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//===-- NVPTXFavorNonGenericAddrSpace.cpp - ---------------------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// When a load/store accesses the generic address space, checks whether the
// address is casted from a non-generic address space. If so, remove this
// addrspacecast because accessing non-generic address spaces is typically
// faster. Besides removing addrspacecasts directly used by loads/stores, this
// optimization also recursively traces into a GEP's pointer operand and a
// bitcast's source to find more eliminable addrspacecasts.
//
// For instance, the code below loads a float from an array allocated in
// addrspace(3).
//
// %0 = addrspacecast [10 x float] addrspace(3)* @a to [10 x float]*
// %1 = gep [10 x float]* %0, i64 0, i64 %i
// %2 = bitcast float* %1 to i32*
// %3 = load i32* %2 ; emits ld.u32
//
// First, function hoistAddrSpaceCastFrom reorders the addrspacecast, the GEP,
// and the bitcast to expose more optimization opportunities to function
// optimizeMemoryInst. The intermediate code looks like:
//
// %0 = gep [10 x float] addrspace(3)* @a, i64 0, i64 %i
// %1 = bitcast float addrspace(3)* %0 to i32 addrspace(3)*
// %2 = addrspacecast i32 addrspace(3)* %1 to i32*
// %3 = load i32* %2 ; still emits ld.u32, but will be optimized shortly
//
// Then, function optimizeMemoryInstruction detects a load from addrspacecast'ed
// generic pointers, and folds the load and the addrspacecast into a load from
// the original address space. The final code looks like:
//
// %0 = gep [10 x float] addrspace(3)* @a, i64 0, i64 %i
// %1 = bitcast float addrspace(3)* %0 to i32 addrspace(3)*
// %3 = load i32 addrspace(3)* %1 ; emits ld.shared.f32
//
// This pass may remove an addrspacecast in a different BB. Therefore, we
// implement it as a FunctionPass.
//
// TODO:
// The current implementation doesn't handle PHINodes. Eliminating
// addrspacecasts used by PHINodes is trickier because PHINodes can introduce
// loops in data flow. 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, ...
//
// Marking %y2 shared depends on marking %y shared, but %y also data-flow
// depends on %y2. We probably need an iterative fix-point algorithm on handle
// this case.
//
//===----------------------------------------------------------------------===//
#include "NVPTX.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Operator.h"
#include "llvm/Support/CommandLine.h"
using namespace llvm;
// An option to disable this optimization. Enable it by default.
static cl::opt<bool> DisableFavorNonGeneric(
"disable-nvptx-favor-non-generic",
cl::init(false),
cl::desc("Do not convert generic address space usage "
"to non-generic address space usage"),
cl::Hidden);
namespace {
/// \brief NVPTXFavorNonGenericAddrSpaces
class NVPTXFavorNonGenericAddrSpaces : public FunctionPass {
public:
static char ID;
NVPTXFavorNonGenericAddrSpaces() : FunctionPass(ID) {}
bool runOnFunction(Function &F) override;
private:
/// Optimizes load/store instructions. Idx is the index of the pointer operand
/// (0 for load, and 1 for store). Returns true if it changes anything.
bool optimizeMemoryInstruction(Instruction *I, unsigned Idx);
/// Recursively traces into a GEP's pointer operand or a bitcast's source to
/// find an eliminable addrspacecast, and hoists that addrspacecast to the
/// outermost level. For example, this function transforms
/// bitcast(gep(gep(addrspacecast(X))))
/// to
/// addrspacecast(bitcast(gep(gep(X)))).
///
/// This reordering exposes to optimizeMemoryInstruction more
/// optimization opportunities on loads and stores.
///
/// If this function successfully hoists an eliminable addrspacecast or V is
/// already such an addrspacecast, it returns the transformed value (which is
/// guaranteed to be an addrspacecast); otherwise, it returns nullptr.
