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//===-- NVPTXInferAddressSpace.cpp - ---------------------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// CUDA C/C++ includes memory space designation as variable type qualifers (such
// as __global__ and __shared__). Knowing the space of a memory access allows
// CUDA compilers to emit faster PTX loads and stores. For example, a load from
// shared memory can be translated to `ld.shared` which is roughly 10% faster
// than a generic `ld` on an NVIDIA Tesla K40c.
//
// Unfortunately, type qualifiers only apply to variable declarations, so CUDA
// compilers must infer the memory space of an address expression from
// type-qualified variables.
//
// LLVM IR uses non-zero (so-called) specific address spaces to represent memory
// spaces (e.g. addrspace(3) means shared memory). The Clang frontend
// places only type-qualified variables in specific address spaces, and then
// conservatively `addrspacecast`s each type-qualified variable to addrspace(0)
// (so-called the generic address space) for other instructions to use.
//
// For example, the Clang translates the following CUDA code
// __shared__ float a[10];
// float v = a[i];
// to
// %0 = addrspacecast [10 x float] addrspace(3)* @a to [10 x float]*
// %1 = gep [10 x float], [10 x float]* %0, i64 0, i64 %i
// %v = load float, float* %1 ; emits ld.f32
// @a is in addrspace(3) since it's type-qualified, but its use from %1 is
// redirected to %0 (the generic version of @a).
//
// The optimization implemented in this file propagates specific address spaces
// from type-qualified variable declarations to its users. For example, it
// optimizes the above IR to
// %1 = gep [10 x float] addrspace(3)* @a, i64 0, i64 %i
// %v = load float addrspace(3)* %1 ; emits ld.shared.f32
// propagating the addrspace(3) from @a to %1. As the result, the NVPTX
// codegen is able to emit ld.shared.f32 for %v.
//
// Address space inference works in two steps. First, it uses a data-flow
// analysis to infer as many generic pointers as possible to point to only one
// specific address space. In the above example, it can prove that %1 only
// points to addrspace(3). This algorithm was published in
// CUDA: Compiling and optimizing for a GPU platform
// Chakrabarti, Grover, Aarts, Kong, Kudlur, Lin, Marathe, Murphy, Wang
// ICCS 2012
//
// Then, address space inference replaces all refinable generic pointers with
// equivalent specific pointers.
//
// The major challenge of implementing this optimization is handling PHINodes,
// which may create loops in the data flow graph. This brings two complications.
//
// First, the data flow analysis in Step 1 needs to be circular. For example,
// %generic.input = addrspacecast float addrspace(3)* %input to float*
// loop:
// %y = phi [ %generic.input, %y2 ]
// %y2 = getelementptr %y, 1
// %v = load %y2
// br ..., label %loop, ...
// proving %y specific requires proving both %generic.input and %y2 specific,
// but proving %y2 specific circles back to %y. To address this complication,
// the data flow analysis operates on a lattice:
// uninitialized > specific address spaces > generic.
// All address expressions (our implementation only considers phi, bitcast,
// addrspacecast, and getelementptr) start with the uninitialized address space.
// The monotone transfer function moves the address space of a pointer down a
// lattice path from uninitialized to specific and then to generic. A join
// operation of two different specific address spaces pushes the expression down
// to the generic address space. The analysis completes once it reaches a fixed
// point.
//
// Second, IR rewriting in Step 2 also needs to be circular. For example,
// converting %y to addrspace(3) requires the compiler to know the converted
// %y2, but converting %y2 needs the converted %y. To address this complication,
// we break these cycles using "undef" placeholders. When converting an
// instruction `I` to a new address space, if its operand `Op` is not converted
// yet, we let `I` temporarily use `undef` and fix all the uses of undef later.
