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Bug fixes for Array Dependence Analysis #116389

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2608131
[DA] disambiguate evolution of base addresses
sebpop Oct 31, 2024
98fb660
[DA] Dependence analysis does not handle array accesses of different …
sebpop Nov 14, 2024
0fa3ca9
[DA] add testcase
sebpop Nov 14, 2024
6ff50fa
[DA] use NSW arithmetic
sebpop Nov 15, 2024
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96 changes: 69 additions & 27 deletions llvm/lib/Analysis/DependenceAnalysis.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -712,10 +712,66 @@ void Dependence::dump(raw_ostream &OS) const {
// tbaa, non-overlapping regions etc), then it is known there is no dependecy.
// Otherwise the underlying objects are checked to see if they point to
// different identifiable objects.
static AliasResult underlyingObjectsAlias(AAResults *AA,
const DataLayout &DL,
const MemoryLocation &LocA,
const MemoryLocation &LocB) {
static AliasResult underlyingObjectsAlias(AAResults *AA, LoopInfo *LI,
ScalarEvolution *SE, Instruction *A,
Instruction *B) {
const MemoryLocation &LocA = MemoryLocation::get(A);
const MemoryLocation &LocB = MemoryLocation::get(B);

// Check the underlying objects are the same
const Value *AObj = getUnderlyingObject(LocA.Ptr);
const Value *BObj = getUnderlyingObject(LocB.Ptr);

if (AObj == BObj) {
// The dependence test gets confused if the size of the memory accesses
// differ.
if (LocA.Size != LocB.Size)
return AliasResult::MayAlias;

// If the underlying objects are the same, they must alias.
return AliasResult::MustAlias;
}

if (auto *APhi = dyn_cast<PHINode>(AObj)) {
if (auto *BPhi = dyn_cast<PHINode>(BObj)) {
Loop *ALoop = LI->getLoopFor(APhi->getParent());
Loop *BLoop = LI->getLoopFor(BPhi->getParent());
if (ALoop == BLoop) {
auto *SCEVa = SE->getSCEV(const_cast<Value *>(AObj));
auto *SCEVb = SE->getSCEV(const_cast<Value *>(BObj));

// If the SCEVs are the same, they must alias.
if (SCEVa == SCEVb)
return AliasResult::MustAlias;

// If SCEV cannot analyze one of the values, then they may alias.
if (isa<SCEVUnknown>(SCEVa) || isa<SCEVUnknown>(SCEVb))
return AliasResult::MayAlias;

// Check whether the start values alias.
const SCEV *StartA = SCEVa;
while (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(StartA))
StartA = AR->getStart();

const SCEV *StartB = SCEVb;
while (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(StartB))
StartB = AR->getStart();

if (const SCEVUnknown *UA = dyn_cast<SCEVUnknown>(StartA)) {
if (const SCEVUnknown *UB = dyn_cast<SCEVUnknown>(StartB)) {
MemoryLocation LocAS =
MemoryLocation::getBeforeOrAfter(UA->getValue());
MemoryLocation LocBS =
MemoryLocation::getBeforeOrAfter(UB->getValue());
if (AA->isNoAlias(LocAS, LocBS))
return AliasResult::NoAlias;
}
}
return AliasResult::MayAlias;
}
}
}

// Check the original locations (minus size) for noalias, which can happen for
// tbaa, incompatible underlying object locations, etc.
MemoryLocation LocAS =
Expand All @@ -725,14 +781,6 @@ static AliasResult underlyingObjectsAlias(AAResults *AA,
if (AA->isNoAlias(LocAS, LocBS))
return AliasResult::NoAlias;

// Check the underlying objects are the same
const Value *AObj = getUnderlyingObject(LocA.Ptr);
const Value *BObj = getUnderlyingObject(LocB.Ptr);

// If the underlying objects are the same, they must alias
if (AObj == BObj)
return AliasResult::MustAlias;