Value *hoistAddrSpaceCastFrom(Value *V, int Depth = 0);
/// Helper function for GEPs.
Value *hoistAddrSpaceCastFromGEP(GEPOperator *GEP, int Depth);
/// Helper function for bitcasts.
Value *hoistAddrSpaceCastFromBitCast(BitCastOperator *BC, int Depth);
};
}
char NVPTXFavorNonGenericAddrSpaces::ID = 0;
namespace llvm {
void initializeNVPTXFavorNonGenericAddrSpacesPass(PassRegistry &);
}
INITIALIZE_PASS(NVPTXFavorNonGenericAddrSpaces, "nvptx-favor-non-generic",
"Remove unnecessary non-generic-to-generic addrspacecasts",
false, false)
// Decides whether V is an addrspacecast and shortcutting V in load/store is
// valid and beneficial.
static bool isEliminableAddrSpaceCast(Value *V) {
// Returns false if V is not even an addrspacecast.
Operator *Cast = dyn_cast<Operator>(V);
if (Cast == nullptr || Cast->getOpcode() != Instruction::AddrSpaceCast)
return false;
Value *Src = Cast->getOperand(0);
PointerType *SrcTy = cast<PointerType>(Src->getType());
PointerType *DestTy = cast<PointerType>(Cast->getType());
// TODO: For now, we only handle the case where the addrspacecast only changes
// the address space but not the type. If the type also changes, we could
// still get rid of the addrspacecast by adding an extra bitcast, but we
// rarely see such scenarios.
if (SrcTy->getElementType() != DestTy->getElementType())
return false;
// Checks whether the addrspacecast is from a non-generic address space to the
// generic address space.
return (SrcTy->getAddressSpace() != AddressSpace::ADDRESS_SPACE_GENERIC &&
DestTy->getAddressSpace() == AddressSpace::ADDRESS_SPACE_GENERIC);
}
Value *NVPTXFavorNonGenericAddrSpaces::hoistAddrSpaceCastFromGEP(
GEPOperator *GEP, int Depth) {
Value *NewOperand =
hoistAddrSpaceCastFrom(GEP->getPointerOperand(), Depth + 1);
if (NewOperand == nullptr)
return nullptr;
// hoistAddrSpaceCastFrom returns an eliminable addrspacecast or nullptr.
assert(isEliminableAddrSpaceCast(NewOperand));
Operator *Cast = cast<Operator>(NewOperand);
SmallVector<Value *, 8> Indices(GEP->idx_begin(), GEP->idx_end());
Value *NewASC;
if (Instruction *GEPI = dyn_cast<Instruction>(GEP)) {
// GEP = gep (addrspacecast X), indices
// =>
// NewGEP = gep X, indices
// NewASC = addrspacecast NewGEP
GetElementPtrInst *NewGEP = GetElementPtrInst::Create(
GEP->getSourceElementType(), Cast->getOperand(0), Indices,
"", GEPI);
NewGEP->setIsInBounds(GEP->isInBounds());
NewASC = new AddrSpaceCastInst(NewGEP, GEP->getType(), "", GEPI);
NewASC->takeName(GEP);
// Without RAUWing GEP, the compiler would visit GEP again and emit
// redundant instructions. This is exercised in test @rauw in
// access-non-generic.ll.
GEP->replaceAllUsesWith(NewASC);
} else {
// GEP is a constant expression.