// For instance, our algorithm first converts %y to
// %y' = phi float addrspace(3)* [ %input, undef ]
// Then, it converts %y2 to
// %y2' = getelementptr %y', 1
// Finally, it fixes the undef in %y' so that
// %y' = phi float addrspace(3)* [ %input, %y2' ]
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "nvptx-infer-addrspace"
#include "NVPTX.h"
#include "MCTargetDesc/NVPTXBaseInfo.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Operator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
using namespace llvm;
namespace {
const unsigned ADDRESS_SPACE_UNINITIALIZED = (unsigned)-1;
using ValueToAddrSpaceMapTy = DenseMap<const Value *, unsigned>;
/// \brief NVPTXInferAddressSpaces
class NVPTXInferAddressSpaces: public FunctionPass {
public:
static char ID;
NVPTXInferAddressSpaces() : FunctionPass(ID) {}
bool runOnFunction(Function &F) override;
private:
// Returns the new address space of V if updated; otherwise, returns None.
Optional<unsigned>
updateAddressSpace(const Value &V,
const ValueToAddrSpaceMapTy &InferredAddrSpace);
// Tries to infer the specific address space of each address expression in
// Postorder.
void inferAddressSpaces(const std::vector<Value *> &Postorder,
ValueToAddrSpaceMapTy *InferredAddrSpace);
// Changes the generic address expressions in function F to point to specific
// address spaces if InferredAddrSpace says so. Postorder is the postorder of
// all generic address expressions in the use-def graph of function F.
bool
rewriteWithNewAddressSpaces(const std::vector<Value *> &Postorder,
const ValueToAddrSpaceMapTy &InferredAddrSpace,
Function *F);
};
} // end anonymous namespace
char NVPTXInferAddressSpaces::ID = 0;
namespace llvm {
void initializeNVPTXInferAddressSpacesPass(PassRegistry &);
}
INITIALIZE_PASS(NVPTXInferAddressSpaces, "nvptx-infer-addrspace",
"Infer address spaces",
false, false)
// Returns true if V is an address expression.
// TODO: Currently, we consider only phi, bitcast, addrspacecast, and
// getelementptr operators.
static bool isAddressExpression(const Value &V) {
if (!isa<Operator>(V))
return false;
switch (cast<Operator>(V).getOpcode()) {
case Instruction::PHI:
case Instruction::BitCast:
case Instruction::AddrSpaceCast:
case Instruction::GetElementPtr:
return true;
default:
return false;
}
}
// Returns the pointer operands of V.
//
// Precondition: V is an address expression.
static SmallVector<Value *, 2> getPointerOperands(const Value &V) {
assert(isAddressExpression(V));
const Operator& Op = cast<Operator>(V);
switch (Op.getOpcode()) {
case Instruction::PHI: {
auto IncomingValues = cast<PHINode>(Op).incoming_values();
return SmallVector<Value *, 2>(IncomingValues.begin(),
IncomingValues.end());
}
case Instruction::BitCast:
case Instruction::AddrSpaceCast:
case Instruction::GetElementPtr:
return {Op.getOperand(0)};
default:
llvm_unreachable("Unexpected instruction type.");
}
}
// If V is an unvisited generic address expression, appends V to PostorderStack
// and marks it as visited.
static void appendsGenericAddressExpressionToPostorderStack(
Value *V, std::vector<std::pair<Value *, bool>> *PostorderStack,
DenseSet<Value *> *Visited) {
assert(V->getType()->isPointerTy());
if (isAddressExpression(*V) &&
V->getType()->getPointerAddressSpace() ==
AddressSpace::ADDRESS_SPACE_GENERIC) {
if (Visited->insert(V).second)
PostorderStack->push_back(std::make_pair(V, false));
}
}
// Returns all generic address expressions in function F. The elements are
// ordered in postorder.
static std::vector<Value *> collectGenericAddressExpressions(Function &F) {
// This function implements a non-recursive postorder traversal of a partial
// use-def graph of function F.
std::vector<std::pair<Value*, bool>> PostorderStack;
// The set of visited expressions.
