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[SCEVExp] Keep NUW/NSW if both original inc and isomporphic inc agree. (
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llvm#79512)

We are replacing with a wider increment. If both OrigInc and
IsomorphicInc are NUW/NSW, then we can preserve them on the wider
increment; the narrower IsomorphicInc would wrap before the wider
OrigInc, so the replacement won't make IsomorphicInc's uses more
poisonous.

PR: llvm#79512
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@fhahn
fhahn authored Feb 1, 2024
1 parent b5c0b67 commit da43733
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Showing 2 changed files with 33 additions and 8 deletions.
29 changes: 27 additions & 2 deletions llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1604,11 +1604,36 @@ void SCEVExpander::replaceCongruentIVInc(
const SCEV *TruncExpr =
SE.getTruncateOrNoop(SE.getSCEV(OrigInc), IsomorphicInc->getType());
if (OrigInc == IsomorphicInc || TruncExpr != SE.getSCEV(IsomorphicInc) ||
!SE.LI.replacementPreservesLCSSAForm(IsomorphicInc, OrigInc) ||
!hoistIVInc(OrigInc, IsomorphicInc,
!SE.LI.replacementPreservesLCSSAForm(IsomorphicInc, OrigInc))
return;

bool BothHaveNUW = false;
bool BothHaveNSW = false;
auto *OBOIncV = dyn_cast<OverflowingBinaryOperator>(OrigInc);
auto *OBOIsomorphic = dyn_cast<OverflowingBinaryOperator>(IsomorphicInc);
if (OBOIncV && OBOIsomorphic) {
BothHaveNUW =
OBOIncV->hasNoUnsignedWrap() && OBOIsomorphic->hasNoUnsignedWrap();
BothHaveNSW =
OBOIncV->hasNoSignedWrap() && OBOIsomorphic->hasNoSignedWrap();
}

if (!hoistIVInc(OrigInc, IsomorphicInc,
/*RecomputePoisonFlags*/ true))
return;

// We are replacing with a wider increment. If both OrigInc and IsomorphicInc
// are NUW/NSW, then we can preserve them on the wider increment; the narrower
// IsomorphicInc would wrap before the wider OrigInc, so the replacement won't
// make IsomorphicInc's uses more poisonous.
assert(OrigInc->getType()->getScalarSizeInBits() >=
IsomorphicInc->getType()->getScalarSizeInBits() &&
"Should only replace an increment with a wider one.");
if (BothHaveNUW || BothHaveNSW) {
OrigInc->setHasNoUnsignedWrap(OBOIncV->hasNoUnsignedWrap() || BothHaveNUW);
OrigInc->setHasNoSignedWrap(OBOIncV->hasNoSignedWrap() || BothHaveNSW);
}

SCEV_DEBUG_WITH_TYPE(DebugType,
dbgs() << "INDVARS: Eliminated congruent iv.inc: "
<< *IsomorphicInc << '\n');
Expand Down
12 changes: 6 additions & 6 deletions llvm/test/Transforms/IndVarSimplify/iv-poison.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -64,7 +64,7 @@ define i2 @iv_hoist_both_adds_nsw(i2 %arg) {
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV_0:%.*]] = phi i2 [ 1, [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[IV_0_NEXT]] = add nuw i2 [[IV_0]], 1
; CHECK-NEXT: [[IV_0_NEXT]] = add nuw nsw i2 [[IV_0]], 1
; CHECK-NEXT: [[DOTNOT_NOT:%.*]] = icmp ult i2 1, [[ARG:%.*]]
; CHECK-NEXT: br i1 [[DOTNOT_NOT]], label [[EXIT:%.*]], label [[LOOP]]
; CHECK: exit:
Expand Down Expand Up @@ -92,7 +92,7 @@ define i4 @iv_hoist_both_adds_nsw_extra_use(i4 %arg) {
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV_0:%.*]] = phi i4 [ 1, [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[IV_0_NEXT]] = add nuw i4 [[IV_0]], 1
; CHECK-NEXT: [[IV_0_NEXT]] = add nuw nsw i4 [[IV_0]], 1
; CHECK-NEXT: call void @use(i4 [[IV_0_NEXT]])
; CHECK-NEXT: call void @use(i4 [[IV_0_NEXT]])
; CHECK-NEXT: [[DOTNOT_NOT:%.*]] = icmp ult i4 1, [[ARG:%.*]]
Expand Down Expand Up @@ -124,7 +124,7 @@ define i4 @iv_hoist_both_adds_nsw_extra_use_incs_reordered(i4 %arg) {
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV_0:%.*]] = phi i4 [ 1, [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[IV_0_NEXT]] = add nuw i4 [[IV_0]], 1
; CHECK-NEXT: [[IV_0_NEXT]] = add nuw nsw i4 [[IV_0]], 1
; CHECK-NEXT: call void @use(i4 [[IV_0_NEXT]])
; CHECK-NEXT: call void @use(i4 [[IV_0_NEXT]])
; CHECK-NEXT: [[DOTNOT_NOT:%.*]] = icmp ult i4 1, [[ARG:%.*]]
Expand Down Expand Up @@ -244,7 +244,7 @@ define i2 @iv_hoist_both_adds_nuw(i2 %arg, i2 %start) {
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV_0:%.*]] = phi i2 [ [[START:%.*]], [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[IV_0_NEXT]] = add i2 [[IV_0]], 1
; CHECK-NEXT: [[IV_0_NEXT]] = add nuw i2 [[IV_0]], 1
; CHECK-NEXT: [[DOTNOT_NOT:%.*]] = icmp ult i2 [[START]], [[ARG:%.*]]
; CHECK-NEXT: br i1 [[DOTNOT_NOT]], label [[EXIT:%.*]], label [[LOOP]]
; CHECK: exit:
Expand Down Expand Up @@ -272,7 +272,7 @@ define i4 @iv_hoist_both_adds_nuw_extra_use(i4 %arg, i4 %start) {
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV_0:%.*]] = phi i4 [ [[START:%.*]], [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[IV_0_NEXT]] = add i4 [[IV_0]], 1
; CHECK-NEXT: [[IV_0_NEXT]] = add nuw i4 [[IV_0]], 1
; CHECK-NEXT: call void @use(i4 [[IV_0_NEXT]])
; CHECK-NEXT: call void @use(i4 [[IV_0_NEXT]])
; CHECK-NEXT: [[DOTNOT_NOT:%.*]] = icmp ult i4 [[START]], [[ARG:%.*]]
Expand Down Expand Up @@ -304,7 +304,7 @@ define i4 @iv_hoist_both_adds_nuw_extra_use_incs_reordered(i4 %arg, i4 %start) {
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV_0:%.*]] = phi i4 [ [[START:%.*]], [[BB:%.*]] ], [ [[IV_0_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[IV_0_NEXT]] = add i4 [[IV_0]], 1
; CHECK-NEXT: [[IV_0_NEXT]] = add nuw i4 [[IV_0]], 1
; CHECK-NEXT: call void @use(i4 [[IV_0_NEXT]])
; CHECK-NEXT: call void @use(i4 [[IV_0_NEXT]])
; CHECK-NEXT: [[DOTNOT_NOT:%.*]] = icmp ult i4 [[START]], [[ARG:%.*]]
Expand Down

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