[llvm] [BasicAA] Use nuw attribute of GEPs (PR #98608)
Hari Limaye via llvm-commits
llvm-commits at lists.llvm.org
Tue Aug 13 06:16:31 PDT 2024
https://github.com/hazzlim updated https://github.com/llvm/llvm-project/pull/98608
>From be98e95588b9c8cdd095cc25df1a0f8c417e3ca4 Mon Sep 17 00:00:00 2001
From: Hari Limaye <hari.limaye at arm.com>
Date: Wed, 26 Jun 2024 15:09:35 +0000
Subject: [PATCH 1/3] [BasicAA] Use nuw attribute of GEPs
Use the nuw attribute of GEPs to prove that pointers do not alias, in
cases matching the following:
+ + +
| BaseOffset | +<nuw> Indices |
---------------->|-------------------->|
|-->VLeftSize | |------->VRightSize
LHS RHS
If the difference between pointers is Offset +<nuw> Indices (the variable
indices all come from nuw GEPs) then we know that the addition does not
wrap the pointer index type (add nuw) and the constant Offset is a lower
bound on the distance between the pointers. We can then prove NoAlias via
Offset u>= VLeftSize.
---
llvm/lib/Analysis/BasicAliasAnalysis.cpp | 45 +++++--
llvm/test/Analysis/BasicAA/gep-nuw-alias.ll | 142 ++++++++++++++++++++
2 files changed, 177 insertions(+), 10 deletions(-)
create mode 100644 llvm/test/Analysis/BasicAA/gep-nuw-alias.ll
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 7bfb23e14aaa79..c531d266a61d6a 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -552,9 +552,9 @@ struct BasicAAResult::DecomposedGEP {
APInt Offset;
// Scaled variable (non-constant) indices.
SmallVector<VariableGEPIndex, 4> VarIndices;
- // Are all operations inbounds GEPs or non-indexing operations?
+ // Nowrap flags common to all GEP operations involved in expression.
// (std::nullopt iff expression doesn't involve any geps)
- std::optional<bool> InBounds;
+ std::optional<GEPNoWrapFlags> NWFlags;
void dump() const {
print(dbgs());
@@ -641,12 +641,11 @@ BasicAAResult::DecomposeGEPExpression(const Value *V, const DataLayout &DL,
return Decomposed;
}
- // Track whether we've seen at least one in bounds gep, and if so, whether
- // all geps parsed were in bounds.
- if (Decomposed.InBounds == std::nullopt)
- Decomposed.InBounds = GEPOp->isInBounds();
- else if (!GEPOp->isInBounds())
- Decomposed.InBounds = false;
+ // Track the common nowrap flags for all GEPs we see.
+ if (Decomposed.NWFlags == std::nullopt)
+ Decomposed.NWFlags = GEPOp->getNoWrapFlags();
+ else
+ *Decomposed.NWFlags &= GEPOp->getNoWrapFlags();
assert(GEPOp->getSourceElementType()->isSized() && "GEP must be sized");
@@ -1117,7 +1116,7 @@ AliasResult BasicAAResult::aliasGEP(
// for the two to alias, then we can assume noalias.
// TODO: Remove !isScalable() once BasicAA fully support scalable location
// size
- if (*DecompGEP1.InBounds && DecompGEP1.VarIndices.empty() &&
+ if (DecompGEP1.NWFlags->isInBounds() && DecompGEP1.VarIndices.empty() &&
V2Size.hasValue() && !V2Size.isScalable() &&
DecompGEP1.Offset.sge(V2Size.getValue()) &&
isBaseOfObject(DecompGEP2.Base))
@@ -1125,7 +1124,7 @@ AliasResult BasicAAResult::aliasGEP(
if (isa<GEPOperator>(V2)) {
// Symmetric case to above.