DenseSet<Value*> Visited;
// We only explore address expressions that are reachable from loads and
// stores for now because we aim at generating faster loads and stores.
for (Instruction &I : instructions(F)) {
if (isa<LoadInst>(I)) {
appendsGenericAddressExpressionToPostorderStack(
I.getOperand(0), &PostorderStack, &Visited);
} else if (isa<StoreInst>(I)) {
appendsGenericAddressExpressionToPostorderStack(
I.getOperand(1), &PostorderStack, &Visited);
}
}
std::vector<Value *> Postorder; // The resultant postorder.
while (!PostorderStack.empty()) {
// If the operands of the expression on the top are already explored,
// adds that expression to the resultant postorder.
if (PostorderStack.back().second) {
Postorder.push_back(PostorderStack.back().first);
PostorderStack.pop_back();
continue;
}
// Otherwise, adds its operands to the stack and explores them.
PostorderStack.back().second = true;
for (Value *PtrOperand : getPointerOperands(*PostorderStack.back().first)) {
appendsGenericAddressExpressionToPostorderStack(
PtrOperand, &PostorderStack, &Visited);
}
}
return Postorder;
}
// A helper function for cloneInstructionWithNewAddressSpace. Returns the clone
// of OperandUse.get() in the new address space. If the clone is not ready yet,
// returns an undef in the new address space as a placeholder.
static Value *operandWithNewAddressSpaceOrCreateUndef(
const Use &OperandUse, unsigned NewAddrSpace,
const ValueToValueMapTy &ValueWithNewAddrSpace,
SmallVectorImpl<const Use *> *UndefUsesToFix) {
Value *Operand = OperandUse.get();
if (Value *NewOperand = ValueWithNewAddrSpace.lookup(Operand))
return NewOperand;
UndefUsesToFix->push_back(&OperandUse);
return UndefValue::get(
Operand->getType()->getPointerElementType()->getPointerTo(NewAddrSpace));
}
// Returns a clone of `I` with its operands converted to those specified in
// ValueWithNewAddrSpace. Due to potential cycles in the data flow graph, an
// operand whose address space needs to be modified might not exist in
// ValueWithNewAddrSpace. In that case, uses undef as a placeholder operand and
// adds that operand use to UndefUsesToFix so that caller can fix them later.
//
// Note that we do not necessarily clone `I`, e.g., if it is an addrspacecast
// from a pointer whose type already matches. Therefore, this function returns a
// Value* instead of an Instruction*.
static Value *cloneInstructionWithNewAddressSpace(
Instruction *I, unsigned NewAddrSpace,
const ValueToValueMapTy &ValueWithNewAddrSpace,
SmallVectorImpl<const Use *> *UndefUsesToFix) {
Type *NewPtrType =
I->getType()->getPointerElementType()->getPointerTo(NewAddrSpace);
if (I->getOpcode() == Instruction::AddrSpaceCast) {
Value *Src = I->getOperand(0);
// Because `I` is generic, the source address space must be specific.
// Therefore, the inferred address space must be the source space, according
// to our algorithm.
assert(Src->getType()->getPointerAddressSpace() == NewAddrSpace);
if (Src->getType() != NewPtrType)
return new BitCastInst(Src, NewPtrType);
return Src;
}
// Computes the converted pointer operands.