// We may have hit the recursion limit for underlying objects, or have
// underlying objects where we don't know they will alias.
if (!isIdentifiedObject(AObj) || !isIdentifiedObject(BObj))
Expand All @@ -743,7 +791,6 @@ static AliasResult underlyingObjectsAlias(AAResults *AA,
return AliasResult::NoAlias;
}


// Returns true if the load or store can be analyzed. Atomic and volatile
// operations have properties which this analysis does not understand.
static
Expand Down Expand Up @@ -3116,11 +3163,9 @@ const SCEV *DependenceInfo::addToCoefficient(const SCEV *Expr,
const Loop *TargetLoop,
const SCEV *Value) const {
const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(Expr);
if (!AddRec) // create a new addRec
return SE->getAddRecExpr(Expr,
Value,
TargetLoop,
SCEV::FlagAnyWrap); // Worst case, with no info.
if (!AddRec)
return SE->getAddRecExpr(Expr, Value, TargetLoop, SCEV::FlagNSW);

if (AddRec->getLoop() == TargetLoop) {
const SCEV *Sum = SE->getAddExpr(AddRec->getStepRecurrence(*SE), Value);
if (Sum->isZero())
Expand All @@ -3131,7 +3176,7 @@ const SCEV *DependenceInfo::addToCoefficient(const SCEV *Expr,
AddRec->getNoWrapFlags());
}
if (SE->isLoopInvariant(AddRec, TargetLoop))
return SE->getAddRecExpr(AddRec, Value, TargetLoop, SCEV::FlagAnyWrap);
return SE->getAddRecExpr(AddRec, Value, TargetLoop, SCEV::FlagNSW);
return SE->getAddRecExpr(
addToCoefficient(AddRec->getStart(), TargetLoop, Value),
AddRec->getStepRecurrence(*SE), AddRec->getLoop(),
Expand Down Expand Up @@ -3606,9 +3651,7 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
Value *SrcPtr = getLoadStorePointerOperand(Src);
Value *DstPtr = getLoadStorePointerOperand(Dst);

switch (underlyingObjectsAlias(AA, F->getDataLayout(),
MemoryLocation::get(Dst),
MemoryLocation::get(Src))) {
switch (underlyingObjectsAlias(AA, LI, SE, Dst, Src)) {
case AliasResult::MayAlias:
case AliasResult::PartialAlias:
// cannot analyse objects if we don't understand their aliasing.
Expand Down Expand Up @@ -4030,11 +4073,8 @@ const SCEV *DependenceInfo::getSplitIteration(const Dependence &Dep,
assert(Dst->mayReadFromMemory() || Dst->mayWriteToMemory());
assert(isLoadOrStore(Src));
assert(isLoadOrStore(Dst));
Value *SrcPtr = getLoadStorePointerOperand(Src);
Value *DstPtr = getLoadStorePointerOperand(Dst);
assert(underlyingObjectsAlias(
AA, F->getDataLayout(), MemoryLocation::get(Dst),
MemoryLocation::get(Src)) == AliasResult::MustAlias);
assert(underlyingObjectsAlias(AA, LI, SE, Dst, Src) ==
AliasResult::MustAlias);

// establish loop nesting levels
establishNestingLevels(Src, Dst);
Expand All @@ -4043,6 +4083,8 @@ const SCEV *DependenceInfo::getSplitIteration(const Dependence &Dep,

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);
Pair[0].Src = SrcSCEV;
Expand Down
15 changes: 15 additions & 0 deletions llvm/test/Analysis/DependenceAnalysis/DifferentAccessSize.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
; 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
; CHECK-LABEL: bug16183_alias
; CHECK: da analyze - confused!

define i64 @bug16183_alias(i32* nocapture %A) {
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
}
136 changes: 136 additions & 0 deletions llvm/test/Analysis/DependenceAnalysis/FlipFlopBaseAddress.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,136 @@
; RUN: opt < %s -disable-output "-passes=print<da>" -aa-pipeline=basic-aa 2>&1 \
; RUN: | FileCheck %s

