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[LoopPeel] Remove known trip count restriction when peeling last. #140792

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merged 3 commits into from
May 26, 2025
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[LoopPeel] Remove known trip count restriction when peeling last. #140792

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c651b2c
[LoopPeel] Remove known trip count restriction when peeling last.
fhahn May 19, 2025
5ccea79
!fixup address comments, thanks
fhahn May 21, 2025
756e563
!fixup fix formatting
fhahn May 23, 2025
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3 changes: 2 additions & 1 deletion llvm/include/llvm/Transforms/Utils/LoopPeel.h
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Expand Up @@ -44,7 +44,8 @@ gatherPeelingPreferences(Loop *L, ScalarEvolution &SE,
void computePeelCount(Loop *L, unsigned LoopSize,
TargetTransformInfo::PeelingPreferences &PP,
unsigned TripCount, DominatorTree &DT,
ScalarEvolution &SE, AssumptionCache *AC = nullptr,
ScalarEvolution &SE, const TargetTransformInfo &TTI,
AssumptionCache *AC = nullptr,
unsigned Threshold = UINT_MAX);

} // end namespace llvm
Expand Down
2 changes: 1 addition & 1 deletion llvm/lib/Transforms/Scalar/LoopUnrollPass.cpp
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Expand Up @@ -1014,7 +1014,7 @@ bool llvm::computeUnrollCount(
}

// 5th priority is loop peeling.
computePeelCount(L, LoopSize, PP, TripCount, DT, SE, AC, UP.Threshold);
computePeelCount(L, LoopSize, PP, TripCount, DT, SE, TTI, AC, UP.Threshold);
if (PP.PeelCount) {
UP.Runtime = false;
UP.Count = 1;
Expand Down
82 changes: 61 additions & 21 deletions llvm/lib/Transforms/Utils/LoopPeel.cpp
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Expand Up @@ -38,6 +38,7 @@
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/LoopSimplify.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
Expand Down Expand Up @@ -330,11 +331,7 @@ static unsigned peelToTurnInvariantLoadsDerefencebale(Loop &L,

bool llvm::canPeelLastIteration(const Loop &L, ScalarEvolution &SE) {
const SCEV *BTC = SE.getBackedgeTakenCount(&L);
// The loop must execute at least 2 iterations to guarantee that peeled
// iteration executes.
// TODO: Add checks during codegen.
if (isa<SCEVCouldNotCompute>(BTC) ||
!SE.isKnownPredicate(CmpInst::ICMP_UGT, BTC, SE.getZero(BTC->getType())))
if (isa<SCEVCouldNotCompute>(BTC))
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@efriedma-quic efriedma-quic May 20, 2025

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Should we be checking isHighCostExpansion() or something like that somewhere?

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@fhahn fhahn May 21, 2025

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Yep, updated thanks!

return false;

// Check if the exit condition of the loop can be adjusted by the peeling
Expand Down Expand Up @@ -364,12 +361,18 @@ bool llvm::canPeelLastIteration(const Loop &L, ScalarEvolution &SE) {
/// is known at the second-to-last.
static bool shouldPeelLastIteration(Loop &L, CmpPredicate Pred,
const SCEVAddRecExpr *LeftAR,
const SCEV *RightSCEV,
ScalarEvolution &SE) {
const SCEV *RightSCEV, ScalarEvolution &SE,
const TargetTransformInfo &TTI) {
if (!canPeelLastIteration(L, SE))
return false;

const SCEV *BTC = SE.getBackedgeTakenCount(&L);
SCEVExpander Expander(SE, L.getHeader()->getDataLayout(), "loop-peel");
if (!SE.isKnownNonZero(BTC) &&
Expander.isHighCostExpansion(BTC, &L, SCEVCheapExpansionBudget, &TTI,
L.getLoopPredecessor()->getTerminator()))
return false;