SmallVector<Value *, 4> NewPointerOperands;
for (const Use &OperandUse : I->operands()) {
if (!OperandUse.get()->getType()->isPointerTy())
NewPointerOperands.push_back(nullptr);
else
NewPointerOperands.push_back(operandWithNewAddressSpaceOrCreateUndef(
OperandUse, NewAddrSpace, ValueWithNewAddrSpace, UndefUsesToFix));
}
switch (I->getOpcode()) {
case Instruction::BitCast:
return new BitCastInst(NewPointerOperands[0], NewPtrType);
case Instruction::PHI: {
assert(I->getType()->isPointerTy());
PHINode *PHI = cast<PHINode>(I);
PHINode *NewPHI = PHINode::Create(NewPtrType, PHI->getNumIncomingValues());
for (unsigned Index = 0; Index < PHI->getNumIncomingValues(); ++Index) {
unsigned OperandNo = PHINode::getOperandNumForIncomingValue(Index);
NewPHI->addIncoming(NewPointerOperands[OperandNo],
PHI->getIncomingBlock(Index));
}
return NewPHI;
}
case Instruction::GetElementPtr: {
GetElementPtrInst *GEP = cast<GetElementPtrInst>(I);
GetElementPtrInst *NewGEP = GetElementPtrInst::Create(
GEP->getSourceElementType(), NewPointerOperands[0],
SmallVector<Value *, 4>(GEP->idx_begin(), GEP->idx_end()));
NewGEP->setIsInBounds(GEP->isInBounds());
return NewGEP;
}
default:
llvm_unreachable("Unexpected opcode");
}
}
// Similar to cloneInstructionWithNewAddressSpace, returns a clone of the
// constant expression `CE` with its operands replaced as specified in
// ValueWithNewAddrSpace.
static Value *cloneConstantExprWithNewAddressSpace(
ConstantExpr *CE, unsigned NewAddrSpace,
const ValueToValueMapTy &ValueWithNewAddrSpace) {
Type *TargetType =
CE->getType()->getPointerElementType()->getPointerTo(NewAddrSpace);
if (CE->getOpcode() == Instruction::AddrSpaceCast) {
// Because CE is generic, the source address space must be specific.
// Therefore, the inferred address space must be the source space according
// to our algorithm.
assert(CE->getOperand(0)->getType()->getPointerAddressSpace() ==
NewAddrSpace);
return ConstantExpr::getBitCast(CE->getOperand(0), TargetType);
}
// Computes the operands of the new constant expression.
SmallVector<Constant *, 4> NewOperands;
for (unsigned Index = 0; Index < CE->getNumOperands(); ++Index) {
Constant *Operand = CE->getOperand(Index);
// If the address space of `Operand` needs to be modified, the new operand
// with the new address space should already be in ValueWithNewAddrSpace
// because (1) the constant expressions we consider (i.e. addrspacecast,
// bitcast, and getelementptr) do not incur cycles in the data flow graph
// and (2) this function is called on constant expressions in postorder.
if (Value *NewOperand = ValueWithNewAddrSpace.lookup(Operand)) {
NewOperands.push_back(cast<Constant>(NewOperand));
} else {
// Otherwise, reuses the old operand.
NewOperands.push_back(Operand);
}
}
if (CE->getOpcode() == Instruction::GetElementPtr) {
// Needs to specify the source type while constructing a getelementptr
// constant expression.
return CE->getWithOperands(
NewOperands, TargetType, /*OnlyIfReduced=*/false,
NewOperands[0]->getType()->getPointerElementType());
}
return CE->getWithOperands(NewOperands, TargetType);
}
// Returns a clone of the value `V`, with its operands replaced as specified in
// ValueWithNewAddrSpace. This function is called on every generic address
// expression whose address space needs to be modified, in postorder.
//
// See cloneInstructionWithNewAddressSpace for the meaning of UndefUsesToFix.
static Value *
cloneValueWithNewAddressSpace(Value *V, unsigned NewAddrSpace,
const ValueToValueMapTy &ValueWithNewAddrSpace,
SmallVectorImpl<const Use *> *UndefUsesToFix) {
// All values in Postorder are generic address expressions.
assert(isAddressExpression(*V) &&
V->getType()->getPointerAddressSpace() ==
AddressSpace::ADDRESS_SPACE_GENERIC);
if (Instruction *I = dyn_cast<Instruction>(V)) {
Value *NewV = cloneInstructionWithNewAddressSpace(
I, NewAddrSpace, ValueWithNewAddrSpace, UndefUsesToFix);
if (Instruction *NewI = dyn_cast<Instruction>(NewV)) {
if (NewI->getParent() == nullptr) {
NewI->insertBefore(I);
NewI->takeName(I);
}
}
return NewV;
}
return cloneConstantExprWithNewAddressSpace(
cast<ConstantExpr>(V), NewAddrSpace, ValueWithNewAddrSpace);
}
// Defines the join operation on the address space lattice (see the file header
// comments).