; Check that dependence analysis correctly handles flip-flop of base addresses.
; Bug 41488 - https://github.com/llvm/llvm-project/issues/41488

; CHECK-LABEL: bug41488_test1
; CHECK-NOT: da analyze - none!

define float @bug41488_test1() {
entry:
%g = alloca float, align 4
%h = alloca float, align 4
br label %for.body

for.body:
%p = phi float* [ %g, %entry ], [ %q, %for.body ]
%q = phi float* [ %h, %entry ], [ %p, %for.body ]
%0 = load float, float* %p, align 4
store float undef, float* %q, align 4
%branch_cond = fcmp ugt float %0, 0.0
br i1 %branch_cond, label %for.cond.cleanup, label %for.body

for.cond.cleanup:
ret float undef
}

; CHECK-LABEL: bug41488_test2
; CHECK-NOT: da analyze - none!

define void @bug41488_test2(i32 %n) {
entry:
%g = alloca float, align 4
%h = alloca float, align 4
br label %for.body

for.body:
%i = phi i32 [0, %entry ], [ %inc, %for.body ]
%p = phi float* [ %g, %entry ], [ %q, %for.body ]
%q = phi float* [ %h, %entry ], [ %p, %for.body ]
%0 = load float, float* %p, align 4
store float 0.0, float* %q, align 4
%inc = add nuw i32 %i, 1
%branch_cond = icmp ult i32 %i, %n
br i1 %branch_cond, label %for.body, label %for.cond.cleanup

for.cond.cleanup:
ret void
}

; Bug 53942 - https://github.com/llvm/llvm-project/issues/53942
; CHECK-LABEL: bug53942_foo
; CHECK-NOT: da analyze - none!

define void @bug53942_foo(i32 noundef %n, ptr noalias nocapture noundef writeonly %A, ptr noalias nocapture noundef %B) {
entry:
%cmp8 = icmp sgt i32 %n, 1
br i1 %cmp8, label %for.body.preheader, label %for.cond.cleanup

for.body.preheader: ; preds = %entry
%wide.trip.count = zext nneg i32 %n to i64
br label %for.body

for.cond.cleanup: ; preds = %for.body, %entry
ret void

for.body: ; preds = %for.body.preheader, %for.body
%indvars.iv = phi i64 [ 1, %for.body.preheader ], [ %indvars.iv.next, %for.body ]
%ptr1.011 = phi ptr [ %A, %for.body.preheader ], [ %ptr2.09, %for.body ]
%ptr2.09 = phi ptr [ %B, %for.body.preheader ], [ %ptr1.011, %for.body ]
%.pre = load double, ptr %B, align 8
%arrayidx2 = getelementptr inbounds double, ptr %ptr1.011, i64 %indvars.iv
store double %.pre, ptr %arrayidx2, align 8
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%exitcond.not = icmp eq i64 %indvars.iv.next, %wide.trip.count
br i1 %exitcond.not, label %for.cond.cleanup, label %for.body
}


; Bug 53942 - https://github.com/llvm/llvm-project/issues/53942
; CHECK-LABEL: bug53942_bar
; CHECK-NOT: da analyze - none!

define void @bug53942_bar(i32 noundef %n, ptr noalias noundef %A, ptr noalias noundef %B) {
entry:
br label %for.cond

for.cond: ; preds = %for.inc, %entry
%i.0 = phi i32 [ 1, %entry ], [ %inc, %for.inc ]
%cmp = icmp slt i32 %i.0, %n
br i1 %cmp, label %for.body, label %for.cond.cleanup

for.cond.cleanup: ; preds = %for.cond
br label %for.end

for.body: ; preds = %for.cond
%and = and i32 %i.0, 2
%tobool.not = icmp eq i32 %and, 0
br i1 %tobool.not, label %cond.false, label %cond.true