- if (*DecompGEP2.InBounds && DecompGEP1.VarIndices.empty() &&
+ if (DecompGEP2.NWFlags->isInBounds() && DecompGEP1.VarIndices.empty() &&
V1Size.hasValue() && !V1Size.isScalable() &&
DecompGEP1.Offset.sle(-V1Size.getValue()) &&
isBaseOfObject(DecompGEP1.Base))
@@ -1239,6 +1238,32 @@ AliasResult BasicAAResult::aliasGEP(
}
}
+ // If the difference between pointers is Offset +<nuw> Indices (the variable
+ // indices all come from nuw GEPs) then we know that the addition does not
+ // wrap the pointer index type (add nuw) and the constant Offset is a lower
+ // bound on the distance between the pointers. We can then prove NoAlias via
+ // Offset u>= VLeftSize.
+ // + + +
+ // | BaseOffset | +<nuw> Indices |
+ // ---------------->|-------------------->|
+ // |-->VLeftSize | |-------> VRightSize
+ // LHS RHS
+ if (!DecompGEP1.VarIndices.empty() &&
+ llvm::all_of(DecompGEP1.VarIndices, [&](const VariableGEPIndex &V) {
+ return V.IsNegated == DecompGEP1.VarIndices.front().IsNegated;
+ })) {
+ APInt Off = DecompGEP1.Offset;
+ bool Swapped = Off.isNegative();
+ LocationSize VLeftSize = Swapped ? V1Size : V2Size;
+ DecomposedGEP &DecompRight = Swapped ? DecompGEP2 : DecompGEP1;
+
+ bool IndicesFromRight = DecompGEP1.VarIndices.front().IsNegated == Swapped;
+ if (IndicesFromRight && DecompRight.NWFlags->hasNoUnsignedWrap())
+ if (!VLeftSize.isScalable() && VLeftSize.hasValue() &&
+ Off.abs().uge(VLeftSize.getValue()))
+ return AliasResult::NoAlias;
+ }
+
// Bail on analysing scalable LocationSize
if (V1Size.isScalable() || V2Size.isScalable())
return AliasResult::MayAlias;
diff --git a/llvm/test/Analysis/BasicAA/gep-nuw-alias.ll b/llvm/test/Analysis/BasicAA/gep-nuw-alias.ll
new file mode 100644
index 00000000000000..79899d60640d6e
--- /dev/null
+++ b/llvm/test/Analysis/BasicAA/gep-nuw-alias.ll
@@ -0,0 +1,142 @@
+; RUN: opt < %s -aa-pipeline=basic-aa -passes=aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; CHECK-LABEL: test_no_lower_bound
+;
+; CHECK-DAG: MayAlias: i32* %a, i32* %b
+define void @test_no_lower_bound(ptr %p, i64 %i) {
+ %a = getelementptr i8, ptr %p, i64 4
+ %b = getelementptr nuw i8, ptr %p, i64 %i
+
+ load i32, ptr %a
+ load i32, ptr %b
+
+ ret void
+}
+
+; CHECK-LABEL: test_lower_bound_lt_size
+;
+; CHECK-DAG: MayAlias: i32* %a, i32* %b
+define void @test_lower_bound_lt_size(ptr %p, i64 %i) {
+ %a = getelementptr i8, ptr %p
+ %add = getelementptr nuw i8, ptr %p, i64 2
+ %b = getelementptr nuw i8, ptr %add, i64 %i
+
+ load i32, ptr %a
+ load i32, ptr %b
+
+ ret void
+}
+
+; CHECK-LABEL: test_lower_bound_ge_size
+;
+; CHECK-DAG: NoAlias: i32* %a, i32* %b
+define void @test_lower_bound_ge_size(ptr %p, i64 %i) {
+ %a = getelementptr i8, ptr %p
+ %add = getelementptr nuw i8, ptr %p, i64 4
+ %b = getelementptr nuw i8, ptr %add, i64 %i
+
+ load i32, ptr %a
+ load i32, ptr %b
+
+ ret void
+}
+
+; CHECK-LABEL: test_not_all_nuw
+;
+; If part of the addressing is done with non-nuw GEPs, we can't use properties
+; implied by the last GEP with the whole offset. In this case, the calculation
+; of %add (%p + 4) could wrap the pointer index type, such that %add +<nuw> %i
+; could still alias with %p.
