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/OCamlLangImpl7.html | 1904 ------------------------------------- 1 file changed, 1904 deletions(-) delete mode 100644 docs/tutorial/OCamlLangImpl7.html (limited to 'docs/tutorial/OCamlLangImpl7.html') diff --git a/docs/tutorial/OCamlLangImpl7.html b/docs/tutorial/OCamlLangImpl7.html deleted file mode 100644 index aa30555..0000000 --- a/docs/tutorial/OCamlLangImpl7.html +++ /dev/null @@ -1,1904 +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 - and Erick Tryzelaar -

- - -

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 -'named_values' 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 -named_values 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 -named_values 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:

- -

Note: the ocaml bindings currently model both Value*s and -AllocInst*s as Llvm.llvalues, but this may change in the -future to be more type safe.

- -

-let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
-

- -

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:

- -

-(* Create an alloca instruction in the entry block of the function. This
- * is used for mutable variables etc. *)
-let create_entry_block_alloca the_function var_name =
-  let builder = builder_at (instr_begin (entry_block the_function)) in
-  build_alloca double_type var_name builder
-

- -

This funny looking code creates an Llvm.llbuilder object that is -pointing at the first instruction 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:

- -

-let rec codegen_expr = function
-  ...
-  | Ast.Variable name ->
-      let v = try Hashtbl.find named_values name with
-        | Not_found -> raise (Error "unknown variable name")
-      in
-      (* Load the value. *)
-      build_load v name builder
-

- -

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 -codegen_expr Ast.For ... (see the full code listing -for the unabridged code):

- -

-  | Ast.For (var_name, start, end_, step, body) ->
-      let the_function = block_parent (insertion_block builder) in
-
-      (* Create an alloca for the variable in the entry block. *)
-      let alloca = create_entry_block_alloca the_function var_name in
-
-      (* Emit the start code first, without 'variable' in scope. *)
-      let start_val = codegen_expr start in
-
-      (* Store the value into the alloca. *)
-      ignore(build_store start_val alloca builder);
-
-      ...
-
-      (* 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. *)
-      let old_val =
-        try Some (Hashtbl.find named_values var_name) with Not_found -> None
-      in
-      Hashtbl.add named_values var_name alloca;
-
-      ...
-
-      (* Compute the end condition. *)
-      let end_cond = codegen_expr end_ in
-
-      (* Reload, increment, and restore the alloca. This handles the case where
-       * the body of the loop mutates the variable. *)
-      let cur_var = build_load alloca var_name builder in
-      let next_var = build_add cur_var step_val "nextvar" builder in
-      ignore(build_store next_var alloca builder);
-      ...
-

- -

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:

- -

-(* Create an alloca for each argument and register the argument in the symbol
- * table so that references to it will succeed. *)
-let create_argument_allocas the_function proto =
-  let args = match proto with
-    | Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -> args
-  in
-  Array.iteri (fun i ai ->
-    let var_name = args.(i) in
-    (* Create an alloca for this variable. *)
-    let alloca = create_entry_block_alloca the_function var_name in
-
-    (* Store the initial value into the alloca. *)
-    ignore(build_store ai alloca builder);
-
-    (* Add arguments to variable symbol table. *)
-    Hashtbl.add named_values var_name alloca;
-  ) (params the_function)
-

- -

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 Codegen.codegen_func 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:

- -

-let main () =
-  ...
-  let the_fpm = PassManager.create_function Codegen.the_module in
-
-  (* Set up the optimizer pipeline.  Start with registering info about how the
-   * target lays out data structures. *)
-  DataLayout.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
-
-  (* Promote allocas to registers. *)
-  add_memory_to_register_promotion the_fpm;
-
-  (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
-  add_instruction_combining the_fpm;
-
-  (* reassociate expressions. *)
-  add_reassociation the_fpm;
-

- -

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:

- -

-let main () =
-  (* Install standard binary operators.
-   * 1 is the lowest precedence. *)
-  Hashtbl.add Parser.binop_precedence '=' 2;
-  Hashtbl.add Parser.binop_precedence '<' 10;
-  Hashtbl.add Parser.binop_precedence '+' 20;
-  Hashtbl.add Parser.binop_precedence '-' 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:

- -

-let rec codegen_expr = function
-      begin match op with
-      | '=' ->
-          (* Special case '=' because we don't want to emit the LHS as an
-           * expression. *)
-          let name =
-            match lhs with
-            | Ast.Variable name -> name
-            | _ -> raise (Error "destination of '=' must be a variable")
-          in
-

