[llvm] Feat/sink gep constant offset (PR #140027)
via llvm-commits
llvm-commits at lists.llvm.org
Thu May 15 01:48:13 PDT 2025
https://github.com/StevenYangCC created https://github.com/llvm/llvm-project/pull/140027
Sink constant offset in a GEP chain to tail. For example,
%gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 512
%gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 %ofst0
%gep2 = getelementptr half, ptr addrspace(3) %gep1, i32 %ofst1
%data = load half, ptr addrspace(3) %gep2, align 2
==>
%gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 %ofst0
%gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 %ofst1
%gep2 = getelementptr half, ptr addrspace(3) %gep1, i32 512
%data = load half, ptr addrspace(3) %gep2, align 2
>From 95821a818bb1d665d972e503ec88aa1aa6322e9a Mon Sep 17 00:00:00 2001
From: "chengcang.yang" <chengcang.yang at iluvatar.com>
Date: Thu, 15 May 2025 18:14:25 +0800
Subject: [PATCH 1/2] [TMP]
---
.../Scalar/SeparateConstOffsetFromGEP.cpp | 15 +++++++++++++++
1 file changed, 15 insertions(+)
diff --git a/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp b/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
index 320b79203c0b3..d6e9463043661 100644
--- a/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
+++ b/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
@@ -456,6 +456,8 @@ class SeparateConstOffsetFromGEP {
/// A helper that reunites sexts in an instruction.
bool reuniteExts(Instruction *I);
+ bool sinkGEPConstantOffset(Function &F);
+
/// Find the closest dominator of <Dominatee> that is equivalent to <Key>.
Instruction *findClosestMatchingDominator(
ExprKey Key, Instruction *Dominatee,
@@ -1255,6 +1257,8 @@ bool SeparateConstOffsetFromGEP::run(Function &F) {
Changed |= reuniteExts(F);
+ Changed |= sinkGEPConstantOffset(F);
+
if (VerifyNoDeadCode)
verifyNoDeadCode(F);
@@ -1344,6 +1348,17 @@ bool SeparateConstOffsetFromGEP::reuniteExts(Function &F) {
return Changed;
}
+bool SeparateConstOffsetFromGEP::sinkGEPConstantOffset(Function &F) {
+ bool Changed = false;
+ for (BasicBlock &B : F) {
+ for (Instruction &I : B) {
+
+ }
+ }
+
+ return Changed;
+}
+
void SeparateConstOffsetFromGEP::verifyNoDeadCode(Function &F) {
for (BasicBlock &B : F) {
for (Instruction &I : B) {
>From 1cba4d4d365ca6f66e46707e6c3b6bc45ec9147c Mon Sep 17 00:00:00 2001
From: "chengcang.yang" <chengcang.yang at iluvatar.com>
Date: Thu, 15 May 2025 18:14:25 +0800
Subject: [PATCH 2/2] [SeparateConstOffsetFromGEP] Sink constant offset in GEP
chain to tail.
Summary:
Sink constant offsets down the GEP chain to the tail helps reduce
register usage. For example:
%gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 512
%gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 %ofst0
%gep2 = getelementptr half, ptr addrspace(3) %gep1, i32 %ofst1
%data = load half, ptr addrspace(3) %gep2, align 2
==>
%gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 %ofst0
%gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 %ofst1
%gep2 = getelementptr half, ptr addrspace(3) %gep1, i32 512
%data = load half, ptr addrspace(3) %gep2, align 2
---
.../Scalar/SeparateConstOffsetFromGEP.cpp | 132 +++++++++++++++++-
1 file changed, 131 insertions(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp b/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
index d6e9463043661..a7bac1bb2d3fc 100644
--- a/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
+++ b/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
@@ -456,8 +456,22 @@ class SeparateConstOffsetFromGEP {
/// A helper that reunites sexts in an instruction.
bool reuniteExts(Instruction *I);
+ /// Sink constant offset in a GEP chain to tail. For example,
+ /// %gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 512
+ /// %gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 %ofst0
+ /// %gep2 = getelementptr half, ptr addrspace(3) %gep1, i32 %ofst1
+ /// %data = load half, ptr addrspace(3) %gep2, align 2
+ /// ==>
+ /// %gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 %ofst0
+ /// %gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 %ofst1
+ /// %gep2 = getelementptr half, ptr addrspace(3) %gep1, i32 512
+ /// %data = load half, ptr addrspace(3) %gep2, align 2
bool sinkGEPConstantOffset(Function &F);
+ /// A helper that does sink action for a root in a gep chain.
+ /// Return true if Ptr is a candidate for upper GEP in recursive calling.
+ bool sinkGEPConstantOffset(Value *Ptr, bool &Changed);
+
/// Find the closest dominator of <Dominatee> that is equivalent to <Key>.
Instruction *findClosestMatchingDominator(
ExprKey Key, Instruction *Dominatee,
@@ -1348,14 +1362,130 @@ bool SeparateConstOffsetFromGEP::reuniteExts(Function &F) {
return Changed;
}
+bool SeparateConstOffsetFromGEP::sinkGEPConstantOffset(Value *Ptr,
+ bool &Changed) {
+ // The purpose of this function is to sink the constant offsets in the GEP
+ // chain to the tail of the chain.