const SCEV *ValAtLastIter = LeftAR->evaluateAtIteration(BTC, SE);
const SCEV *ValAtSecondToLastIter = LeftAR->evaluateAtIteration(
SE.getMinusSCEV(BTC, SE.getOne(BTC->getType())), SE);
Expand All @@ -391,7 +394,8 @@ static bool shouldPeelLastIteration(Loop &L, CmpPredicate Pred,
// ..
// }
static std::pair<unsigned, unsigned>
countToEliminateCompares(Loop &L, unsigned MaxPeelCount, ScalarEvolution &SE) {
countToEliminateCompares(Loop &L, unsigned MaxPeelCount, ScalarEvolution &SE,
const TargetTransformInfo &TTI) {
assert(L.isLoopSimplifyForm() && "Loop needs to be in loop simplify form");
unsigned DesiredPeelCount = 0;
unsigned DesiredPeelCountLast = 0;
Expand Down Expand Up @@ -479,7 +483,7 @@ countToEliminateCompares(Loop &L, unsigned MaxPeelCount, ScalarEvolution &SE) {
const SCEV *Step = LeftAR->getStepRecurrence(SE);
if (!PeelWhilePredicateIsKnown(NewPeelCount, IterVal, RightSCEV, Step,
Pred)) {
if (shouldPeelLastIteration(L, Pred, LeftAR, RightSCEV, SE))
if (shouldPeelLastIteration(L, Pred, LeftAR, RightSCEV, SE, TTI))
DesiredPeelCountLast = 1;
return;
}
Expand Down Expand Up @@ -593,8 +597,8 @@ static bool violatesLegacyMultiExitLoopCheck(Loop *L) {
void llvm::computePeelCount(Loop *L, unsigned LoopSize,
TargetTransformInfo::PeelingPreferences &PP,
unsigned TripCount, DominatorTree &DT,
ScalarEvolution &SE, AssumptionCache *AC,
unsigned Threshold) {
ScalarEvolution &SE, const TargetTransformInfo &TTI,
AssumptionCache *AC, unsigned Threshold) {
assert(LoopSize > 0 && "Zero loop size is not allowed!");
// Save the PP.PeelCount value set by the target in
// TTI.getPeelingPreferences or by the flag -unroll-peel-count.
Expand Down Expand Up @@ -656,7 +660,7 @@ void llvm::computePeelCount(Loop *L, unsigned LoopSize,
}

const auto &[CountToEliminateCmps, CountToEliminateCmpsLast] =
countToEliminateCompares(*L, MaxPeelCount, SE);
countToEliminateCompares(*L, MaxPeelCount, SE, TTI);
DesiredPeelCount = std::max(DesiredPeelCount, CountToEliminateCmps);

if (DesiredPeelCount == 0)
Expand Down Expand Up @@ -822,7 +826,7 @@ static void initBranchWeights(DenseMap<Instruction *, WeightInfo> &WeightInfos,
/// instructions in the last peeled-off iteration.
static void cloneLoopBlocks(
Loop *L, unsigned IterNumber, bool PeelLast, BasicBlock *InsertTop,
BasicBlock *InsertBot,
BasicBlock *InsertBot, BasicBlock *OrigPreHeader,
SmallVectorImpl<std::pair<BasicBlock *, BasicBlock *>> &ExitEdges,
SmallVectorImpl<BasicBlock *> &NewBlocks, LoopBlocksDFS &LoopBlocks,
ValueToValueMapTy &VMap, ValueToValueMapTy &LVMap, DominatorTree *DT,
Expand Down Expand Up @@ -914,12 +918,22 @@ static void cloneLoopBlocks(
// loop iteration. Since this copy is no longer part of the loop, we
// resolve this statically:
if (PeelLast) {
// For the last iteration, we use the value from the latch of the original
// loop directly.
// For the last iteration, we introduce new phis for each header phi in
// InsertTop, using the incoming value from the preheader for the original
// preheader (when skipping the main loop) and the incoming value from the
// latch for the latch (when continuing from the main loop).
IRBuilder<> B(InsertTop->getTerminator());
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
PHINode *NewPHI = cast<PHINode>(VMap[&*I]);
VMap[&*I] = NewPHI->getIncomingValueForBlock(Latch);
PHINode *PN = B.CreatePHI(NewPHI->getType(), 2);
NewPHI->eraseFromParent();
if (OrigPreHeader)
PN->addIncoming(cast<PHINode>(&*I)->getIncomingValueForBlock(PreHeader),
OrigPreHeader);