cond.true: ; preds = %for.body
br label %cond.end

cond.false: ; preds = %for.body
br label %cond.end

cond.end: ; preds = %cond.false, %cond.true
%cond = phi ptr [ %A, %cond.true ], [ %B, %cond.false ]
%and1 = and i32 %i.0, 2
%tobool2.not = icmp eq i32 %and1, 0
br i1 %tobool2.not, label %cond.false4, label %cond.true3

cond.true3: ; preds = %cond.end
br label %cond.end5

cond.false4: ; preds = %cond.end
br label %cond.end5

cond.end5: ; preds = %cond.false4, %cond.true3
%cond6 = phi ptr [ %B, %cond.true3 ], [ %A, %cond.false4 ]
%sub = add nsw i32 %i.0, -1
%idxprom = sext i32 %sub to i64
%arrayidx = getelementptr inbounds double, ptr %cond6, i64 %idxprom
%0 = load double, ptr %arrayidx, align 8
%idxprom7 = zext nneg i32 %i.0 to i64
%arrayidx8 = getelementptr inbounds double, ptr %cond, i64 %idxprom7
store double %0, ptr %arrayidx8, align 8
br label %for.inc

for.inc: ; preds = %cond.end5
%inc = add nuw nsw i32 %i.0, 1
br label %for.cond

for.end: ; preds = %for.cond.cleanup
ret void
}
67 changes: 67 additions & 0 deletions llvm/test/Analysis/DependenceAnalysis/PR31848.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,67 @@
; RUN: opt < %s -disable-output "-passes=print<da>" -aa-pipeline=basic-aa 2>&1 \
; RUN: | FileCheck %s

define void @foo(i32* nocapture %A, i32 %n) {
entry:
%n.cmp = icmp sgt i32 %n, 0
br i1 %n.cmp, label %for.j.header, label %exit

for.j.header:
%j= phi i32 [ %j.inc, %for.j.latch ], [ 0, %entry ]
br label %for.i.header

for.i.header:
%i = phi i32 [ %i.inc, %for.i.latch ], [ 0, %for.j.header ]
br label %for.di.header

for.di.header:
%di = phi i32 [ -1, %for.i.header ], [ %di.inc, %for.di.latch ]
%di.add = add nsw i32 %di, %i
br label %for.dj.header

for.dj.header:
%dj = phi i32 [ -1, %for.di.header ], [ %dj.inc, %body ]
%dj.add = add nsw i32 %dj, %j
br label %while.x

while.x:
%x = phi i32 [ %di.add, %for.dj.header ], [ %x.inc, %while.x ]
%x.cmp = icmp slt i32 %x, 0
%x.inc = add nsw i32 %x, %n
br i1 %x.cmp, label %while.x, label %while.y

while.y:
%y = phi i32 [ %y.inc, %while.y ], [ %dj.add, %while.x ]
%y.cmp = icmp slt i32 %y, 0
%y.inc = add nsw i32 %y, %n
br i1 %y.cmp, label %while.y, label %body

body:
%mul = mul nsw i32 %y, %n
%add = add nsw i32 %mul, %x
%idxprom = sext i32 %add to i64
%arrayidx = getelementptr inbounds i32, i32* %A, i64 %idxprom
; CHECK: da analyze - output [* * * *]
store i32 7, i32* %arrayidx, align 4
%dj.inc = add nsw i32 %dj, 1
%dj.exitcond = icmp eq i32 %dj.inc, 2
br i1 %dj.exitcond, label %for.di.latch, label %for.dj.header

for.di.latch:
%di.inc = add nsw i32 %di, 1
%di.exitcond = icmp eq i32 %di.inc, 2
br i1 %di.exitcond, label %for.i.latch, label %for.di.header

for.i.latch:
%i.inc = add nuw nsw i32 %i, 1
%i.exitcond = icmp eq i32 %i.inc, %n
br i1 %i.exitcond, label %for.j.latch, label %for.i.header

for.j.latch:
%j.inc = add nuw nsw i32 %j, 1
%j.exitcond = icmp eq i32 %j.inc, %n
br i1 %j.exitcond, label %exit, label %for.j.header

exit:
ret void
}
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