[llvm] [DA] do not handle array accesses of different offsets (PR #123436)
Sebastian Pop via llvm-commits
llvm-commits at lists.llvm.org
Tue Feb 18 23:00:20 PST 2025
https://github.com/sebpop updated https://github.com/llvm/llvm-project/pull/123436
>From c3e9f44e6e5cc112d25c7bc1ddc40e630e429566 Mon Sep 17 00:00:00 2001
From: Sebastian Pop <spop at nvidia.com>
Date: Thu, 14 Nov 2024 18:17:04 +0000
Subject: [PATCH 1/2] [DA] Dependence analysis does not handle array accesses
of different sizes
This fixes bug https://github.com/llvm/llvm-project/issues/16183
where the elements of a same array are accesses as i32 and i64.
This is not handled by the array dependence analysis.
---
llvm/lib/Analysis/DependenceAnalysis.cpp | 38 +++++++++++--------
.../DependenceAnalysis/DifferentAccessSize.ll | 22 +++++++++++
2 files changed, 44 insertions(+), 16 deletions(-)
create mode 100644 llvm/test/Analysis/DependenceAnalysis/DifferentAccessSize.ll
diff --git a/llvm/lib/Analysis/DependenceAnalysis.cpp b/llvm/lib/Analysis/DependenceAnalysis.cpp
index cd252c62ba9cd..5ca565e74682e 100644
--- a/llvm/lib/Analysis/DependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/DependenceAnalysis.cpp
@@ -3604,14 +3604,10 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst) {
return std::make_unique<Dependence>(Src, Dst);
}
- assert(isLoadOrStore(Src) && "instruction is not load or store");
- assert(isLoadOrStore(Dst) && "instruction is not load or store");
- Value *SrcPtr = getLoadStorePointerOperand(Src);
- Value *DstPtr = getLoadStorePointerOperand(Dst);
+ const MemoryLocation &DstLoc = MemoryLocation::get(Dst);
+ const MemoryLocation &SrcLoc = MemoryLocation::get(Src);
- switch (underlyingObjectsAlias(AA, F->getDataLayout(),
- MemoryLocation::get(Dst),
- MemoryLocation::get(Src))) {
+ switch (underlyingObjectsAlias(AA, F->getDataLayout(), DstLoc, SrcLoc)) {
case AliasResult::MayAlias:
case AliasResult::PartialAlias:
// cannot analyse objects if we don't understand their aliasing.
@@ -3625,16 +3621,15 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst) {
break; // The underlying objects alias; test accesses for dependence.
}
- // establish loop nesting levels
- establishNestingLevels(Src, Dst);
- LLVM_DEBUG(dbgs() << " common nesting levels = " << CommonLevels << "\n");
- LLVM_DEBUG(dbgs() << " maximum nesting levels = " << MaxLevels << "\n");
-
- FullDependence Result(Src, Dst, PossiblyLoopIndependent, CommonLevels);
- ++TotalArrayPairs;
+ if (DstLoc.Size != SrcLoc.Size) {
+ // The dependence test gets confused if the size of the memory accesses
+ // differ.