static unsigned joinAddressSpaces(unsigned AS1, unsigned AS2) {
if (AS1 == AddressSpace::ADDRESS_SPACE_GENERIC ||
AS2 == AddressSpace::ADDRESS_SPACE_GENERIC)
return AddressSpace::ADDRESS_SPACE_GENERIC;
if (AS1 == ADDRESS_SPACE_UNINITIALIZED)
return AS2;
if (AS2 == ADDRESS_SPACE_UNINITIALIZED)
return AS1;
// The join of two different specific address spaces is generic.
return AS1 == AS2 ? AS1 : (unsigned)AddressSpace::ADDRESS_SPACE_GENERIC;
}
bool NVPTXInferAddressSpaces::runOnFunction(Function &F) {
if (skipFunction(F))
return false;
// Collects all generic address expressions in postorder.
std::vector<Value *> Postorder = collectGenericAddressExpressions(F);
// Runs a data-flow analysis to refine the address spaces of every expression
// in Postorder.
ValueToAddrSpaceMapTy InferredAddrSpace;
inferAddressSpaces(Postorder, &InferredAddrSpace);
// Changes the address spaces of the generic address expressions who are
// inferred to point to a specific address space.
return rewriteWithNewAddressSpaces(Postorder, InferredAddrSpace, &F);
}
void NVPTXInferAddressSpaces::inferAddressSpaces(
const std::vector<Value *> &Postorder,
ValueToAddrSpaceMapTy *InferredAddrSpace) {
SetVector<Value *> Worklist(Postorder.begin(), Postorder.end());
// Initially, all expressions are in the uninitialized address space.
for (Value *V : Postorder)
(*InferredAddrSpace)[V] = ADDRESS_SPACE_UNINITIALIZED;
while (!Worklist.empty()) {
Value* V = Worklist.pop_back_val();
// Tries to update the address space of the stack top according to the
// address spaces of its operands.
DEBUG(dbgs() << "Updating the address space of\n"
<< " " << *V << "\n");
Optional<unsigned> NewAS = updateAddressSpace(*V, *InferredAddrSpace);
if (!NewAS.hasValue())
continue;
// If any updates are made, grabs its users to the worklist because
// their address spaces can also be possibly updated.
DEBUG(dbgs() << " to " << NewAS.getValue() << "\n");
(*InferredAddrSpace)[V] = NewAS.getValue();
for (Value *User : V->users()) {
// Skip if User is already in the worklist.
if (Worklist.count(User))
continue;
auto Pos = InferredAddrSpace->find(User);
// Our algorithm only updates the address spaces of generic address
// expressions, which are those in InferredAddrSpace.
if (Pos == InferredAddrSpace->end())
continue;
// Function updateAddressSpace moves the address space down a lattice
// path. Therefore, nothing to do if User is already inferred as
// generic (the bottom element in the lattice).
if (Pos->second == AddressSpace::ADDRESS_SPACE_GENERIC)
continue;
Worklist.insert(User);
}
}
}
Optional<unsigned> NVPTXInferAddressSpaces::updateAddressSpace(
const Value &V, const ValueToAddrSpaceMapTy &InferredAddrSpace) {
assert(InferredAddrSpace.count(&V));
// The new inferred address space equals the join of the address spaces
// of all its pointer operands.
unsigned NewAS = ADDRESS_SPACE_UNINITIALIZED;
for (Value *PtrOperand : getPointerOperands(V)) {
unsigned OperandAS;
if (InferredAddrSpace.count(PtrOperand))
OperandAS = InferredAddrSpace.lookup(PtrOperand);
else
OperandAS = PtrOperand->getType()->getPointerAddressSpace();
NewAS = joinAddressSpaces(NewAS, OperandAS);
// join(generic, *) = generic. So we can break if NewAS is already generic.