- -

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. *)
-          let val_ = codegen_expr rhs in
-
-          (* Lookup the name. *)
-          let variable = try Hashtbl.find named_values name with
-          | Not_found -> raise (Error "unknown variable name")
-          in
-          ignore(build_store val_ variable builder);
-          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:

- -

-type token =
-  ...
-  (* var definition *)
-  | Var
-
-...
-
-and lex_ident buffer = parser
-      ...
-      | "in" -> [< 'Token.In; stream >]
-      | "binary" -> [< 'Token.Binary; stream >]
-      | "unary" -> [< 'Token.Unary; stream >]
-      | "var" -> [< 'Token.Var; stream >]
-      ...
-

- -

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

- -

-type expr =
-  ...
-  (* variant for var/in. *)
-  | Var of (string * expr option) array * expr
-  ...
-

- -

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
- *   ::= identifier
- *   ::= numberexpr
- *   ::= parenexpr
- *   ::= ifexpr
- *   ::= forexpr
- *   ::= varexpr *)
-let rec parse_primary = parser
-  ...
-  (* varexpr
-   *   ::= 'var' identifier ('=' expression?
-   *             (',' identifier ('=' expression)?)* 'in' expression *)
-  | [< 'Token.Var;
-       (* At least one variable name is required. *)
-       'Token.Ident id ?? "expected identifier after var";
-       init=parse_var_init;
-       var_names=parse_var_names [(id, init)];
-       (* At this point, we have to have 'in'. *)
-       'Token.In ?? "expected 'in' keyword after 'var'";
-       body=parse_expr >] ->
-      Ast.Var (Array.of_list (List.rev var_names), body)
-
-...
-
-and parse_var_init = parser
-  (* read in the optional initializer. *)
-  | [< 'Token.Kwd '='; e=parse_expr >] -> Some e
-  | [< >] -> None
-
-and parse_var_names accumulator = parser
-  | [< 'Token.Kwd ',';
-       'Token.Ident id ?? "expected identifier list after var";
-       init=parse_var_init;
-       e=parse_var_names ((id, init) :: accumulator) >] -> e
-  | [< >] -> accumulator
-

- -

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

- -

-let rec codegen_expr = function
-  ...
-  | Ast.Var (var_names, body)
-      let old_bindings = ref [] in
-
-      let the_function = block_parent (insertion_block builder) in
-
-      (* Register all variables and emit their initializer. *)
-      Array.iter (fun (var_name, init) ->
-

- -

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'. *)
-        let init_val =
-          match init with
-          | Some init -> codegen_expr init
-          (* If not specified, use 0.0. *)
-          | None -> const_float double_type 0.0
-        in
-
-        let alloca = create_entry_block_alloca the_function var_name in
-        ignore(build_store init_val alloca builder);
-
-        (* Remember the old variable binding so that we can restore the binding
-         * when we unrecurse. *)
-
-        begin
-          try
-            let old_value = Hashtbl.find named_values var_name in
-            old_bindings := (var_name, old_value) :: !old_bindings;
-          with Not_found > ()
-        end;
-
-        (* Remember this binding. *)
-        Hashtbl.add named_values var_name alloca;
-      ) var_names;
-

- -

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. *)
-      let body_val = codegen_expr body in
-

- -

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

- -

-      (* Pop all our variables from scope. *)
-      List.iter (fun (var_name, old_value) ->
-        Hashtbl.add named_values var_name old_value
-      ) !old_bindings;
-
-      (* Return the body computation. *)
-      body_val
-

- -

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
-ocamlbuild toy.byte
-# Run
-./toy.byte
-

- -

Here is the code:

- -

_tags:

-<{lexer,parser}.ml>: use_camlp4, pp(camlp4of)
-<*.{byte,native}>: g++, use_llvm, use_llvm_analysis
-<*.{byte,native}>: use_llvm_executionengine, use_llvm_target
-<*.{byte,native}>: use_llvm_scalar_opts, use_bindings
-

myocamlbuild.ml:

-open Ocamlbuild_plugin;;
-
-ocaml_lib ~extern:true "llvm";;
-ocaml_lib ~extern:true "llvm_analysis";;
-ocaml_lib ~extern:true "llvm_executionengine";;
-ocaml_lib ~extern:true "llvm_target";;
-ocaml_lib ~extern:true "llvm_scalar_opts";;
-
-flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"; A"-cclib"; A"-rdynamic"]);;
-dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
-

token.ml:

-(*===----------------------------------------------------------------------===
- * Lexer Tokens
- *===----------------------------------------------------------------------===*)
-
-(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
- * these others for known things. *)
-type token =
-  (* commands *)
-  | Def | Extern
-
-  (* primary *)
-  | Ident of string | Number of float
-
-  (* unknown *)
-  | Kwd of char
-
-  (* control *)
-  | If | Then | Else
-  | For | In
-
-  (* operators *)
-  | Binary | Unary
-
-  (* var definition *)
-  | Var
-

lexer.ml:

-(*===----------------------------------------------------------------------===
- * Lexer
- *===----------------------------------------------------------------------===*)
-
-let rec lex = parser
-  (* Skip any whitespace. *)
-  | [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
-
-  (* identifier: [a-zA-Z][a-zA-Z0-9] *)
-  | [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
-      let buffer = Buffer.create 1 in
-      Buffer.add_char buffer c;
-      lex_ident buffer stream
-
-  (* number: [0-9.]+ *)
-  | [< ' ('0' .. '9' as c); stream >] ->
-      let buffer = Buffer.create 1 in
-      Buffer.add_char buffer c;
-      lex_number buffer stream
-
-  (* Comment until end of line. *)
-  | [< ' ('#'); stream >] ->
-      lex_comment stream
-
-  (* Otherwise, just return the character as its ascii value. *)
-  | [< 'c; stream >] ->
-      [< 'Token.Kwd c; lex stream >]
-
-  (* end of stream. *)
-  | [< >] -> [< >]
-
-and lex_number buffer = parser
-  | [< ' ('0' .. '9' | '.' as c); stream >] ->
-      Buffer.add_char buffer c;
-      lex_number buffer stream
-  | [< stream=lex >] ->
-      [< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
-
-and lex_ident buffer = parser
-  | [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
-      Buffer.add_char buffer c;
-      lex_ident buffer stream
-  | [< stream=lex >] ->
-      match Buffer.contents buffer with
-      | "def" -> [< 'Token.Def; stream >]
-      | "extern" -> [< 'Token.Extern; stream >]
-      | "if" -> [< 'Token.If; stream >]
-      | "then" -> [< 'Token.Then; stream >]
-      | "else" -> [< 'Token.Else; stream >]
-      | "for" -> [< 'Token.For; stream >]
-      | "in" -> [< 'Token.In; stream >]
-      | "binary" -> [< 'Token.Binary; stream >]
-      | "unary" -> [< 'Token.Unary; stream >]
-      | "var" -> [< 'Token.Var; stream >]
-      | id -> [< 'Token.Ident id; stream >]
-
-and lex_comment = parser
-  | [< ' ('\n'); stream=lex >] -> stream
-  | [< 'c; e=lex_comment >] -> e
-  | [< >] -> [< >]
-

ast.ml:

-(*===----------------------------------------------------------------------===
- * Abstract Syntax Tree (aka Parse Tree)
- *===----------------------------------------------------------------------===*)
-
-(* expr - Base type for all expression nodes. *)
-type expr =
-  (* variant for numeric literals like "1.0". *)
-  | Number of float
-
-  (* variant for referencing a variable, like "a". *)
-  | Variable of string
-
-  (* variant for a unary operator. *)
-  | Unary of char * expr
-
-  (* variant for a binary operator. *)
-  | Binary of char * expr * expr
-
-  (* variant for function calls. *)
-  | Call of string * expr array
-
-  (* variant for if/then/else. *)
-  | If of expr * expr * expr
-
-  (* variant for for/in. *)
-  | For of string * expr * expr * expr option * expr
-
-  (* variant for var/in. *)
-  | Var of (string * expr option) array * expr
-
-(* proto - This type represents the "prototype" for a function, which captures
- * its name, and its argument names (thus implicitly the number of arguments the
- * function takes). *)
-type proto =
-  | Prototype of string * string array
-  | BinOpPrototype of string * string array * int
-
-(* func - This type represents a function definition itself. *)
-type func = Function of proto * expr
-

parser.ml:

-(*===---------------------------------------------------------------------===
- * Parser
- *===---------------------------------------------------------------------===*)
-
-(* binop_precedence - This holds the precedence for each binary operator that is
- * defined *)
-let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
-
-(* precedence - Get the precedence of the pending binary operator token. *)
-let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1
-
-(* primary
- *   ::= identifier
- *   ::= numberexpr
- *   ::= parenexpr
- *   ::= ifexpr
- *   ::= forexpr
- *   ::= varexpr *)
-let rec parse_primary = parser
-  (* numberexpr ::= number *)
-  | [< 'Token.Number n >] -> Ast.Number n
-
-  (* parenexpr ::= '(' expression ')' *)
-  | [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e
-
-  (* identifierexpr
-   *   ::= identifier
-   *   ::= identifier '(' argumentexpr ')' *)
-  | [< 'Token.Ident id; stream >] ->
-      let rec parse_args accumulator = parser
-        | [< e=parse_expr; stream >] ->
-            begin parser
-              | [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
-              | [< >] -> e :: accumulator
-            end stream
-        | [< >] -> accumulator
-      in
-      let rec parse_ident id = parser
-        (* Call. *)
-        | [< 'Token.Kwd '(';
-             args=parse_args [];
-             'Token.Kwd ')' ?? "expected ')'">] ->
-            Ast.Call (id, Array.of_list (List.rev args))
-
-        (* Simple variable ref. *)
-        | [< >] -> Ast.Variable id
-      in
-      parse_ident id stream
-
-  (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
-  | [< 'Token.If; c=parse_expr;
-       'Token.Then ?? "expected 'then'"; t=parse_expr;
-       'Token.Else ?? "expected 'else'"; e=parse_expr >] ->
-      Ast.If (c, t, e)
-
-  (* forexpr
-        ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
-  | [< 'Token.For;
-       'Token.Ident id ?? "expected identifier after for";
-       'Token.Kwd '=' ?? "expected '=' after for";
-       stream >] ->
-      begin parser
-        | [<
-             start=parse_expr;
-             'Token.Kwd ',' ?? "expected ',' after for";
-             end_=parse_expr;
-             stream >] ->
-            let step =
-              begin parser
-              | [< 'Token.Kwd ','; step=parse_expr >] -> Some step
-              | [< >] -> None
-              end stream
-            in
-            begin parser
-            | [< 'Token.In; body=parse_expr >] ->
-                Ast.For (id, start, end_, step, body)
-            | [< >] ->
-                raise (Stream.Error "expected 'in' after for")
-            end stream
-        | [< >] ->
-            raise (Stream.Error "expected '=' after for")
-      end stream
-
-  (* varexpr
-   *   ::= 'var' identifier ('=' expression?
-   *             (',' identifier ('=' expression)?)* 'in' expression *)
-  | [< 'Token.Var;
-       (* At least one variable name is required. *)
-       'Token.Ident id ?? "expected identifier after var";
-       init=parse_var_init;
-       var_names=parse_var_names [(id, init)];
-       (* At this point, we have to have 'in'. *)
-       'Token.In ?? "expected 'in' keyword after 'var'";
-       body=parse_expr >] ->
-      Ast.Var (Array.of_list (List.rev var_names), body)
-
-  | [< >] -> raise (Stream.Error "unknown token when expecting an expression.")
-
-(* unary
- *   ::= primary
- *   ::= '!' unary *)
-and parse_unary = parser
-  (* If this is a unary operator, read it. *)
-  | [< 'Token.Kwd op when op != '(' && op != ')'; operand=parse_expr >] ->
-      Ast.Unary (op, operand)
-
-  (* If the current token is not an operator, it must be a primary expr. *)
-  | [< stream >] -> parse_primary stream
-
-(* binoprhs
- *   ::= ('+' primary)* *)
-and parse_bin_rhs expr_prec lhs stream =