PN->addIncoming(cast<PHINode>(&*I)->getIncomingValueForBlock(Latch),
Latch);
VMap[&*I] = PN;
}
} else {
// For the first iteration, we use the value from the preheader directly.
Expand Down Expand Up @@ -1053,7 +1067,7 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, bool PeelLast, LoopInfo *LI,
// Set up all the necessary basic blocks.
BasicBlock *InsertTop;
BasicBlock *InsertBot;
BasicBlock *NewPreHeader;
BasicBlock *NewPreHeader = nullptr;
DenseMap<Instruction *, Value *> ExitValues;
if (PeelLast) {
// It is convenient to split the single exit block from the latch the
Expand Down Expand Up @@ -1084,11 +1098,34 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, bool PeelLast, LoopInfo *LI,
for (PHINode &P : Exit->phis())
ExitValues[&P] = P.getIncomingValueForBlock(Latch);

const SCEV *BTC = SE->getBackedgeTakenCount(L);

InsertTop = SplitEdge(Latch, Exit, &DT, LI);
InsertBot = SplitBlock(InsertTop, InsertTop->getTerminator(), &DT, LI);

InsertTop->setName(Exit->getName() + ".peel.begin");
InsertBot->setName(Exit->getName() + ".peel.next");
NewPreHeader = nullptr;

// If the original loop may only execute a single iteration we need to
// insert a trip count check and skip the original loop with the last
// iteration peeled off if necessary.
if (!SE->isKnownNonZero(BTC)) {
NewPreHeader = SplitEdge(PreHeader, Header, &DT, LI);
SCEVExpander Expander(*SE, Latch->getDataLayout(), "loop-peel");

BranchInst *PreHeaderBR = cast<BranchInst>(PreHeader->getTerminator());
Value *BTCValue =
Expander.expandCodeFor(BTC, BTC->getType(), PreHeaderBR);
IRBuilder<> B(PreHeaderBR);
Value *Cond =
B.CreateICmpNE(BTCValue, ConstantInt::get(BTCValue->getType(), 0));
B.CreateCondBr(Cond, NewPreHeader, InsertTop);
PreHeaderBR->eraseFromParent();

// PreHeader now dominates InsertTop.
DT.changeImmediateDominator(InsertTop, PreHeader);
}
} else {
// It is convenient to split the preheader into 3 parts - two blocks to
// anchor the peeled copy of the loop body, and a new preheader for the
Expand Down Expand Up @@ -1162,8 +1199,9 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, bool PeelLast, LoopInfo *LI,
for (unsigned Iter = 0; Iter < PeelCount; ++Iter) {
SmallVector<BasicBlock *, 8> NewBlocks;

cloneLoopBlocks(L, Iter, PeelLast, InsertTop, InsertBot, ExitEdges,
NewBlocks, LoopBlocks, VMap, LVMap, &DT, LI,
cloneLoopBlocks(L, Iter, PeelLast, InsertTop, InsertBot,
NewPreHeader ? PreHeader : nullptr, ExitEdges, NewBlocks,
LoopBlocks, VMap, LVMap, &DT, LI,
LoopLocalNoAliasDeclScopes, *SE);