+;
+; CHECK-DAG: MayAlias: i32* %a, i32* %b
+define void @test_not_all_nuw(ptr %p, i64 %i) {
+ %a = getelementptr i8, ptr %p
+ %add = getelementptr i8, ptr %p, i64 4
+ %b = getelementptr nuw i8, ptr %add, i64 %i
+
+ load i32, ptr %a
+ load i32, ptr %b
+
+ ret void
+}
+
+; CHECK-LABEL: test_multi_step_not_all_nuw
+;
+; CHECK-DAG: MayAlias: i32* %a, i32* %b
+define void @test_multi_step_not_all_nuw(ptr %p, i64 %i, i64 %j, i64 %k) {
+ %a = getelementptr i8, ptr %p
+ %add = getelementptr i8, ptr %p, i64 4
+ %step1 = getelementptr i8, ptr %add, i64 %i
+ %step2 = getelementptr i8, ptr %step1, i64 %j
+ %b = getelementptr nuw i8, ptr %step2, i64 %k
+
+ load i32, ptr %a
+ load i32, ptr %b
+
+ ret void
+}
+
+; CHECK-LABEL: test_multi_step_all_nuw
+;
+; CHECK-DAG: NoAlias: i32* %a, i32* %b
+define void @test_multi_step_all_nuw(ptr %p, i64 %i, i64 %j, i64 %k) {
+ %a = getelementptr i8, ptr %p
+ %add = getelementptr nuw i8, ptr %p, i64 4
+ %step1 = getelementptr nuw i8, ptr %add, i64 %i
+ %step2 = getelementptr nuw i8, ptr %step1, i64 %j
+ %b = getelementptr nuw i8, ptr %step2, i64 %k
+
+ load i32, ptr %a
+ load i32, ptr %b
+
+ ret void
+}
+
+%struct = type { i64, [2 x i32], i64 }
+
+; CHECK-LABEL: test_struct_no_nuw
+;
+; The array access may alias with the struct elements before and after, because
+; we cannot prove that (%arr + %i) does not alias with the base pointer %p.
+;
+; CHECK-DAG: MayAlias: i32* %arrayidx, i64* %st
+; CHECK-DAG: NoAlias: i64* %after, i64* %st
+; CHECK-DAG: MayAlias: i64* %after, i32* %arrayidx
+
+define void @test_struct_no_nuw(ptr %st, i64 %i) {
+ %arr = getelementptr i8, ptr %st, i64 8
+ %arrayidx = getelementptr [2 x i32], ptr %arr, i64 0, i64 %i
+ %after = getelementptr i8, ptr %st, i64 16
+
+ load i64, ptr %st
+ load i32, ptr %arrayidx
+ load i64, ptr %after
+
+ ret void
+}
+
+; CHECK-LABEL: test_struct_nuw
+;
+; We can prove that the array access does not alias with struct element before,
+; because we can prove that (%arr +<nuw> %i) does not wrap the pointer index
+; type (add nuw). The array access may still alias with the struct element
+; after, as the add nuw property does not preclude this.