-  match Stream.peek stream with
-  (* If this is a binop, find its precedence. *)
-  | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
-      let token_prec = precedence c in
-
-      (* If this is a binop that binds at least as tightly as the current binop,
-       * consume it, otherwise we are done. *)
-      if token_prec < expr_prec then lhs else begin
-        (* Eat the binop. *)
-        Stream.junk stream;
-
-        (* Parse the primary expression after the binary operator. *)
-        let rhs = parse_unary stream in
-
-        (* Okay, we know this is a binop. *)
-        let rhs =
-          match Stream.peek stream with
-          | Some (Token.Kwd c2) ->
-              (* If BinOp binds less tightly with rhs than the operator after
-               * rhs, let the pending operator take rhs as its lhs. *)
-              let next_prec = precedence c2 in
-              if token_prec < next_prec
-              then parse_bin_rhs (token_prec + 1) rhs stream
-              else rhs
-          | _ -> rhs
-        in
-
-        (* Merge lhs/rhs. *)
-        let lhs = Ast.Binary (c, lhs, rhs) in
-        parse_bin_rhs expr_prec lhs stream
-      end
-  | _ -> lhs
-
-and parse_var_init = parser
-  (* read in the optional initializer. *)
-  | [< 'Token.Kwd '='; e=parse_expr >] -> Some e
-  | [< >] -> None
-
-and parse_var_names accumulator = parser
-  | [< 'Token.Kwd ',';
-       'Token.Ident id ?? "expected identifier list after var";
-       init=parse_var_init;
-       e=parse_var_names ((id, init) :: accumulator) >] -> e
-  | [< >] -> accumulator
-
-(* expression
- *   ::= primary binoprhs *)
-and parse_expr = parser
-  | [< lhs=parse_unary; stream >] -> parse_bin_rhs 0 lhs stream
-
-(* prototype
- *   ::= id '(' id* ')'
- *   ::= binary LETTER number? (id, id)
- *   ::= unary LETTER number? (id) *)
-let parse_prototype =
-  let rec parse_args accumulator = parser
-    | [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
-    | [< >] -> accumulator
-  in
-  let parse_operator = parser
-    | [< 'Token.Unary >] -> "unary", 1
-    | [< 'Token.Binary >] -> "binary", 2
-  in
-  let parse_binary_precedence = parser
-    | [< 'Token.Number n >] -> int_of_float n
-    | [< >] -> 30
-  in
-  parser
-  | [< 'Token.Ident id;
-       'Token.Kwd '(' ?? "expected '(' in prototype";
-       args=parse_args [];
-       'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
-      (* success. *)
-      Ast.Prototype (id, Array.of_list (List.rev args))
-  | [< (prefix, kind)=parse_operator;
-       'Token.Kwd op ?? "expected an operator";
-       (* Read the precedence if present. *)
-       binary_precedence=parse_binary_precedence;
-       'Token.Kwd '(' ?? "expected '(' in prototype";
-        args=parse_args [];
-       'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
-      let name = prefix ^ (String.make 1 op) in
-      let args = Array.of_list (List.rev args) in
-
-      (* Verify right number of arguments for operator. *)
-      if Array.length args != kind
-      then raise (Stream.Error "invalid number of operands for operator")
-      else
-        if kind == 1 then
-          Ast.Prototype (name, args)
-        else
-          Ast.BinOpPrototype (name, args, binary_precedence)
-  | [< >] ->
-      raise (Stream.Error "expected function name in prototype")
-
-(* definition ::= 'def' prototype expression *)
-let parse_definition = parser
-  | [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
-      Ast.Function (p, e)
-
-(* toplevelexpr ::= expression *)
-let parse_toplevel = parser
-  | [< e=parse_expr >] ->
-      (* Make an anonymous proto. *)
-      Ast.Function (Ast.Prototype ("", [||]), e)
-
-(*  external ::= 'extern' prototype *)
-let parse_extern = parser
-  | [< 'Token.Extern; e=parse_prototype >] -> e
-