if (NewAS == AddressSpace::ADDRESS_SPACE_GENERIC)
break;
}
unsigned OldAS = InferredAddrSpace.lookup(&V);
assert(OldAS != AddressSpace::ADDRESS_SPACE_GENERIC);
if (OldAS == NewAS)
return None;
return NewAS;
}
bool NVPTXInferAddressSpaces::rewriteWithNewAddressSpaces(
const std::vector<Value *> &Postorder,
const ValueToAddrSpaceMapTy &InferredAddrSpace, Function *F) {
// For each address expression to be modified, creates a clone of it with its
// pointer operands converted to the new address space. Since the pointer
// operands are converted, the clone is naturally in the new address space by
// construction.
ValueToValueMapTy ValueWithNewAddrSpace;
SmallVector<const Use *, 32> UndefUsesToFix;
for (Value* V : Postorder) {
unsigned NewAddrSpace = InferredAddrSpace.lookup(V);
if (V->getType()->getPointerAddressSpace() != NewAddrSpace) {
ValueWithNewAddrSpace[V] = cloneValueWithNewAddressSpace(
V, NewAddrSpace, ValueWithNewAddrSpace, &UndefUsesToFix);
}
}
if (ValueWithNewAddrSpace.empty())
return false;
// Fixes all the undef uses generated by cloneInstructionWithNewAddressSpace.
for (const Use* UndefUse : UndefUsesToFix) {
User *V = UndefUse->getUser();
User *NewV = cast<User>(ValueWithNewAddrSpace.lookup(V));
unsigned OperandNo = UndefUse->getOperandNo();
assert(isa<UndefValue>(NewV->getOperand(OperandNo)));
NewV->setOperand(OperandNo, ValueWithNewAddrSpace.lookup(UndefUse->get()));
}
// Replaces the uses of the old address expressions with the new ones.
for (Value *V : Postorder) {
Value *NewV = ValueWithNewAddrSpace.lookup(V);
if (NewV == nullptr)
continue;
SmallVector<Use *, 4> Uses;
for (Use &U : V->uses())
Uses.push_back(&U);
DEBUG(dbgs() << "Replacing the uses of " << *V << "\n to\n " << *NewV
<< "\n");
for (Use *U : Uses) {
if (isa<LoadInst>(U->getUser()) ||
(isa<StoreInst>(U->getUser()) && U->getOperandNo() == 1)) {
// If V is used as the pointer operand of a load/store, sets the pointer
// operand to NewV. This replacement does not change the element type,
// so the resultant load/store is still valid.
U->set(NewV);
} else if (isa<Instruction>(U->getUser())) {
// Otherwise, replaces the use with generic(NewV).
// TODO: Some optimization opportunities are missed. For example, in
// %0 = icmp eq float* %p, %q
// if both p and q are inferred to be shared, we can rewrite %0 as
// %0 = icmp eq float addrspace(3)* %new_p, %new_q
// instead of currently
// %generic_p = addrspacecast float addrspace(3)* %new_p to float*
// %generic_q = addrspacecast float addrspace(3)* %new_q to float*
// %0 = icmp eq float* %generic_p, %generic_q
if (Instruction *I = dyn_cast<Instruction>(V)) {
BasicBlock::iterator InsertPos = std::next(I->getIterator());
while (isa<PHINode>(InsertPos))
++InsertPos;
U->set(new AddrSpaceCastInst(NewV, V->getType(), "", &*InsertPos));
} else {
U->set(ConstantExpr::getAddrSpaceCast(cast<Constant>(NewV),
V->getType()));
}
}
}
if (V->use_empty())
RecursivelyDeleteTriviallyDeadInstructions(V);
}
return true;
}
FunctionPass *llvm::createNVPTXInferAddressSpacesPass() {
return new NVPTXInferAddressSpaces();
}
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