Constant *NewGEP = ConstantExpr::getGetElementPtr(
GEP->getSourceElementType(), cast<Constant>(Cast->getOperand(0)),
Indices, GEP->isInBounds());
NewASC = ConstantExpr::getAddrSpaceCast(NewGEP, GEP->getType());
}
return NewASC;
}
Value *NVPTXFavorNonGenericAddrSpaces::hoistAddrSpaceCastFromBitCast(
BitCastOperator *BC, int Depth) {
Value *NewOperand = hoistAddrSpaceCastFrom(BC->getOperand(0), Depth + 1);
if (NewOperand == nullptr)
return nullptr;
// hoistAddrSpaceCastFrom returns an eliminable addrspacecast or nullptr.
assert(isEliminableAddrSpaceCast(NewOperand));
Operator *Cast = cast<Operator>(NewOperand);
// Cast = addrspacecast Src
// BC = bitcast Cast
// =>
// Cast' = bitcast Src
// BC' = addrspacecast Cast'
Value *Src = Cast->getOperand(0);
Type *TypeOfNewCast =
PointerType::get(BC->getType()->getPointerElementType(),
Src->getType()->getPointerAddressSpace());
Value *NewBC;
if (BitCastInst *BCI = dyn_cast<BitCastInst>(BC)) {
Value *NewCast = new BitCastInst(Src, TypeOfNewCast, "", BCI);
NewBC = new AddrSpaceCastInst(NewCast, BC->getType(), "", BCI);
NewBC->takeName(BC);
// Without RAUWing BC, the compiler would visit BC again and emit
// redundant instructions. This is exercised in test @rauw in
// access-non-generic.ll.
BC->replaceAllUsesWith(NewBC);
} else {
// BC is a constant expression.
Constant *NewCast =
ConstantExpr::getBitCast(cast<Constant>(Src), TypeOfNewCast);
NewBC = ConstantExpr::getAddrSpaceCast(NewCast, BC->getType());
}
return NewBC;
}
Value *NVPTXFavorNonGenericAddrSpaces::hoistAddrSpaceCastFrom(Value *V,
int Depth) {
// Returns V if V is already an eliminable addrspacecast.
if (isEliminableAddrSpaceCast(V))
return V;
// Limit the depth to prevent this recursive function from running too long.
const int MaxDepth = 20;
if (Depth >= MaxDepth)
return nullptr;
// If V is a GEP or bitcast, hoist the addrspacecast if any from its pointer
// operand. This enables optimizeMemoryInstruction to shortcut addrspacecasts
// that are not directly used by the load/store.
if (GEPOperator *GEP = dyn_cast<GEPOperator>(V))
return hoistAddrSpaceCastFromGEP(GEP, Depth);
if (BitCastOperator *BC = dyn_cast<BitCastOperator>(V))
return hoistAddrSpaceCastFromBitCast(BC, Depth);
return nullptr;
}
bool NVPTXFavorNonGenericAddrSpaces::optimizeMemoryInstruction(Instruction *MI,
unsigned Idx) {
Value *NewOperand = hoistAddrSpaceCastFrom(MI->getOperand(Idx));
if (NewOperand == nullptr)
return false;
// load/store (addrspacecast X) => load/store X if shortcutting the
// addrspacecast is valid and can improve performance.
//
// e.g.,
// %1 = addrspacecast float addrspace(3)* %0 to float*
// %2 = load float* %1
// ->
// %2 = load float addrspace(3)* %0
//
// Note: the addrspacecast can also be a constant expression.
assert(isEliminableAddrSpaceCast(NewOperand));
Operator *ASC = dyn_cast<Operator>(NewOperand);
MI->setOperand(Idx, ASC->getOperand(0));
return true;
}
bool NVPTXFavorNonGenericAddrSpaces::runOnFunction(Function &F) {
if (DisableFavorNonGeneric)
return false;
bool Changed = false;
for (BasicBlock &B : F) {
for (Instruction &I : B) {
if (isa<LoadInst>(I)) {
// V = load P
Changed |= optimizeMemoryInstruction(&I, 0);
} else if (isa<StoreInst>(I)) {
// store V, P
Changed |= optimizeMemoryInstruction(&I, 1);
}
}
}
return Changed;
}
FunctionPass *llvm::createNVPTXFavorNonGenericAddrSpacesPass() {
return new NVPTXFavorNonGenericAddrSpaces();
}