+;
+; CHECK-DAG: NoAlias: i32* %arrayidx, i64* %st
+; CHECK-DAG: NoAlias: i64* %after, i64* %st
+; CHECK-DAG: MayAlias: i64* %after, i32* %arrayidx
+
+define void @test_struct_nuw(ptr %st, i64 %i) {
+ %arr = getelementptr nuw i8, ptr %st, i64 8
+ %arrayidx = getelementptr nuw [2 x i32], ptr %arr, i64 0, i64 %i
+ %after = getelementptr nuw i8, ptr %st, i64 16
+
+ load i64, ptr %st
+ load i32, ptr %arrayidx
+ load i64, ptr %after
+
+ ret void
+}
+
>From f2fad1bc9f33a7cb1a49682c0c4fb991e63bea44 Mon Sep 17 00:00:00 2001
From: Hari Limaye <hari.limaye at arm.com>
Date: Mon, 15 Jul 2024 08:24:59 +0000
Subject: [PATCH 2/3] Reorder checks (Addressing review comment)
---
llvm/lib/Analysis/BasicAliasAnalysis.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index c531d266a61d6a..36402d06537a97 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1259,7 +1259,7 @@ AliasResult BasicAAResult::aliasGEP(
bool IndicesFromRight = DecompGEP1.VarIndices.front().IsNegated == Swapped;
if (IndicesFromRight && DecompRight.NWFlags->hasNoUnsignedWrap())
- if (!VLeftSize.isScalable() && VLeftSize.hasValue() &&
+ if (VLeftSize.hasValue() && !VLeftSize.isScalable() &&
Off.abs().uge(VLeftSize.getValue()))
return AliasResult::NoAlias;
}
>From 846411b4dba7326aa5a5fab28ed4e14a770bc9ac Mon Sep 17 00:00:00 2001
From: Hari Limaye <hari.limaye at arm.com>
Date: Thu, 8 Aug 2024 23:17:26 +0000
Subject: [PATCH 3/3] Drop nuw flag in some cases when subtracting GEPs
---
.../llvm/Analysis/BasicAliasAnalysis.h | 3 +-
llvm/lib/Analysis/BasicAliasAnalysis.cpp | 41 ++++++---
llvm/test/Analysis/BasicAA/gep-nuw-alias.ll | 92 +++++++++++++++++++
3 files changed, 121 insertions(+), 15 deletions(-)
diff --git a/llvm/include/llvm/Analysis/BasicAliasAnalysis.h b/llvm/include/llvm/Analysis/BasicAliasAnalysis.h
index 7eca82729430dd..801d55d09731c5 100644
--- a/llvm/include/llvm/Analysis/BasicAliasAnalysis.h
+++ b/llvm/include/llvm/Analysis/BasicAliasAnalysis.h
@@ -122,8 +122,7 @@ class BasicAAResult : public AAResultBase {
bool isValueEqualInPotentialCycles(const Value *V1, const Value *V2,
const AAQueryInfo &AAQI);
- void subtractDecomposedGEPs(DecomposedGEP &DestGEP,
- const DecomposedGEP &SrcGEP,
+ void subtractDecomposedGEPs(DecomposedGEP &DestGEP, DecomposedGEP &SrcGEP,
const AAQueryInfo &AAQI);
AliasResult aliasGEP(const GEPOperator *V1, LocationSize V1Size,
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 36402d06537a97..95b46cac81a05e 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -527,6 +527,8 @@ struct VariableGEPIndex {
return Scale == Other.Scale;
}
+ bool isSubtracted() const { return IsNegated || Scale.isNegative(); }
+
void dump() const {
print(dbgs());
dbgs() << "\n";
@@ -561,8 +563,8 @@ struct BasicAAResult::DecomposedGEP {
dbgs() << "\n";
}
void print(raw_ostream &OS) const {
- OS << "(DecomposedGEP Base=" << Base->getName()
- << ", Offset=" << Offset
+ OS << "(DecomposedGEP Base=" << Base->getName() << ", Offset=" << Offset
+ << ", nuw=" << ((NWFlags && NWFlags->hasNoUnsignedWrap()) ? "1" : "0")
<< ", VarIndices=[";
for (size_t i = 0; i < VarIndices.size(); i++) {
if (i != 0)
@@ -1238,11 +1240,10 @@ AliasResult BasicAAResult::aliasGEP(
}
}
- // If the difference between pointers is Offset +<nuw> Indices (the variable
- // indices all come from nuw GEPs) then we know that the addition does not
- // wrap the pointer index type (add nuw) and the constant Offset is a lower
- // bound on the distance between the pointers. We can then prove NoAlias via
- // Offset u>= VLeftSize.