+ LLVM_DEBUG(dbgs() << "can't analyze must alias with different sizes\n");
+ return std::make_unique<Dependence>(Src, Dst);
+ }
- unsigned Pairs = 1;
- SmallVector<Subscript, 2> Pair(Pairs);
+ Value *SrcPtr = getLoadStorePointerOperand(Src);
+ Value *DstPtr = getLoadStorePointerOperand(Dst);
const SCEV *SrcSCEV = SE->getSCEV(SrcPtr);
const SCEV *DstSCEV = SE->getSCEV(DstPtr);
LLVM_DEBUG(dbgs() << " SrcSCEV = " << *SrcSCEV << "\n");
@@ -3649,6 +3644,17 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst) {
LLVM_DEBUG(dbgs() << "can't analyze SCEV with different pointer base\n");
return std::make_unique<Dependence>(Src, Dst);
}
+
+ // establish loop nesting levels
+ establishNestingLevels(Src, Dst);
+ LLVM_DEBUG(dbgs() << " common nesting levels = " << CommonLevels << "\n");
+ LLVM_DEBUG(dbgs() << " maximum nesting levels = " << MaxLevels << "\n");
+
+ FullDependence Result(Src, Dst, PossiblyLoopIndependent, CommonLevels);
+ ++TotalArrayPairs;
+
+ unsigned Pairs = 1;
+ SmallVector<Subscript, 2> Pair(Pairs);
Pair[0].Src = SrcSCEV;
Pair[0].Dst = DstSCEV;
diff --git a/llvm/test/Analysis/DependenceAnalysis/DifferentAccessSize.ll b/llvm/test/Analysis/DependenceAnalysis/DifferentAccessSize.ll
new file mode 100644
index 0000000000000..2dded8f3b13a1
--- /dev/null
+++ b/llvm/test/Analysis/DependenceAnalysis/DifferentAccessSize.ll
@@ -0,0 +1,22 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -disable-output "-passes=print<da>" -aa-pipeline=basic-aa 2>&1 \
+; RUN: | FileCheck %s
+
+; The dependence test does not handle array accesses of different sizes: i32 and i64.
+; Bug 16183 - https://github.com/llvm/llvm-project/issues/16183
+
+define i64 @bug16183_alias(ptr nocapture %A) {
+; CHECK-LABEL: 'bug16183_alias'
+; CHECK-NEXT: Src: store i32 2, ptr %arrayidx, align 4 --> Dst: store i32 2, ptr %arrayidx, align 4
+; CHECK-NEXT: da analyze - none!
+; CHECK-NEXT: Src: store i32 2, ptr %arrayidx, align 4 --> Dst: %0 = load i64, ptr %A, align 8
+; CHECK-NEXT: da analyze - confused!
+; CHECK-NEXT: Src: %0 = load i64, ptr %A, align 8 --> Dst: %0 = load i64, ptr %A, align 8
+; CHECK-NEXT: da analyze - none!
+;
+entry:
+ %arrayidx = getelementptr inbounds i32, ptr %A, i64 1
+ store i32 2, ptr %arrayidx, align 4
+ %0 = load i64, ptr %A, align 8
+ ret i64 %0
+}
>From c97a56230b082339097ef16f10b5bf6c368a61a4 Mon Sep 17 00:00:00 2001
From: Sebastian Pop <spop at nvidia.com>
Date: Fri, 17 Jan 2025 20:54:19 +0000
Subject: [PATCH 2/2] [DA] check memory offsets are multiples of elements size
Check that the offsets from memory base address are multiples of the array
element size for a pair of memory accesses in a dependence test.
The patch adds a testcase that cannot be disambiguated by the DA.
---
llvm/lib/Analysis/DependenceAnalysis.cpp | 98 ++++++++++++++++++-
.../DependenceAnalysis/DifferentOffsets.ll | 25 +++++
.../DependenceAnalysis/MIVCheckConst.ll | 2 +-
3 files changed, 123 insertions(+), 2 deletions(-)
create mode 100644 llvm/test/Analysis/DependenceAnalysis/DifferentOffsets.ll
diff --git a/llvm/lib/Analysis/DependenceAnalysis.cpp b/llvm/lib/Analysis/DependenceAnalysis.cpp
index 5ca565e74682e..e2617315de937 100644
--- a/llvm/lib/Analysis/DependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/DependenceAnalysis.cpp
@@ -3634,7 +3634,9 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst) {
const SCEV *DstSCEV = SE->getSCEV(DstPtr);
LLVM_DEBUG(dbgs() << " SrcSCEV = " << *SrcSCEV << "\n");
LLVM_DEBUG(dbgs() << " DstSCEV = " << *DstSCEV << "\n");
- if (SE->getPointerBase(SrcSCEV) != SE->getPointerBase(DstSCEV)) {
+ const SCEV *SrcBase = SE->getPointerBase(SrcSCEV);
+ const SCEV *DstBase = SE->getPointerBase(DstSCEV);
+ if (SrcBase != DstBase) {
// If two pointers have different bases, trying to analyze indexes won't
// work; we can't compare them to each other. This can happen, for example,
// if one is produced by an LCSSA PHI node.
