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[lsr][term-fold] Restrict transform to low cost expansions #74747

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Merged
merged 2 commits into from
Jan 31, 2024
Merged

[lsr][term-fold] Restrict transform to low cost expansions #74747

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4d57ba1
[lsr][term-fold] Restrict expansion budget, particularly for profiled…
preames Dec 7, 2023
f63c971
[LSR] Use simplest constant threshold for the moment
preames Jan 31, 2024
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14 changes: 12 additions & 2 deletions llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -6762,7 +6762,7 @@ static llvm::PHINode *GetInductionVariable(const Loop &L, ScalarEvolution &SE,

static std::optional<std::tuple<PHINode *, PHINode *, const SCEV *, bool>>
canFoldTermCondOfLoop(Loop *L, ScalarEvolution &SE, DominatorTree &DT,
const LoopInfo &LI) {
const LoopInfo &LI, const TargetTransformInfo &TTI) {
if (!L->isInnermost()) {
LLVM_DEBUG(dbgs() << "Cannot fold on non-innermost loop\n");
return std::nullopt;
Expand Down Expand Up @@ -6820,6 +6820,7 @@ canFoldTermCondOfLoop(Loop *L, ScalarEvolution &SE, DominatorTree &DT,
PHINode *ToHelpFold = nullptr;
const SCEV *TermValueS = nullptr;
bool MustDropPoison = false;
auto InsertPt = L->getLoopPreheader()->getTerminator();
for (PHINode &PN : L->getHeader()->phis()) {
if (ToFold == &PN)
continue;
Expand Down Expand Up @@ -6861,6 +6862,15 @@ canFoldTermCondOfLoop(Loop *L, ScalarEvolution &SE, DominatorTree &DT,
continue;
}

if (Expander.isHighCostExpansion(TermValueSLocal, L,
2*SCEVCheapExpansionBudget,
&TTI, InsertPt)) {
LLVM_DEBUG(
dbgs() << "Is too expensive to expand terminating value for phi node"
<< PN << "\n");
continue;
}

// The candidate IV may have been otherwise dead and poison from the
// very first iteration. If we can't disprove that, we can't use the IV.
if (!mustExecuteUBIfPoisonOnPathTo(&PN, LoopLatch->getTerminator(), &DT)) {
Expand Down Expand Up @@ -6986,7 +6996,7 @@ static bool ReduceLoopStrength(Loop *L, IVUsers &IU, ScalarEvolution &SE,
}();

if (EnableFormTerm) {
if (auto Opt = canFoldTermCondOfLoop(L, SE, DT, LI)) {
if (auto Opt = canFoldTermCondOfLoop(L, SE, DT, LI, TTI)) {
auto [ToFold, ToHelpFold, TermValueS, MustDrop] = *Opt;

Changed = true;
Expand Down
19 changes: 4 additions & 15 deletions llvm/test/Transforms/LoopStrengthReduce/lsr-term-fold.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -599,26 +599,15 @@ define void @expensive_expand_unknown_tc2(ptr %a, i32 %offset, i32 %n, i32 %step
; CHECK-NEXT: entry:
; CHECK-NEXT: [[OFFSET_NONZERO:%.*]] = or i32 [[OFFSET:%.*]], 1
; CHECK-NEXT: [[UGLYGEP:%.*]] = getelementptr i8, ptr [[A:%.*]], i64 84
; CHECK-NEXT: [[SMAX:%.*]] = call i32 @llvm.smax.i32(i32 [[STEP:%.*]], i32 [[N:%.*]])
; CHECK-NEXT: [[TMP0:%.*]] = sub i32 [[SMAX]], [[STEP]]
; CHECK-NEXT: [[UMIN:%.*]] = call i32 @llvm.umin.i32(i32 [[TMP0]], i32 1)
; CHECK-NEXT: [[TMP1:%.*]] = sub i32 [[TMP0]], [[UMIN]]
; CHECK-NEXT: [[UMAX:%.*]] = call i32 @llvm.umax.i32(i32 [[STEP]], i32 1)
; CHECK-NEXT: [[TMP2:%.*]] = udiv i32 [[TMP1]], [[UMAX]]
; CHECK-NEXT: [[TMP3:%.*]] = add i32 [[UMIN]], [[TMP2]]
; CHECK-NEXT: [[TMP4:%.*]] = zext i32 [[TMP3]] to i64
; CHECK-NEXT: [[TMP5:%.*]] = add nuw nsw i64 [[TMP4]], 1
; CHECK-NEXT: [[TMP6:%.*]] = sext i32 [[OFFSET_NONZERO]] to i64
; CHECK-NEXT: [[TMP7:%.*]] = mul i64 [[TMP5]], [[TMP6]]
; CHECK-NEXT: [[TMP8:%.*]] = add nsw i64 [[TMP7]], 84
; CHECK-NEXT: [[SCEVGEP:%.*]] = getelementptr i8, ptr [[A]], i64 [[TMP8]]
; CHECK-NEXT: br label [[FOR_BODY:%.*]]
; CHECK: for.body:
; CHECK-NEXT: [[LSR_IV1:%.*]] = phi ptr [ [[UGLYGEP2:%.*]], [[FOR_BODY]] ], [ [[UGLYGEP]], [[ENTRY:%.*]] ]
; CHECK-NEXT: [[LSR_IV:%.*]] = phi i32 [ [[LSR_IV_NEXT:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY]] ]
; CHECK-NEXT: store i32 1, ptr [[LSR_IV1]], align 4
; CHECK-NEXT: [[LSR_IV_NEXT]] = add nsw i32 [[LSR_IV]], [[STEP:%.*]]
; CHECK-NEXT: [[UGLYGEP2]] = getelementptr i8, ptr [[LSR_IV1]], i32 [[OFFSET_NONZERO]]
; CHECK-NEXT: [[LSR_FOLD_TERM_COND_REPLACED_TERM_COND:%.*]] = icmp eq ptr [[UGLYGEP2]], [[SCEVGEP]]
; CHECK-NEXT: br i1 [[LSR_FOLD_TERM_COND_REPLACED_TERM_COND]], label [[FOR_END:%.*]], label [[FOR_BODY]]
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp sge i32 [[LSR_IV_NEXT]], [[N:%.*]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label [[FOR_END:%.*]], label [[FOR_BODY]]
; CHECK: for.end:
; CHECK-NEXT: ret void
;
Expand Down