// Remap to use values from the current iteration instead of the
Expand Down Expand Up @@ -1216,9 +1254,11 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, bool PeelLast, LoopInfo *LI,

if (PeelLast) {
// Now adjust users of the original exit values by replacing them with the
// exit value from the peeled iteration.
for (const auto &[P, E] : ExitValues)
// exit value from the peeled iteration and remove them.
for (const auto &[P, E] : ExitValues) {
P->replaceAllUsesWith(isa<Constant>(E) ? E : &*VMap.lookup(E));
P->eraseFromParent();
}
formLCSSA(*L, DT, LI, SE);
} else {
// Now adjust the phi nodes in the loop header to get their initial values
Expand Down
38 changes: 33 additions & 5 deletions llvm/test/Transforms/LoopUnroll/peel-last-iteration-expansion-cost.ll
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Expand Up @@ -25,17 +25,41 @@ define i32 @test_expansion_cost_2(i32 %start, i32 %end) {
; BUDGET3-SAME: i32 [[START:%.*]], i32 [[END:%.*]]) {
; BUDGET3-NEXT: [[ENTRY:.*]]:
; BUDGET3-NEXT: [[SUB:%.*]] = add i32 [[END]], -1
; BUDGET3-NEXT: [[TMP0:%.*]] = sub i32 [[SUB]], [[START]]
; BUDGET3-NEXT: [[TMP1:%.*]] = icmp ne i32 [[TMP0]], 0
; BUDGET3-NEXT: br i1 [[TMP1]], label %[[ENTRY_SPLIT:.*]], label %[[EXIT_PEEL_BEGIN:.*]]
; BUDGET3: [[ENTRY_SPLIT]]:
; BUDGET3-NEXT: br label %[[LOOP_HEADER:.*]]
; BUDGET3: [[LOOP_HEADER]]:
; BUDGET3-NEXT: [[TMP3:%.*]] = phi i32 [ [[START]], %[[ENTRY]] ], [ [[IV_NEXT_PEEL:%.*]], %[[LOOP_LATCH:.*]] ]
; BUDGET3-NEXT: [[C_PEEL:%.*]] = icmp eq i32 [[TMP3]], [[SUB]]
; BUDGET3-NEXT: br i1 [[C_PEEL]], label %[[THEN:.*]], label %[[LOOP_LATCH]]
; BUDGET3-NEXT: [[IV:%.*]] = phi i32 [ [[START]], %[[ENTRY_SPLIT]] ], [ [[IV_NEXT:%.*]], %[[LOOP_LATCH:.*]] ]
; BUDGET3-NEXT: [[C:%.*]] = icmp eq i32 [[IV]], [[SUB]]
; BUDGET3-NEXT: br i1 [[C]], label %[[THEN:.*]], label %[[LOOP_LATCH]]
; BUDGET3: [[THEN]]:
; BUDGET3-NEXT: br label %[[LOOP_LATCH]]
; BUDGET3: [[LOOP_LATCH]]:
; BUDGET3-NEXT: [[IV_NEXT_PEEL]] = add nsw i32 [[TMP3]], 1
; BUDGET3-NEXT: [[IV_NEXT]] = add nsw i32 [[IV]], 1
; BUDGET3-NEXT: [[TMP2:%.*]] = sub i32 [[END]], 1
; BUDGET3-NEXT: [[EC:%.*]] = icmp eq i32 [[IV_NEXT]], [[TMP2]]
; BUDGET3-NEXT: br i1 [[EC]], label %[[EXIT_PEEL_BEGIN_LOOPEXIT:.*]], label %[[LOOP_HEADER]], !llvm.loop [[LOOP0:![0-9]+]]
; BUDGET3: [[EXIT_PEEL_BEGIN_LOOPEXIT]]:
; BUDGET3-NEXT: [[DOTPH:%.*]] = phi i32 [ [[IV_NEXT]], %[[LOOP_LATCH]] ]
