[InstCombine] Support GEP chains in foldCmpLoadFromIndexedGlobal() #157447
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Currently this fold only supports a single GEP. However, in ptradd representation, it may be split across multiple GEPs. In particular, PR llvm#151333 will split off constant offset GEPs. To support this, add a new helper decomposeLinearExpression(), which decomposes a pointer into a linear expression of the form BasePtr + Index * Scale + Offset. I plan to also extend this helper to look through mul/shl on the index and use it in more places that currently use collectOffset() to extract a single index * scale. This will make sure such optimizations are not affected by the ptradd migration.
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@llvm/pr-subscribers-llvm-transforms @llvm/pr-subscribers-llvm-analysis Author: Nikita Popov (nikic) ChangesCurrently this fold only supports a single GEP. However, in ptradd representation, it may be split across multiple GEPs. In particular, PR #151333 will split off constant offset GEPs. To support this, add a new helper decomposeLinearExpression(), which decomposes a pointer into a linear expression of the form BasePtr + Index * Scale + Offset. I plan to also extend this helper to look through mul/shl on the index and use it in more places that currently use collectOffset() to extract a single index * scale. This will make sure such optimizations are not affected by the ptradd migration. Full diff: https://github.com/llvm/llvm-project/pull/157447.diff 5 Files Affected:
diff --git a/llvm/include/llvm/Analysis/Loads.h b/llvm/include/llvm/Analysis/Loads.h
index 4cec8115709d6..4893c0c9934d2 100644
--- a/llvm/include/llvm/Analysis/Loads.h
+++ b/llvm/include/llvm/Analysis/Loads.h
@@ -13,7 +13,9 @@
#ifndef LLVM_ANALYSIS_LOADS_H
#define LLVM_ANALYSIS_LOADS_H
+#include "llvm/ADT/APInt.h"
#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/GEPNoWrapFlags.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
@@ -193,6 +195,26 @@ LLVM_ABI bool canReplacePointersIfEqual(const Value *From, const Value *To,
const DataLayout &DL);
LLVM_ABI bool canReplacePointersInUseIfEqual(const Use &U, const Value *To,
const DataLayout &DL);
+
+/// Linear expression BasePtr + Index * Scale + Offset.
+/// Index, Scale and Offset all have the same bit width, which matches the
+/// pointer index size of BasePtr.
+/// Index may be nullptr if Scale is 0.
+struct LinearExpression {
+ Value *BasePtr;
+ Value *Index = nullptr;
+ APInt Scale;
+ APInt Offset;
+ GEPNoWrapFlags Flags = GEPNoWrapFlags::all();
+
+ LinearExpression(Value *BasePtr, unsigned BitWidth)
+ : BasePtr(BasePtr), Scale(BitWidth, 0), Offset(BitWidth, 0) {}
+};
+
+/// Decompose a pointer into a linear expression. This may look through
+/// multiple GEPs.
+LLVM_ABI LinearExpression decomposeLinearExpression(const DataLayout &DL,
+ Value *Ptr);
}
#endif
diff --git a/llvm/lib/Analysis/Loads.cpp b/llvm/lib/Analysis/Loads.cpp
index 47fdb41e8b445..0c4e3a2e3b233 100644
--- a/llvm/lib/Analysis/Loads.cpp
+++ b/llvm/lib/Analysis/Loads.cpp
@@ -21,6 +21,7 @@
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/GetElementPtrTypeIterator.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Operator.h"
@@ -876,3 +877,66 @@ bool llvm::isReadOnlyLoop(
}
return true;
}
+
+LinearExpression llvm::decomposeLinearExpression(const DataLayout &DL,
+ Value *Ptr) {
+ assert(Ptr->getType()->isPointerTy() && "Must be called with pointer arg");
+
+ unsigned BitWidth = DL.getIndexTypeSizeInBits(Ptr->getType());
+ LinearExpression Expr(Ptr, BitWidth);
+
+ while (true) {
+ auto *GEP = dyn_cast<GEPOperator>(Expr.BasePtr);
+ if (!GEP || GEP->getSourceElementType()->isScalableTy())
+ return Expr;
+
+ Value *VarIndex = nullptr;
+ for (Value *Index : GEP->indices()) {
+ if (isa<ConstantInt>(Index))
+ continue;
+ // Only allow a single variable index. We do not bother to handle the
+ // case of the same variable index appearing multiple times.