+ // If the difference between pointers is Offset +<nuw> Indices then we know
+ // that the addition does not wrap the pointer index type (add nuw) and the
+ // constant Offset is a lower bound on the distance between the pointers. We
+ // can then prove NoAlias via Offset u>= VLeftSize.
// + + +
// | BaseOffset | +<nuw> Indices |
// ---------------->|-------------------->|
@@ -1250,15 +1251,15 @@ AliasResult BasicAAResult::aliasGEP(
// LHS RHS
if (!DecompGEP1.VarIndices.empty() &&
llvm::all_of(DecompGEP1.VarIndices, [&](const VariableGEPIndex &V) {
- return V.IsNegated == DecompGEP1.VarIndices.front().IsNegated;
+ return V.isSubtracted() == DecompGEP1.VarIndices.front().isSubtracted();
})) {
- APInt Off = DecompGEP1.Offset;
+ const APInt &Off = DecompGEP1.Offset;
bool Swapped = Off.isNegative();
LocationSize VLeftSize = Swapped ? V1Size : V2Size;
- DecomposedGEP &DecompRight = Swapped ? DecompGEP2 : DecompGEP1;
+ const DecomposedGEP &DecompRight = Swapped ? DecompGEP2 : DecompGEP1;
- bool IndicesFromRight = DecompGEP1.VarIndices.front().IsNegated == Swapped;
- if (IndicesFromRight && DecompRight.NWFlags->hasNoUnsignedWrap())
+ bool IndicesAdded = DecompGEP1.VarIndices.front().isSubtracted() == Swapped;
+ if (IndicesAdded && DecompRight.NWFlags->hasNoUnsignedWrap())
if (VLeftSize.hasValue() && !VLeftSize.isScalable() &&
Off.abs().uge(VLeftSize.getValue()))
return AliasResult::NoAlias;
@@ -1866,8 +1867,15 @@ bool BasicAAResult::isValueEqualInPotentialCycles(const Value *V,
/// Computes the symbolic difference between two de-composed GEPs.
void BasicAAResult::subtractDecomposedGEPs(DecomposedGEP &DestGEP,
- const DecomposedGEP &SrcGEP,
+ DecomposedGEP &SrcGEP,
const AAQueryInfo &AAQI) {
+ // Drop nuw flag from GEP if subtraction of constant offsets overflows in an
+ // unsigned sense.
+ if (DestGEP.Offset.ult(SrcGEP.Offset))
+ DestGEP.NWFlags = DestGEP.NWFlags->withoutNoUnsignedWrap();
+ else if (SrcGEP.Offset.ult(DestGEP.Offset) && SrcGEP.NWFlags)
+ SrcGEP.NWFlags = SrcGEP.NWFlags->withoutNoUnsignedWrap();
+
DestGEP.Offset -= SrcGEP.Offset;
for (const VariableGEPIndex &Src : SrcGEP.VarIndices) {
// Find V in Dest. This is N^2, but pointer indices almost never have more
@@ -1890,6 +1898,13 @@ void BasicAAResult::subtractDecomposedGEPs(DecomposedGEP &DestGEP,
// If we found it, subtract off Scale V's from the entry in Dest. If it
// goes to zero, remove the entry.
if (Dest.Scale != Src.Scale) {
+ // Drop nuw flag from GEP if subtraction of V's Scale overflows in an
+ // unsigned sense.