codegen.ml:

-(*===----------------------------------------------------------------------===
- * Code Generation
- *===----------------------------------------------------------------------===*)
-
-open Llvm
-
-exception Error of string
-
-let context = global_context ()
-let the_module = create_module context "my cool jit"
-let builder = builder context
-let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
-let double_type = double_type context
-
-(* Create an alloca instruction in the entry block of the function. This
- * is used for mutable variables etc. *)
-let create_entry_block_alloca the_function var_name =
-  let builder = builder_at context (instr_begin (entry_block the_function)) in
-  build_alloca double_type var_name builder
-
-let rec codegen_expr = function
-  | Ast.Number n -> const_float double_type n
-  | Ast.Variable name ->
-      let v = try Hashtbl.find named_values name with
-        | Not_found -> raise (Error "unknown variable name")
-      in
-      (* Load the value. *)
-      build_load v name builder
-  | Ast.Unary (op, operand) ->
-      let operand = codegen_expr operand in
-      let callee = "unary" ^ (String.make 1 op) in
-      let callee =
-        match lookup_function callee the_module with
-        | Some callee -> callee
-        | None -> raise (Error "unknown unary operator")
-      in
-      build_call callee [|operand|] "unop" builder
-  | Ast.Binary (op, lhs, rhs) ->
-      begin match op with
-      | '=' ->
-          (* Special case '=' because we don't want to emit the LHS as an
-           * expression. *)
-          let name =
-            match lhs with
-            | Ast.Variable name -> name
-            | _ -> raise (Error "destination of '=' must be a variable")
-          in
-
-          (* Codegen the rhs. *)
-          let val_ = codegen_expr rhs in
-
-          (* Lookup the name. *)
-          let variable = try Hashtbl.find named_values name with
-          | Not_found -> raise (Error "unknown variable name")
-          in
-          ignore(build_store val_ variable builder);
-          val_
-      | _ ->
-          let lhs_val = codegen_expr lhs in
-          let rhs_val = codegen_expr rhs in
-          begin
-            match op with
-            | '+' -> build_add lhs_val rhs_val "addtmp" builder
-            | '-' -> build_sub lhs_val rhs_val "subtmp" builder
-            | '*' -> build_mul lhs_val rhs_val "multmp" builder
-            | '<' ->
-                (* Convert bool 0/1 to double 0.0 or 1.0 *)
-                let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
-                build_uitofp i double_type "booltmp" builder
-            | _ ->
-                (* If it wasn't a builtin binary operator, it must be a user defined
-                 * one. Emit a call to it. *)
-                let callee = "binary" ^ (String.make 1 op) in
-                let callee =
-                  match lookup_function callee the_module with
-                  | Some callee -> callee
-                  | None -> raise (Error "binary operator not found!")
-                in
-                build_call callee [|lhs_val; rhs_val|] "binop" builder
-          end
-      end
-  | Ast.Call (callee, args) ->
-      (* Look up the name in the module table. *)
-      let callee =
-        match lookup_function callee the_module with
-        | Some callee -> callee
-        | None -> raise (Error "unknown function referenced")
-      in
-      let params = params callee in
-
-      (* If argument mismatch error. *)
-      if Array.length params == Array.length args then () else
-        raise (Error "incorrect # arguments passed");
-      let args = Array.map codegen_expr args in
-      build_call callee args "calltmp" builder
-  | Ast.If (cond, then_, else_) ->
-      let cond = codegen_expr cond in
-
-      (* Convert condition to a bool by comparing equal to 0.0 *)
-      let zero = const_float double_type 0.0 in
-      let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
-
-      (* Grab the first block so that we might later add the conditional branch
-       * to it at the end of the function. *)
-      let start_bb = insertion_block builder in
-      let the_function = block_parent start_bb in
-
-      let then_bb = append_block context "then" the_function in
-
-      (* Emit 'then' value. *)
-      position_at_end then_bb builder;
-      let then_val = codegen_expr then_ in
-
-      (* Codegen of 'then' can change the current block, update then_bb for the
-       * phi. We create a new name because one is used for the phi node, and the
-       * other is used for the conditional branch. *)
-      let new_then_bb = insertion_block builder in
-
-      (* Emit 'else' value. *)
-      let else_bb = append_block context "else" the_function in
-      position_at_end else_bb builder;
-      let else_val = codegen_expr else_ in
-
-      (* Codegen of 'else' can change the current block, update else_bb for the
-       * phi. *)
-      let new_else_bb = insertion_block builder in
-
-      (* Emit merge block. *)
-      let merge_bb = append_block context "ifcont" the_function in
-      position_at_end merge_bb builder;
-      let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
-      let phi = build_phi incoming "iftmp" builder in
-
-      (* Return to the start block to add the conditional branch. *)
-      position_at_end start_bb builder;
-      ignore (build_cond_br cond_val then_bb else_bb builder);
-
-      (* Set a unconditional branch at the end of the 'then' block and the
-       * 'else' block to the 'merge' block. *)
-      position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
-      position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
-
-      (* Finally, set the builder to the end of the merge block. *)
-      position_at_end merge_bb builder;
-
-      phi
-  | Ast.For (var_name, start, end_, step, body) ->
-      (* 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: *)
-
-      let the_function = block_parent (insertion_block builder) in
-
-      (* Create an alloca for the variable in the entry block. *)
-      let alloca = create_entry_block_alloca the_function var_name in
-
-      (* Emit the start code first, without 'variable' in scope. *)
-      let start_val = codegen_expr start in
-
-      (* Store the value into the alloca. *)
-      ignore(build_store start_val alloca builder);
-
-      (* Make the new basic block for the loop header, inserting after current
-       * block. *)