+ if (Expr.Index || VarIndex)
+ return Expr;
+ VarIndex = Index;
+ }
+
+ // Don't return non-canonical indexes.
+ if (VarIndex && !VarIndex->getType()->isIntegerTy(BitWidth))
+ return Expr;
+
+ // We have verified that we can fully handle this GEP, so we can update Expr
+ // members past this point.
+ Expr.BasePtr = GEP->getPointerOperand();
+ Expr.Flags = Expr.Flags.intersectForOffsetAdd(GEP->getNoWrapFlags());
+ for (gep_type_iterator GTI = gep_type_begin(GEP), GTE = gep_type_end(GEP);
+ GTI != GTE; ++GTI) {
+ Value *Index = GTI.getOperand();
+ if (auto *ConstOffset = dyn_cast<ConstantInt>(Index)) {
+ if (ConstOffset->isZero())
+ continue;
+ if (StructType *STy = GTI.getStructTypeOrNull()) {
+ unsigned ElementIdx = ConstOffset->getZExtValue();
+ const StructLayout *SL = DL.getStructLayout(STy);
+ Expr.Offset += SL->getElementOffset(ElementIdx);
+ continue;
+ }
+ // Truncate if type size exceeds index space.
+ APInt IndexedSize(BitWidth, GTI.getSequentialElementStride(DL),
+ /*isSigned=*/false,
+ /*implcitTrunc=*/true);
+ Expr.Offset += ConstOffset->getValue() * IndexedSize;
+ continue;
+ }
+
+ // FIXME: Also look through a mul/shl in the index.
+ assert(Expr.Index == nullptr && "Shouldn't have index yet");
+ Expr.Index = Index;
+ // Truncate if type size exceeds index space.
+ Expr.Scale = APInt(BitWidth, GTI.getSequentialElementStride(DL),
+ /*isSigned=*/false, /*implicitTrunc=*/true);
+ }
+ }
+
+ return Expr;
+}
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 01b0da3469c18..6a778726df729 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -19,6 +19,7 @@
#include "llvm/Analysis/CmpInstAnalysis.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/InstructionSimplify.h"
+#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/Utils/Local.h"
#include "llvm/Analysis/VectorUtils.h"
#include "llvm/IR/ConstantRange.h"
@@ -110,30 +111,27 @@ static bool isSignTest(ICmpInst::Predicate &Pred, const APInt &C) {
/// If AndCst is non-null, then the loaded value is masked with that constant
/// before doing the comparison. This handles cases like "A[i]&4 == 0".
Instruction *InstCombinerImpl::foldCmpLoadFromIndexedGlobal(
- LoadInst *LI, GetElementPtrInst *GEP, GlobalVariable *GV, CmpInst &ICI,
- ConstantInt *AndCst) {
- if (LI->isVolatile() || !GV->isConstant() || !GV->hasDefinitiveInitializer())
+ LoadInst *LI, GetElementPtrInst *GEP, CmpInst &ICI, ConstantInt *AndCst) {
+ auto *GV = dyn_cast<GlobalVariable>(getUnderlyingObject(GEP));
+ if (LI->isVolatile() || !GV || !GV->isConstant() ||
+ !GV->hasDefinitiveInitializer())
return nullptr;
- Constant *Init = GV->getInitializer();
- TypeSize GlobalSize = DL.getTypeAllocSize(Init->getType());
Type *EltTy = LI->getType();
TypeSize EltSize = DL.getTypeStoreSize(EltTy);
if (EltSize.isScalable())
return nullptr;
- unsigned IndexBW = DL.getIndexTypeSizeInBits(GEP->getType());
- SmallMapVector<Value *, APInt, 4> VarOffsets;
- APInt ConstOffset(IndexBW, 0);
- if (!GEP->collectOffset(DL, IndexBW, VarOffsets, ConstOffset) ||
- VarOffsets.size() != 1 || IndexBW > 64)
+ LinearExpression Expr = decomposeLinearExpression(DL, GEP);
+ if (!Expr.Index || Expr.BasePtr != GV || Expr.Offset.getBitWidth() > 64)
return nullptr;
- Value *Idx = VarOffsets.front().first;
- const APInt &Stride = VarOffsets.front().second;
- // If the index type is non-canonical, wait for it to be canonicalized.