+ if (Dest.Scale.ult(Src.Scale))
+ DestGEP.NWFlags = DestGEP.NWFlags->withoutNoUnsignedWrap();
+ else if (SrcGEP.NWFlags)
+ SrcGEP.NWFlags = SrcGEP.NWFlags->withoutNoUnsignedWrap();
+
Dest.Scale -= Src.Scale;
Dest.IsNSW = false;
} else {
diff --git a/llvm/test/Analysis/BasicAA/gep-nuw-alias.ll b/llvm/test/Analysis/BasicAA/gep-nuw-alias.ll
index 79899d60640d6e..fc888f5e29eded 100644
--- a/llvm/test/Analysis/BasicAA/gep-nuw-alias.ll
+++ b/llvm/test/Analysis/BasicAA/gep-nuw-alias.ll
@@ -140,3 +140,95 @@ define void @test_struct_nuw(ptr %st, i64 %i) {
ret void
}
+; CHECK-LABEL: constant_offset_overflow
+;
+; If subtraction of constant offsets could overflow in an unsigned sense, we
+; cannot prove the lower bound between the GEPs and so they may still alias.
+;
+; CHECK-DAG: MayAlias: i32* %a, i32* %b
+
+define void @constant_offset_overflow(ptr %p, i64 %i) {
+ %a = getelementptr i8, ptr %p, i64 -8
+ %add = getelementptr nuw i8, ptr %p, i64 4
+ %b = getelementptr nuw i8, ptr %add, i64 %i
+
+ load i32, ptr %a
+ load i32, ptr %b
+
+ ret void
+}
+
+; CHECK-LABEL: constant_offset_overflow_rev_order
+;
+; Same as above test, just verifying correct behaviour when GEPs encountered
+; in the reverse order;
+;
+; CHECK-DAG: MayAlias: i32* %a, i32* %b
+
+define void @constant_offset_overflow_rev_order(ptr %p, i64 %i) {
+ %a = getelementptr i8, ptr %p, i64 -8
+ %add = getelementptr nuw i8, ptr %p, i64 4
+ %b = getelementptr nuw i8, ptr %add, i64 %i
+
+ load i32, ptr %b
+ load i32, ptr %a
+
+ ret void
+}
+
+; CHECK-LABEL: equal_var_idx_noalias
+;
+; If GEPs have equal variable indices, we can prove NoAlias when the Scale of
+; the RHS GEP is greater, as in this scenario the constant lower bound holds.
+;
+; CHECK-DAG: NoAlias: i32* %a, i32* %b
+
+define void @equal_var_idx_noalias(ptr %p, i64 %i) {
+ %a = getelementptr i8, ptr %p, i64 %i
+
+ %add = getelementptr nuw i8, ptr %p, i64 4
+ %b = getelementptr nuw i16, ptr %add, i64 %i
+
+ load i32, ptr %a
+ load i32, ptr %b
+
+ ret void
+}
+
+; CHECK-LABEL: equal_var_idx_alias
+;
+; If GEPs have equal variable indices, we cannot prove NoAlias when the Scale of
+; the RHS GEP is ult Scale of the LHS GEP.
+;
+; CHECK-DAG: MayAlias: i32* %a, i32* %b
+
+define void @equal_var_idx_alias(ptr %p, i64 %i) {
+ %a = getelementptr i32, ptr %p, i64 %i
+
+ %add = getelementptr nuw i8, ptr %p, i64 4
+ %b = getelementptr nuw i16, ptr %add, i64 %i
+
+ load i32, ptr %b
+ load i32, ptr %a
+
+ ret void
+}
+
+; CHECK-LABEL: both_var_idx
+;
+; If the RHS GEP has unmatched variable indices, we cannot prove a constant
+; lower bound between GEPs.
+;
+; CHECK-DAG: MayAlias: i32* %a, i32* %b
+
+define void @both_var_idx(ptr %p, i64 %i, i64 %j) {
+ %a = getelementptr i8, ptr %p, i64 %i
+
+ %add = getelementptr nuw i8, ptr %p, i64 4
+ %b = getelementptr nuw i8, ptr %add, i64 %j
+
+ load i32, ptr %a
+ load i32, ptr %b
+
+ ret void
+}
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