@@ -3645,6 +3647,100 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst) {
return std::make_unique<Dependence>(Src, Dst);
}
+ auto EltSize = SrcLoc.Size.toRaw();
+ assert(EltSize == DstLoc.Size.toRaw() && "Array element size differ");
+
+ // Check that memory access offsets in V are multiples of array EltSize.
+ std::function<bool(const SCEV *, const SCEV *&)> checkOffsets =
+ [&](const SCEV *V, const SCEV *&Param) -> bool {
+ if (auto *AddRec = dyn_cast<SCEVAddRecExpr>(V)) {
+ if (!checkOffsets(AddRec->getStart(), Param))
+ return false;
+ return checkOffsets(AddRec->getStepRecurrence(*SE), Param);
+ }
+ if (auto *Cst = dyn_cast<SCEVConstant>(V)) {
+ APInt C = Cst->getAPInt();
+ return C.srem(EltSize) == 0;
+ }
+
+ auto checkParamsMultipleOfSize = [&](const SCEV *V,
+ const SCEV *&Param) -> bool {
+ if (EltSize == 1)
+ return true;
+ if (!Param) {
+ Param = V;
+ return true;
+ }
+ if (Param == V)
+ return true;
+
+ // Check whether "(Param - V) % Size == 0".
+ const SCEV *Diff = SE->getMinusSCEV(Param, V);
+ if (const SCEVConstant *Cst = dyn_cast<SCEVConstant>(Diff)) {
+ APInt Val = Cst->getAPInt();
+ if (Val.isZero())
+ return true;
+ auto Rem =
+ Val.srem(APInt(Val.getBitWidth(), EltSize, /*isSigned=*/true));
+ if (Rem.isZero())
+ return true;
+ LLVM_DEBUG(dbgs() << "SCEV with different offsets: " << *Param << " - "
+ << *V << " = " << *Diff << " % " << EltSize << " = "
+ << Rem << " != 0\n");
+ return false;
+ }
+ // Check if the symbolic difference is a multiple of Size.
+ const SCEV *Val = SE->getConstant(
+ APInt(Diff->getType()->getScalarSizeInBits(), EltSize));
+
+ // Check by using the remainder computation.
+ const SCEV *Remainder = SE->getURemExpr(Diff, Val);
+ if (const SCEVConstant *C = dyn_cast<SCEVConstant>(Remainder))
+ if (C->getValue()->isZero())
+ return true;
+
+ // Check by using the division computation.
+ const SCEV *Q = SE->getUDivExpr(Diff, Val);
+ const SCEV *Product = SE->getMulExpr(Q, Val);
+ if (Diff == Product)
+ return true;
+ LLVM_DEBUG(dbgs() << "SCEV with different offsets:\n"
+ << *Param << " - " << *V << " = " << *Diff << "\n"
+ << "Remainder = " << *Remainder << "\n"
+ << "Q = " << *Q << " Product = " << *Product << "\n");
+ return false;
+ };
+ // Expressions like "n".
+ if (isa<SCEVUnknown>(V))
+ return checkParamsMultipleOfSize(V, Param);
+ // Expressions like "n + 1".
+ if (isa<SCEVAddExpr>(V))
+ return !SCEVExprContains(V, [](const SCEV *S) {
+ return isa<SCEVUnknown>(S);
+ }) || checkParamsMultipleOfSize(V, Param);
+
+ if (isa<SCEVMulExpr>(V))
+ return !SCEVExprContains(V, [](const SCEV *S) {
+ return isa<SCEVUnknown>(S);
+ }) || checkParamsMultipleOfSize(V, Param);
+
+ LLVM_DEBUG(dbgs() << "SCEV node not handled yet: " << *V << "\n");
+ return false;
+ };
+
+ // Check that memory access offsets are multiples of element sizes.