-      let loop_bb = append_block context "loop" the_function in
-
-      (* Insert an explicit fall through from the current block to the
-       * loop_bb. *)
-      ignore (build_br loop_bb builder);
-
-      (* Start insertion in loop_bb. *)
-      position_at_end loop_bb builder;
-
-      (* 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. *)
-      let old_val =
-        try Some (Hashtbl.find named_values var_name) with Not_found -> None
-      in
-      Hashtbl.add named_values var_name 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 *)
-      ignore (codegen_expr body);
-
-      (* Emit the step value. *)
-      let step_val =
-        match step with
-        | Some step -> codegen_expr step
-        (* If not specified, use 1.0. *)
-        | None -> const_float double_type 1.0
-      in
-
-      (* Compute the end condition. *)
-      let end_cond = codegen_expr end_ in
-
-      (* Reload, increment, and restore the alloca. This handles the case where
-       * the body of the loop mutates the variable. *)
-      let cur_var = build_load alloca var_name builder in
-      let next_var = build_add cur_var step_val "nextvar" builder in
-      ignore(build_store next_var alloca builder);
-
-      (* Convert condition to a bool by comparing equal to 0.0. *)
-      let zero = const_float double_type 0.0 in
-      let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
-
-      (* Create the "after loop" block and insert it. *)
-      let after_bb = append_block context "afterloop" the_function in
-
-      (* Insert the conditional branch into the end of loop_end_bb. *)
-      ignore (build_cond_br end_cond loop_bb after_bb builder);
-
-      (* Any new code will be inserted in after_bb. *)
-      position_at_end after_bb builder;
-
-      (* Restore the unshadowed variable. *)
-      begin match old_val with
-      | Some old_val -> Hashtbl.add named_values var_name old_val
-      | None -> ()
-      end;
-
-      (* for expr always returns 0.0. *)
-      const_null double_type
-  | Ast.Var (var_names, body) ->
-      let old_bindings = ref [] in
-
-      let the_function = block_parent (insertion_block builder) in
-
-      (* Register all variables and emit their initializer. *)
-      Array.iter (fun (var_name, init) ->
-        (* 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'. *)
-        let init_val =
-          match init with
-          | Some init -> codegen_expr init
-          (* If not specified, use 0.0. *)
-          | None -> const_float double_type 0.0
-        in
-
-        let alloca = create_entry_block_alloca the_function var_name in
-        ignore(build_store init_val alloca builder);
-
-        (* Remember the old variable binding so that we can restore the binding
-         * when we unrecurse. *)
-        begin
-          try
-            let old_value = Hashtbl.find named_values var_name in
-            old_bindings := (var_name, old_value) :: !old_bindings;
-          with Not_found -> ()
-        end;
-
-        (* Remember this binding. *)
-        Hashtbl.add named_values var_name alloca;
-      ) var_names;
-
-      (* Codegen the body, now that all vars are in scope. *)
-      let body_val = codegen_expr body in
-
-      (* Pop all our variables from scope. *)
-      List.iter (fun (var_name, old_value) ->
-        Hashtbl.add named_values var_name old_value
-      ) !old_bindings;
-
-      (* Return the body computation. *)
-      body_val
-
-let codegen_proto = function
-  | Ast.Prototype (name, args) | Ast.BinOpPrototype (name, args, _) ->
-      (* Make the function type: double(double,double) etc. *)
-      let doubles = Array.make (Array.length args) double_type in
-      let ft = function_type double_type doubles in
-      let f =
-        match lookup_function name the_module with
-        | None -> declare_function name ft the_module
-
-        (* If 'f' conflicted, there was already something named 'name'. If it
-         * has a body, don't allow redefinition or reextern. *)
-        | Some f ->
-            (* If 'f' already has a body, reject this. *)
-            if block_begin f <> At_end f then
-              raise (Error "redefinition of function");
-
-            (* If 'f' took a different number of arguments, reject. *)
-            if element_type (type_of f) <> ft then
-              raise (Error "redefinition of function with different # args");
-            f
-      in
-
-      (* Set names for all arguments. *)
-      Array.iteri (fun i a ->
-        let n = args.(i) in
-        set_value_name n a;
-        Hashtbl.add named_values n a;
-      ) (params f);
-      f
-
-(* Create an alloca for each argument and register the argument in the symbol
- * table so that references to it will succeed. *)
-let create_argument_allocas the_function proto =
-  let args = match proto with
-    | Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -> args
-  in
-  Array.iteri (fun i ai ->
-    let var_name = args.(i) in
-    (* Create an alloca for this variable. *)
-    let alloca = create_entry_block_alloca the_function var_name in
-
-    (* Store the initial value into the alloca. *)
-    ignore(build_store ai alloca builder);
-
-    (* Add arguments to variable symbol table. *)
-    Hashtbl.add named_values var_name alloca;
-  ) (params the_function)
-
-let codegen_func the_fpm = function
-  | Ast.Function (proto, body) ->
-      Hashtbl.clear named_values;
-      let the_function = codegen_proto proto in
-
-      (* If this is an operator, install it. *)
-      begin match proto with
-      | Ast.BinOpPrototype (name, args, prec) ->
-          let op = name.[String.length name - 1] in
-          Hashtbl.add Parser.binop_precedence op prec;
-      | _ -> ()
-      end;
-
-      (* Create a new basic block to start insertion into. *)
-      let bb = append_block context "entry" the_function in
-      position_at_end bb builder;
-
-      try
-        (* Add all arguments to the symbol table and create their allocas. *)
-        create_argument_allocas the_function proto;
-
-        let ret_val = codegen_expr body in
-
-        (* Finish off the function. *)
-        let _ = build_ret ret_val builder in
-
-        (* Validate the generated code, checking for consistency. *)
-        Llvm_analysis.assert_valid_function the_function;
-
-        (* Optimize the function. *)
-        let _ = PassManager.run_function the_function the_fpm in
-
-        the_function
-      with e ->
-        delete_function the_function;
-        raise e
-