; BUDGET3-NEXT: br label %[[EXIT_PEEL_BEGIN]]
; BUDGET3: [[EXIT_PEEL_BEGIN]]:
; BUDGET3-NEXT: [[TMP3:%.*]] = phi i32 [ [[START]], %[[ENTRY]] ], [ [[DOTPH]], %[[EXIT_PEEL_BEGIN_LOOPEXIT]] ]
; BUDGET3-NEXT: br label %[[LOOP_HEADER_PEEL:.*]]
; BUDGET3: [[LOOP_HEADER_PEEL]]:
; BUDGET3-NEXT: [[C_PEEL:%.*]] = icmp eq i32 [[TMP3]], [[SUB]]
; BUDGET3-NEXT: br i1 [[C_PEEL]], label %[[THEN_PEEL:.*]], label %[[LOOP_LATCH_PEEL:.*]]
; BUDGET3: [[THEN_PEEL]]:
; BUDGET3-NEXT: br label %[[LOOP_LATCH_PEEL]]
; BUDGET3: [[LOOP_LATCH_PEEL]]:
; BUDGET3-NEXT: [[IV_NEXT_PEEL:%.*]] = add nsw i32 [[TMP3]], 1
; BUDGET3-NEXT: [[EC_PEEL:%.*]] = icmp eq i32 [[IV_NEXT_PEEL]], [[END]]
; BUDGET3-NEXT: br i1 [[EC_PEEL]], label %[[EXIT:.*]], label %[[LOOP_HEADER]]
; BUDGET3-NEXT: br i1 [[EC_PEEL]], label %[[EXIT_PEEL_NEXT:.*]], label %[[EXIT_PEEL_NEXT]]
; BUDGET3: [[EXIT_PEEL_NEXT]]:
; BUDGET3-NEXT: br label %[[LOOP_HEADER_PEEL_NEXT:.*]]
; BUDGET3: [[LOOP_HEADER_PEEL_NEXT]]:
; BUDGET3-NEXT: br label %[[EXIT:.*]]
; BUDGET3: [[EXIT]]:
; BUDGET3-NEXT: ret i32 0
;
Expand All @@ -59,3 +83,7 @@ loop.latch:
exit:
ret i32 0
}
;.
; BUDGET3: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]]}
; BUDGET3: [[META1]] = !{!"llvm.loop.peeled.count", i32 1}
;.
8 changes: 1 addition & 7 deletions llvm/test/Transforms/LoopUnroll/peel-last-iteration-with-constant-trip-count.ll
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Expand Up @@ -13,7 +13,6 @@ define i64 @peel_single_block_loop_iv_step_1() {
; CHECK-NEXT: br i1 [[EC1]], label %[[LOOP]], label %[[EXIT_PEEL_BEGIN:.*]], !llvm.loop [[LOOP0:![0-9]+]]
; CHECK: [[EXIT_PEEL_BEGIN]]:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ [[IV_NEXT1]], %[[LOOP]] ]
; CHECK-NEXT: [[IV_LCSSA:%.*]] = phi i64 [ [[IV1]], %[[LOOP]] ]
; CHECK-NEXT: br label %[[LOOP_PEEL:.*]]
; CHECK: [[LOOP_PEEL]]:
; CHECK-NEXT: [[CMP18_NOT:%.*]] = icmp eq i64 [[IV]], 63
Expand Down Expand Up @@ -92,7 +91,6 @@ define i64 @peel_single_block_loop_iv_step_1_eq_pred() {
; CHECK-NEXT: br i1 [[CMP_PEEL]], label %[[EXIT_PEEL_BEGIN:.*]], label %[[LOOP]], !llvm.loop [[LOOP2:![0-9]+]]
; CHECK: [[EXIT_PEEL_BEGIN]]:
; CHECK-NEXT: [[IV_NEXT_LCSSA:%.*]] = phi i64 [ [[IV_LCSSA]], %[[LOOP]] ]
; CHECK-NEXT: [[IV_LCSSA1:%.*]] = phi i64 [ [[IV]], %[[LOOP]] ]
; CHECK-NEXT: br label %[[LOOP_PEEL:.*]]
; CHECK: [[LOOP_PEEL]]:
; CHECK-NEXT: [[CMP_PEEL1:%.*]] = icmp eq i64 [[IV_NEXT_LCSSA]], 63
Expand Down Expand Up @@ -171,7 +169,6 @@ define i64 @peel_single_block_loop_iv_step_1_nested_loop() {
; CHECK-NEXT: br i1 [[EC]], label %[[LOOP]], label %[[OUTER_LATCH_PEEL_BEGIN:.*]], !llvm.loop [[LOOP3:![0-9]+]]
; CHECK: [[OUTER_LATCH_PEEL_BEGIN]]:
; CHECK-NEXT: [[IV_NEXT_LCSSA:%.*]] = phi i64 [ [[IV_NEXT]], %[[LOOP]] ]
; CHECK-NEXT: [[IV_LCSSA:%.*]] = phi i64 [ [[IV]], %[[LOOP]] ]
; CHECK-NEXT: br label %[[LOOP_PEEL:.*]]
; CHECK: [[LOOP_PEEL]]:
; CHECK-NEXT: [[CMP_PEEL:%.*]] = icmp eq i64 [[IV_NEXT_LCSSA]], 63
Expand Down Expand Up @@ -237,7 +234,6 @@ define i64 @peel_multi_block_loop_iv_step_1() {
; CHECK-NEXT: br i1 [[EC]], label %[[LOOP]], label %[[EXIT_PEEL_BEGIN:.*]], !llvm.loop [[LOOP4:![0-9]+]]
; CHECK: [[EXIT_PEEL_BEGIN]]:
; CHECK-NEXT: [[IV_NEXT_LCSSA:%.*]] = phi i64 [ [[IV_NEXT]], %[[LATCH]] ]
; CHECK-NEXT: [[IV_LCSSA:%.*]] = phi i64 [ [[IV]], %[[LATCH]] ]
; CHECK-NEXT: br label %[[LOOP_PEEL:.*]]
; CHECK: [[LOOP_PEEL]]:
; CHECK-NEXT: [[CMP_PEEL:%.*]] = icmp eq i64 [[IV_NEXT_LCSSA]], 63
Expand Down Expand Up @@ -365,7 +361,6 @@ define i64 @peel_single_block_loop_iv_step_1_btc_1() {
; CHECK-NEXT: br i1 false, label %[[LOOP]], label %[[EXIT_PEEL_BEGIN:.*]], !llvm.loop [[LOOP5:![0-9]+]]
; CHECK: [[EXIT_PEEL_BEGIN]]:
; CHECK-NEXT: [[IV:%.*]] = phi i64 [ [[IV_NEXT1]], %[[LOOP]] ]
; CHECK-NEXT: [[IV_LCSSA:%.*]] = phi i64 [ [[IV1]], %[[LOOP]] ]
; CHECK-NEXT: br label %[[LOOP_PEEL:.*]]
; CHECK: [[LOOP_PEEL]]:
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i64 [[IV]], 1
Expand Down Expand Up @@ -483,9 +478,8 @@ define i32 @peel_loop_with_branch_and_phi_uses(ptr %x, i1 %c) {
; CHECK-NEXT: [[EC1:%.*]] = icmp ne i32 [[IV_NEXT1]], 99
; CHECK-NEXT: br i1 [[EC1]], label %[[LOOP_HEADER]], label %[[LOOPEXIT_PEEL_BEGIN:.*]], !llvm.loop [[LOOP6:![0-9]+]]
; CHECK: [[LOOPEXIT_PEEL_BEGIN]]:
; CHECK-NEXT: [[RED:%.*]] = phi i32 [ [[ADD1]], %[[LOOP_LATCH]] ]
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ [[IV_NEXT1]], %[[LOOP_LATCH]] ]
; CHECK-NEXT: [[ADD_LCSSA:%.*]] = phi i32 [ [[ADD1]], %[[LOOP_LATCH]] ]
; CHECK-NEXT: [[RED:%.*]] = phi i32 [ [[ADD1]], %[[LOOP_LATCH]] ]
; CHECK-NEXT: br label %[[LOOP_HEADER_PEEL:.*]]
; CHECK: [[LOOP_HEADER_PEEL]]:
; CHECK-NEXT: [[CMP1:%.*]] = icmp eq i32 [[IV]], 99
Expand Down
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