- if (Idx->getType()->getScalarSizeInBits() != IndexBW)
- return nullptr;
+ Constant *Init = GV->getInitializer();
+ TypeSize GlobalSize = DL.getTypeAllocSize(Init->getType());
+
+ Value *Idx = Expr.Index;
+ const APInt &Stride = Expr.Scale;
+ const APInt &ConstOffset = Expr.Offset;
// Allow an additional context offset, but only within the stride.
if (!ConstOffset.ult(Stride))
@@ -280,7 +278,7 @@ Instruction *InstCombinerImpl::foldCmpLoadFromIndexedGlobal(
// comparison is false if Idx was 0x80..00.
// We need to erase the highest countTrailingZeros(ElementSize) bits of Idx.
auto MaskIdx = [&](Value *Idx) {
- if (!GEP->isInBounds() && Stride.countr_zero() != 0) {
+ if (!Expr.Flags.isInBounds() && Stride.countr_zero() != 0) {
Value *Mask = Constant::getAllOnesValue(Idx->getType());
Mask = Builder.CreateLShr(Mask, Stride.countr_zero());
Idx = Builder.CreateAnd(Idx, Mask);
@@ -1958,10 +1956,8 @@ Instruction *InstCombinerImpl::foldICmpAndConstant(ICmpInst &Cmp,
if (auto *C2 = dyn_cast<ConstantInt>(Y))
if (auto *LI = dyn_cast<LoadInst>(X))
if (auto *GEP = dyn_cast<GetElementPtrInst>(LI->getOperand(0)))
- if (auto *GV = dyn_cast<GlobalVariable>(GEP->getOperand(0)))
- if (Instruction *Res =
- foldCmpLoadFromIndexedGlobal(LI, GEP, GV, Cmp, C2))
- return Res;
+ if (Instruction *Res = foldCmpLoadFromIndexedGlobal(LI, GEP, Cmp, C2))
+ return Res;
if (!Cmp.isEquality())
return nullptr;
@@ -4314,10 +4310,9 @@ Instruction *InstCombinerImpl::foldICmpInstWithConstantNotInt(ICmpInst &I) {
// Try to optimize things like "A[i] > 4" to index computations.
if (GetElementPtrInst *GEP =
dyn_cast<GetElementPtrInst>(LHSI->getOperand(0)))
- if (GlobalVariable *GV = dyn_cast<GlobalVariable>(GEP->getOperand(0)))
- if (Instruction *Res =
- foldCmpLoadFromIndexedGlobal(cast<LoadInst>(LHSI), GEP, GV, I))
- return Res;
+ if (Instruction *Res =
+ foldCmpLoadFromIndexedGlobal(cast<LoadInst>(LHSI), GEP, I))
+ return Res;
break;
}
@@ -8798,10 +8793,9 @@ Instruction *InstCombinerImpl::visitFCmpInst(FCmpInst &I) {
break;
case Instruction::Load:
if (auto *GEP = dyn_cast<GetElementPtrInst>(LHSI->getOperand(0)))
- if (auto *GV = dyn_cast<GlobalVariable>(GEP->getOperand(0)))
- if (Instruction *Res = foldCmpLoadFromIndexedGlobal(
- cast<LoadInst>(LHSI), GEP, GV, I))
- return Res;
+ if (Instruction *Res =
+ foldCmpLoadFromIndexedGlobal(cast<LoadInst>(LHSI), GEP, I))
+ return Res;
break;
case Instruction::FPTrunc:
if (Instruction *NV = foldFCmpFpTrunc(I, *LHSI, *RHSC))
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index 2340028ce93dc..8c64d6398eca9 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -710,7 +710,7 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
bool foldAllocaCmp(AllocaInst *Alloca);
Instruction *foldCmpLoadFromIndexedGlobal(LoadInst *LI,
GetElementPtrInst *GEP,
- GlobalVariable *GV, CmpInst &ICI,
+ CmpInst &ICI,
ConstantInt *AndCst = nullptr);
Instruction *foldFCmpIntToFPConst(FCmpInst &I, Instruction *LHSI,
Constant *RHSC);
diff --git a/llvm/test/Transforms/InstCombine/load-cmp.ll b/llvm/test/Transforms/InstCombine/load-cmp.ll
index 79faefbd5df56..f7d788110b1bc 100644
--- a/llvm/test/Transforms/InstCombine/load-cmp.ll
+++ b/llvm/test/Transforms/InstCombine/load-cmp.ll
@@ -419,6 +419,32 @@ define i1 @load_vs_array_type_mismatch_offset1(i32 %idx) {
ret i1 %cmp
}
+define i1 @load_vs_array_type_mismatch_offset1_separate_gep(i32 %idx) {