+ const SCEV *SrcEv = SE->getMinusSCEV(SrcSCEV, SrcBase);
+ const SCEV *DstEv = SE->getMinusSCEV(DstSCEV, DstBase);
+ const SCEV *Param = nullptr;
+
+ if (Src != Dst) {
+ // Check that memory access offsets are multiples of element sizes.
+ if (!checkOffsets(SrcEv, Param) || !checkOffsets(DstEv, Param)) {
+ LLVM_DEBUG(dbgs() << "can't analyze SCEV with different offsets\n");
+ return std::make_unique<Dependence>(Src, Dst);
+ }
+ }
+
// establish loop nesting levels
establishNestingLevels(Src, Dst);
LLVM_DEBUG(dbgs() << " common nesting levels = " << CommonLevels << "\n");
diff --git a/llvm/test/Analysis/DependenceAnalysis/DifferentOffsets.ll b/llvm/test/Analysis/DependenceAnalysis/DifferentOffsets.ll
new file mode 100644
index 0000000000000..00375da1f26de
--- /dev/null
+++ b/llvm/test/Analysis/DependenceAnalysis/DifferentOffsets.ll
@@ -0,0 +1,25 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -disable-output "-passes=print<da>" -aa-pipeline=basic-aa 2>&1 \
+; RUN: | FileCheck %s
+
+; The dependence test does not handle array accesses with difference between array accesses
+; is not a multiple of the array element size.
+
+; In this test, the element size is i32 = 4 bytes and the difference between the
+; load and the store is 2 bytes.
+
+define i32 @alias_with_different_offsets(ptr nocapture %A) {
+; CHECK-LABEL: 'alias_with_different_offsets'
+; CHECK-NEXT: Src: store i32 2, ptr %arrayidx, align 1 --> Dst: store i32 2, ptr %arrayidx, align 1
+; CHECK-NEXT: da analyze - none!
+; CHECK-NEXT: Src: store i32 2, ptr %arrayidx, align 1 --> Dst: %0 = load i32, ptr %A, align 1
+; CHECK-NEXT: da analyze - confused!
+; CHECK-NEXT: Src: %0 = load i32, ptr %A, align 1 --> Dst: %0 = load i32, ptr %A, align 1
+; CHECK-NEXT: da analyze - none!
+;
+entry:
+ %arrayidx = getelementptr inbounds i8, ptr %A, i64 2
+ store i32 2, ptr %arrayidx, align 1
+ %0 = load i32, ptr %A, align 1
+ ret i32 %0
+}
diff --git a/llvm/test/Analysis/DependenceAnalysis/MIVCheckConst.ll b/llvm/test/Analysis/DependenceAnalysis/MIVCheckConst.ll
index fa58a81d2355b..c8fbbe60b0bf2 100644
--- a/llvm/test/Analysis/DependenceAnalysis/MIVCheckConst.ll
+++ b/llvm/test/Analysis/DependenceAnalysis/MIVCheckConst.ll
@@ -41,7 +41,7 @@ define void @test(ptr %A, ptr %B, i1 %arg, i32 %n, i32 %m) #0 align 2 {
; CHECK-NEXT: Src: %v27 = load <32 x i32>, ptr %v25, align 256 --> Dst: %v27 = load <32 x i32>, ptr %v25, align 256
; CHECK-NEXT: da analyze - consistent input [0 S S]!
; CHECK-NEXT: Src: %v27 = load <32 x i32>, ptr %v25, align 256 --> Dst: %v32 = load <32 x i32>, ptr %v30, align 128
-; CHECK-NEXT: da analyze - input [* S S|<]!
+; CHECK-NEXT: da analyze - confused!
; CHECK-NEXT: Src: %v32 = load <32 x i32>, ptr %v30, align 128 --> Dst: %v32 = load <32 x i32>, ptr %v30, align 128
; CHECK-NEXT: da analyze - consistent input [0 S S]!
;
More information about the llvm-commits
mailing list