+ // This algorithm is implemented recursively, the algorithm starts from the
+ // tail of the chain through the DFS method and shifts the constant offset
+ // of the GEP step by step upwards by bottom-up DFS method, i.e. step by step
+ // down to the tail.
+ // A simple example is given:
+ /// %gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 512
+ /// %gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 %ofst0
+ /// %gep2 = getelementptr half, ptr addrspace(3) %gep1, i32 %ofst1
+ /// %data = load half, ptr addrspace(3) %gep2, align 2
+ /// ==>
+ /// %gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 %ofst0
+ /// %gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 %ofst1
+ /// %gep2 = getelementptr half, ptr addrspace(3) %gep1, i32 512
+ /// %data = load half, ptr addrspace(3) %gep2, align 2
+ GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr);
+ if (!GEP)
+ return false;
+
+ bool BaseResult = sinkGEPConstantOffset(GEP->getPointerOperand(), Changed);
+
+ if (GEP->getNumIndices() != 1)
+ return false;
+
+ ConstantInt *C = nullptr;
+ Value *Idx = GEP->getOperand(1);
+ bool MatchConstant = match(Idx, m_ConstantInt(C));
+
+ if (!BaseResult)
+ return MatchConstant;
+
+ Type *ResTy = GEP->getResultElementType();
+ GetElementPtrInst *BaseGEP =
+ dyn_cast<GetElementPtrInst>(GEP->getPointerOperand());
+ assert(BaseGEP);
+ Value *BaseIdx = BaseGEP->getOperand(1);
+ Type *BaseResTy = BaseGEP->getResultElementType();
+
+ if (MatchConstant) {
+ // %gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 8
+ // %gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 4
+ // as:
+ // %gep1 = getelementptr half, ptr addrspace(3) %ptr, i32 12
+ Type *NewResTy = nullptr;
+ Constant *NewIdx = nullptr;
+ if (ResTy == BaseResTy) {
+ NewResTy = ResTy;
+ int64_t NewIdxValue = cast<ConstantInt>(BaseIdx)->getSExtValue() +
+ cast<ConstantInt>(Idx)->getSExtValue();
+ Type *NewIdxType = (NewIdxValue < std::numeric_limits<int32_t>::min() ||
+ NewIdxValue > std::numeric_limits<int32_t>::max())
+ ? Type::getInt64Ty(GEP->getContext())
+ : Type::getInt32Ty(GEP->getContext());
+ NewIdx = ConstantInt::get(NewIdxType, NewIdxValue);
+ } else {
+ NewResTy = Type::getInt8Ty(GEP->getContext());
+ int64_t NewIdxValue = (cast<ConstantInt>(BaseIdx)->getSExtValue() *
+ DL->getTypeAllocSize(BaseResTy)) +
+ (cast<ConstantInt>(Idx)->getSExtValue() *
+ DL->getTypeAllocSize(ResTy));
+ Type *NewIdxType = (NewIdxValue < std::numeric_limits<int32_t>::min() ||
+ NewIdxValue > std::numeric_limits<int32_t>::max())
+ ? Type::getInt64Ty(GEP->getContext())
+ : Type::getInt32Ty(GEP->getContext());
+ NewIdx = ConstantInt::get(NewIdxType, NewIdxValue);
+ }
+ assert(NewResTy);
+ assert(NewIdx);
+ auto *NewGEP = GetElementPtrInst::Create(
+ NewResTy, BaseGEP->getPointerOperand(), NewIdx);
+ NewGEP->setIsInBounds(GEP->isInBounds());
+ NewGEP->insertBefore(GEP->getIterator());
+ NewGEP->takeName(GEP);
+
+ GEP->replaceAllUsesWith(NewGEP);
+ GEP->eraseFromParent();
+
+ Changed = true;
+ return true;
+ }
+
+ // %gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 8
+ // %gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 %idx
+ // as:
+ // %gepx0 = getelementptr half, ptr addrspace(3) %ptr, i32 %idx
+ // %gepx1 = getelementptr half, ptr addrspace(3) %gepx0, i32 8
+ auto *GEPX0 =
+ GetElementPtrInst::Create(ResTy, BaseGEP->getPointerOperand(), Idx);
+ GEPX0->setIsInBounds(BaseGEP->isInBounds());
+ GEPX0->insertBefore(GEP->getIterator());
+ auto *GEPX1 = GetElementPtrInst::Create(BaseResTy, GEPX0, BaseIdx);
+ GEPX1->setIsInBounds(GEP->isInBounds());
+ GEPX1->insertBefore(GEP->getIterator());
+ GEPX1->takeName(GEP);
+
+ GEP->replaceAllUsesWith(GEPX1);
+ GEP->eraseFromParent();
+
+ Changed = true;
+ return true;
+}
+
bool SeparateConstOffsetFromGEP::sinkGEPConstantOffset(Function &F) {
bool Changed = false;
+ SmallVector<Value *, 4> Candidates;
for (BasicBlock &B : F) {
for (Instruction &I : B) {
-
+ Value *Ptr = nullptr;
+ if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
+ Ptr = LI->getPointerOperand();
+ } else if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
+ Ptr = SI->getPointerOperand();
+ }
+ if (Ptr)
+ Candidates.push_back(Ptr);
}
}
+ for (Value *Ptr : Candidates)
+ sinkGEPConstantOffset(Ptr, Changed);
+
return Changed;
}
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