+; CHECK-LABEL: @load_vs_array_type_mismatch_offset1_separate_gep(
+; CHECK-NEXT: [[TMP1:%.*]] = and i32 [[IDX:%.*]], -3
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[TMP1]], 1
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %gep1 = getelementptr inbounds {i16, i16}, ptr @g_i16_1, i32 %idx
+ %gep2 = getelementptr inbounds i8, ptr %gep1, i32 2
+ %load = load i16, ptr %gep2
+ %cmp = icmp eq i16 %load, 0
+ ret i1 %cmp
+}
+
+define i1 @load_vs_array_type_mismatch_offset1_separate_gep_swapped(i32 %idx) {
+; CHECK-LABEL: @load_vs_array_type_mismatch_offset1_separate_gep_swapped(
+; CHECK-NEXT: [[TMP1:%.*]] = and i32 [[IDX:%.*]], -3
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[TMP1]], 1
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %gep1 = getelementptr inbounds i8, ptr @g_i16_1, i32 2
+ %gep2 = getelementptr inbounds {i16, i16}, ptr %gep1, i32 %idx
+ %load = load i16, ptr %gep2
+ %cmp = icmp eq i16 %load, 0
+ ret i1 %cmp
+}
+
@g_i16_2 = internal constant [8 x i16] [i16 1, i16 0, i16 0, i16 1, i16 1, i16 0, i16 0, i16 1]
; idx == 0 || idx == 2
@@ -554,3 +580,36 @@ entry:
%cond = icmp ult i32 %isOK, 5
ret i1 %cond
}
+
+define i1 @cmp_load_multiple_indices(i32 %idx, i32 %idx2) {
+; CHECK-LABEL: @cmp_load_multiple_indices(
+; CHECK-NEXT: [[GEP1:%.*]] = getelementptr inbounds i16, ptr @g_i16_1, i32 [[IDX:%.*]]
+; CHECK-NEXT: [[GEP2:%.*]] = getelementptr inbounds i16, ptr [[GEP1]], i32 [[IDX2:%.*]]
+; CHECK-NEXT: [[GEP3:%.*]] = getelementptr inbounds nuw i8, ptr [[GEP2]], i32 2
+; CHECK-NEXT: [[LOAD:%.*]] = load i16, ptr [[GEP3]], align 2
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i16 [[LOAD]], 0
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %gep1 = getelementptr inbounds i16, ptr @g_i16_1, i32 %idx
+ %gep2 = getelementptr inbounds i16, ptr %gep1, i32 %idx2
+ %gep3 = getelementptr inbounds i8, ptr %gep2, i32 2
+ %load = load i16, ptr %gep3
+ %cmp = icmp eq i16 %load, 0
+ ret i1 %cmp
+}
+
+define i1 @cmp_load_multiple_indices2(i32 %idx, i32 %idx2) {
+; CHECK-LABEL: @cmp_load_multiple_indices2(
+; CHECK-NEXT: [[GEP1_SPLIT:%.*]] = getelementptr inbounds [1 x i16], ptr @g_i16_1, i32 [[IDX:%.*]]
+; CHECK-NEXT: [[GEP1:%.*]] = getelementptr inbounds i16, ptr [[GEP1_SPLIT]], i32 [[IDX2:%.*]]
+; CHECK-NEXT: [[GEP2:%.*]] = getelementptr inbounds nuw i8, ptr [[GEP1]], i32 2
+; CHECK-NEXT: [[LOAD:%.*]] = load i16, ptr [[GEP2]], align 2
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i16 [[LOAD]], 0
+; CHECK-NEXT: ret i1 [[CMP]]
+;
+ %gep1 = getelementptr inbounds [1 x i16], ptr @g_i16_1, i32 %idx, i32 %idx2
+ %gep2 = getelementptr inbounds i8, ptr %gep1, i32 2
+ %load = load i16, ptr %gep2
+ %cmp = icmp eq i16 %load, 0
+ ret i1 %cmp
+}
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Currently this fold only supports a single GEP. However, in ptradd representation, it may be split across multiple GEPs. In particular, PR #151333 will split off constant offset GEPs.
To support this, add a new helper decomposeLinearExpression(), which decomposes a pointer into a linear expression of the form BasePtr + Index * Scale + Offset.
I plan to also extend this helper to look through mul/shl on the index and use it in more places that currently use collectOffset() to extract a single index * scale. This will make sure such optimizations are not affected by the ptradd migration.