toplevel.ml:

-(*===----------------------------------------------------------------------===
- * Top-Level parsing and JIT Driver
- *===----------------------------------------------------------------------===*)
-
-open Llvm
-open Llvm_executionengine
-
-(* top ::= definition | external | expression | ';' *)
-let rec main_loop the_fpm the_execution_engine stream =
-  match Stream.peek stream with
-  | None -> ()
-
-  (* ignore top-level semicolons. *)
-  | Some (Token.Kwd ';') ->
-      Stream.junk stream;
-      main_loop the_fpm the_execution_engine stream
-
-  | Some token ->
-      begin
-        try match token with
-        | Token.Def ->
-            let e = Parser.parse_definition stream in
-            print_endline "parsed a function definition.";
-            dump_value (Codegen.codegen_func the_fpm e);
-        | Token.Extern ->
-            let e = Parser.parse_extern stream in
-            print_endline "parsed an extern.";
-            dump_value (Codegen.codegen_proto e);
-        | _ ->
-            (* Evaluate a top-level expression into an anonymous function. *)
-            let e = Parser.parse_toplevel stream in
-            print_endline "parsed a top-level expr";
-            let the_function = Codegen.codegen_func the_fpm e in
-            dump_value the_function;
-
-            (* JIT the function, returning a function pointer. *)
-            let result = ExecutionEngine.run_function the_function [||]
-              the_execution_engine in
-
-            print_string "Evaluated to ";
-            print_float (GenericValue.as_float Codegen.double_type result);
-            print_newline ();
-        with Stream.Error s | Codegen.Error s ->
-          (* Skip token for error recovery. *)
-          Stream.junk stream;
-          print_endline s;
-      end;
-      print_string "ready> "; flush stdout;
-      main_loop the_fpm the_execution_engine stream
-

toy.ml:

-(*===----------------------------------------------------------------------===
- * Main driver code.
- *===----------------------------------------------------------------------===*)
-
-open Llvm
-open Llvm_executionengine
-open Llvm_target
-open Llvm_scalar_opts
-
-let main () =
-  ignore (initialize_native_target ());
-
-  (* Install standard binary operators.
-   * 1 is the lowest precedence. *)
-  Hashtbl.add Parser.binop_precedence '=' 2;
-  Hashtbl.add Parser.binop_precedence '<' 10;
-  Hashtbl.add Parser.binop_precedence '+' 20;
-  Hashtbl.add Parser.binop_precedence '-' 20;
-  Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
-
-  (* Prime the first token. *)
-  print_string "ready> "; flush stdout;
-  let stream = Lexer.lex (Stream.of_channel stdin) in
-
-  (* Create the JIT. *)
-  let the_execution_engine = ExecutionEngine.create Codegen.the_module in
-  let the_fpm = PassManager.create_function Codegen.the_module in
-
-  (* Set up the optimizer pipeline.  Start with registering info about how the
-   * target lays out data structures. *)
-  DataLayout.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
-
-  (* Promote allocas to registers. *)
-  add_memory_to_register_promotion the_fpm;
-
-  (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
-  add_instruction_combination the_fpm;
-
-  (* reassociate expressions. *)
-  add_reassociation the_fpm;
-
-  (* Eliminate Common SubExpressions. *)
-  add_gvn the_fpm;
-
-  (* Simplify the control flow graph (deleting unreachable blocks, etc). *)
-  add_cfg_simplification the_fpm;
-
-  ignore (PassManager.initialize the_fpm);
-
-  (* Run the main "interpreter loop" now. *)
-  Toplevel.main_loop the_fpm the_execution_engine stream;
-
-  (* Print out all the generated code. *)
-  dump_module Codegen.the_module
-;;
-
-main ()
-

bindings.c

-#include <stdio.h>
-
-/* putchard - putchar that takes a double and returns 0. */
-extern double putchard(double X) {
-  putchar((char)X);
-  return 0;
-}
-
-/* printd - printf that takes a double prints it as "%f\n", returning 0. */
-extern double printd(double X) {
-  printf("%f\n", X);
-  return 0;
-}
-

- -Next: Conclusion and other useful LLVM tidbits -

- - -

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

- - -- cgit v1.1