx = 0;
-  for (Function::arg_iterator AI = F->arg_begin(); Idx != Args.size();
-       ++AI, ++Idx)
-    AI->setName(Args[Idx]);
-    
-  return F;
-}
-
-/// CreateArgumentAllocas - Create an alloca for each argument and register the
-/// argument in the symbol table so that references to it will succeed.
-void PrototypeAST::CreateArgumentAllocas(Function *F) {
-  Function::arg_iterator AI = F->arg_begin();
-  for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) {
-    // Create an alloca for this variable.
-    AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]);
-
-    // Store the initial value into the alloca.
-    Builder.CreateStore(AI, Alloca);
-
-    // Add arguments to variable symbol table.
-    NamedValues[Args[Idx]] = Alloca;
-  }
-}
-
-Function *FunctionAST::Codegen() {
-  NamedValues.clear();
-  
-  Function *TheFunction = Proto->Codegen();
-  if (TheFunction == 0)
-    return 0;
-  
-  // If this is an operator, install it.
-  if (Proto->isBinaryOp())
-    BinopPrecedence[Proto->getOperatorName()] = Proto->getBinaryPrecedence();
-  
-  // Create a new basic block to start insertion into.
-  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
-  Builder.SetInsertPoint(BB);
-  
-  // Add all arguments to the symbol table and create their allocas.
-  Proto->CreateArgumentAllocas(TheFunction);
-
-  if (Value *RetVal = Body->Codegen()) {
-    // Finish off the function.
-    Builder.CreateRet(RetVal);
-
-    // Validate the generated code, checking for consistency.
-    verifyFunction(*TheFunction);
-
-    // Optimize the function.
-    TheFPM->run(*TheFunction);
-    
-    return TheFunction;
-  }
-  
-  // Error reading body, remove function.
-  TheFunction->eraseFromParent();
-
-  if (Proto->isBinaryOp())
-    BinopPrecedence.erase(Proto->getOperatorName());
-  return 0;
-}
-
-//===----------------------------------------------------------------------===//
-// Top-Level parsing and JIT Driver
-//===----------------------------------------------------------------------===//
-
-static ExecutionEngine *TheExecutionEngine;
-
-static void HandleDefinition() {
-  if (FunctionAST *F = ParseDefinition()) {
-    if (Function *LF = F->Codegen()) {
-      fprintf(stderr, "Read function definition:");
-      LF->dump();
-    }
-  } else {
-    // Skip token for error recovery.
-    getNextToken();
-  }
-}
-
-static void HandleExtern() {
-  if (PrototypeAST *P = ParseExtern()) {
-    if (Function *F = P->Codegen()) {
-      fprintf(stderr, "Read extern: ");
-      F->dump();
-    }
-  } else {
-    // Skip token for error recovery.
-    getNextToken();
-  }
-}
-
-static void HandleTopLevelExpression() {
-  // Evaluate a top-level expression into an anonymous function.
-  if (FunctionAST *F = ParseTopLevelExpr()) {
-    if (Function *LF = F->Codegen()) {
-      // JIT the function, returning a function pointer.
-      void *FPtr = TheExecutionEngine->getPointerToFunction(LF);
-      
-      // Cast it to the right type (takes no arguments, returns a double) so we
-      // can call it as a native function.
-      double (*FP)() = (double (*)())(intptr_t)FPtr;
-      fprintf(stderr, "Evaluated to %f\n", FP());
-    }
-  } else {
-    // Skip token for error recovery.
-    getNextToken();
-  }
-}
-
-/// top ::= definition | external | expression | ';'
-static void MainLoop() {
-  while (1) {
-    fprintf(stderr, "ready> ");
-    switch (CurTok) {
-    case tok_eof:    return;
-    case ';':        getNextToken(); break;  // ignore top-level semicolons.
-    case tok_def:    HandleDefinition(); break;
-    case tok_extern: HandleExtern(); break;
-    default:         HandleTopLevelExpression(); break;
-    }
-  }
-}
-
-//===----------------------------------------------------------------------===//
-// "Library" functions that can be "extern'd" from user code.
-//===----------------------------------------------------------------------===//
-
-/// putchard - putchar that takes a double and returns 0.
-extern "C" 
-double putchard(double X) {
-  putchar((char)X);
-  return 0;
-}
-
-/// printd - printf that takes a double prints it as "%f\n", returning 0.
-extern "C" 
-double printd(double X) {
-  printf("%f\n", X);
-  return 0;
-}
-
-//===----------------------------------------------------------------------===//
-// Main driver code.
-//===----------------------------------------------------------------------===//
-
-int main() {
-  InitializeNativeTarget();
-  LLVMContext &Context = getGlobalContext();
-
-  // Install standard binary operators.
-  // 1 is lowest precedence.
-  BinopPrecedence['='] = 2;
-  BinopPrecedence['<'] = 10;
-  BinopPrecedence['+'] = 20;
-  BinopPrecedence['-'] = 20;
-  BinopPrecedence['*'] = 40;  // highest.
-
-  // Prime the first token.
-  fprintf(stderr, "ready> ");
-  getNextToken();
-
-  // Make the module, which holds all the code.
-  TheModule = new Module("my cool jit", Context);
-
-  // Create the JIT.  This takes ownership of the module.
-  std::string ErrStr;
-  TheExecutionEngine = EngineBuilder(TheModule).setErrorStr(&ErrStr).create();
-  if (!TheExecutionEngine) {
-    fprintf(stderr, "Could not create ExecutionEngine: %s\n", ErrStr.c_str());
-    exit(1);
-  }
-
-  FunctionPassManager OurFPM(TheModule);
-
-  // Set up the optimizer pipeline.  Start with registering info about how the
-  // target lays out data structures.
-  OurFPM.add(new DataLayout(*TheExecutionEngine->getDataLayout()));
-  // Provide basic AliasAnalysis support for GVN.
-  OurFPM.add(createBasicAliasAnalysisPass());
-  // Promote allocas to registers.
-  OurFPM.add(createPromoteMemoryToRegisterPass());
-  // Do simple "peephole" optimizations and bit-twiddling optzns.
-  OurFPM.add(createInstructionCombiningPass());
-  // Reassociate expressions.
-  OurFPM.add(createReassociatePass());
-  // Eliminate Common SubExpressions.
-  OurFPM.add(createGVNPass());
-  // Simplify the control flow graph (deleting unreachable blocks, etc).
-  OurFPM.add(createCFGSimplificationPass());
-
-  OurFPM.doInitialization();
-
-  // Set the global so the code gen can use this.
-  TheFPM = &OurFPM;
-
-  // Run the main "interpreter loop" now.
-  MainLoop();
-
-  TheFPM = 0;
-
-  // Print out all of the generated code.
-  TheModule->dump();
-
-  return 0;
-}
-

- -Next: Conclusion and other useful LLVM tidbits -

- - -

- - Chris Lattner
- The LLVM Compiler Infrastructure
- Last modified: $Date: 2012-10-08 18:39:34 +0200 (Mon, 08 Oct 2012) $ -

- - -- cgit v1.1