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[LoopPeel] Avoid unprofitable last iteration peeling with equalities #143588

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[LoopPeel] Avoid unprofitable last iteration peeling with equalities #143588

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[LoopPeel] Avoid unprofitable last iteration peeling with equalitis
preames Jun 10, 2025
318af92
Merge branch 'main' into pr-peel-last-equality-edge-case
preames Jun 17, 2025
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20 changes: 17 additions & 3 deletions llvm/lib/Transforms/Utils/LoopPeel.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -381,9 +381,23 @@ static bool shouldPeelLastIteration(Loop &L, CmpPredicate Pred,
const SCEV *ValAtSecondToLastIter = LeftAR->evaluateAtIteration(
SE.getMinusSCEV(BTC, SE.getOne(BTC->getType())), SE);

return SE.isKnownPredicate(ICmpInst::getInversePredicate(Pred), ValAtLastIter,
RightSCEV) &&
SE.isKnownPredicate(Pred, ValAtSecondToLastIter, RightSCEV);
CmpPredicate InvPred = ICmpInst::getInversePredicate(Pred);
if (!SE.isKnownPredicate(InvPred, ValAtLastIter, RightSCEV) ||
!SE.isKnownPredicate(Pred, ValAtSecondToLastIter, RightSCEV))
return false;

// For a monotonic predicate, we've found the single transition point and
// don't need to check further.
if (!ICmpInst::isEquality(Pred))
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Should we use SE.getMonotonicPredicateType additionally here, to handle the case where the AddRec may wrap, which could make something like i < x not motontonic?

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@preames preames Jun 16, 2025 •
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I'm confused. In order to get here, we have to have identified a predicate which is either a nssw equality, or a monotonic predicate. (Along the min/max intrinsic, we check the same conditions, but using different names, etc..). What's the point of adding the additional monotonic check here?

return true;

// For an equality, the condition can toggle twice. For the peeling to be
// profitable, we need to know that this is the first toggle. The two
// toggles are separate by one iteration. If the BTC isn't at least two
// this will overflow, but that's okay correctness wise.
const SCEV *ValAtPriorIter = LeftAR->evaluateAtIteration(
SE.getMinusSCEV(BTC, SE.getConstant(BTC->getType(), 2)), SE);
return SE.isKnownPredicate(Pred, ValAtPriorIter, RightSCEV);
}

// Return the number of iterations to peel off from the beginning and end of the
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36 changes: 7 additions & 29 deletions llvm/test/Transforms/LoopUnroll/peel-last-iteration-with-variable-trip-count.ll
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Expand Up @@ -176,39 +176,18 @@ define i32 @peel_last_with_trip_count_check_lcssa_phi_cmp_not_invar2(i32 %n) {
; CHECK-SAME: i32 [[N:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: [[SUB:%.*]] = add i32 [[N]], -2
; CHECK-NEXT: [[TMP0:%.*]] = add i32 [[N]], -1
; CHECK-NEXT: [[TMP1:%.*]] = icmp ne i32 [[TMP0]], 0
; CHECK-NEXT: br i1 [[TMP1]], label %[[ENTRY_SPLIT:.*]], label %[[EXIT_PEEL_BEGIN:.*]]
; CHECK: [[ENTRY_SPLIT]]:
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, %[[ENTRY_SPLIT]] ], [ [[IV_NEXT1:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[C:%.*]] = icmp ne i32 [[IV]], [[SUB]]
; CHECK-NEXT: [[SEL:%.*]] = select i1 [[C]], i32 1, i32 2
; CHECK-NEXT: call void @foo(i32 [[SEL]])
; CHECK-NEXT: [[IV_NEXT1]] = add nuw i32 [[IV]], 1
; CHECK-NEXT: [[TMP2:%.*]] = sub i32 [[N]], 1
; CHECK-NEXT: [[EC1:%.*]] = icmp ne i32 [[IV_NEXT1]], [[TMP2]]
; CHECK-NEXT: br i1 [[EC1]], label %[[LOOP]], label %[[EXIT_PEEL_BEGIN_LOOPEXIT:.*]], !llvm.loop [[LOOP2:![0-9]+]]
; CHECK: [[EXIT_PEEL_BEGIN_LOOPEXIT]]:
; CHECK-NEXT: [[DOTPH:%.*]] = phi i32 [ [[IV_NEXT1]], %[[LOOP]] ]
; CHECK-NEXT: br label %[[EXIT_PEEL_BEGIN]]
; CHECK: [[EXIT_PEEL_BEGIN]]:
; CHECK-NEXT: [[TMP3:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[DOTPH]], %[[EXIT_PEEL_BEGIN_LOOPEXIT]] ]
; CHECK-NEXT: br label %[[LOOP_PEEL:.*]]
; CHECK: [[LOOP_PEEL]]:
; CHECK-NEXT: [[TMP3:%.*]] = phi i32 [ 0, %[[ENTRY]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[C_PEEL:%.*]] = icmp ne i32 [[TMP3]], [[SUB]]
; CHECK-NEXT: [[SEL_LCSSA:%.*]] = select i1 [[C_PEEL]], i32 1, i32 2
; CHECK-NEXT: call void @foo(i32 [[SEL_LCSSA]])
; CHECK-NEXT: [[IV_NEXT:%.*]] = add i32 [[TMP3]], 1
; CHECK-NEXT: [[IV_NEXT]] = add i32 [[TMP3]], 1
; CHECK-NEXT: [[EC:%.*]] = icmp ne i32 [[IV_NEXT]], [[N]]
; CHECK-NEXT: br i1 [[EC]], label %[[EXIT_PEEL_NEXT:.*]], label %[[EXIT_PEEL_NEXT]]
; CHECK: [[EXIT_PEEL_NEXT]]:
; CHECK-NEXT: br label %[[LOOP_PEEL_NEXT:.*]]
; CHECK: [[LOOP_PEEL_NEXT]]:
; CHECK-NEXT: br label %[[EXIT:.*]]
; CHECK-NEXT: br i1 [[EC]], label %[[LOOP]], label %[[EXIT:.*]]
; CHECK: [[EXIT]]:
; CHECK-NEXT: ret i32 [[SEL_LCSSA]]
; CHECK-NEXT: [[SEL_LCSSA1:%.*]] = phi i32 [ [[SEL_LCSSA]], %[[LOOP]] ]
; CHECK-NEXT: ret i32 [[SEL_LCSSA1]]
;
entry:
%sub = add i32 %n, -2
Expand Down Expand Up @@ -245,7 +224,7 @@ define i32 @peel_last_with_trip_count_check_lcssa_phi_cmp_not_invar3(i32 %n) {
; CHECK-NEXT: [[IV_NEXT]] = add nuw i32 [[IV]], 1
; CHECK-NEXT: [[TMP1:%.*]] = sub i32 [[N]], 1
; CHECK-NEXT: [[EC:%.*]] = icmp ne i32 [[IV_NEXT]], [[TMP1]]
; CHECK-NEXT: br i1 [[EC]], label %[[LOOP]], label %[[EXIT_PEEL_BEGIN_LOOPEXIT:.*]], !llvm.loop [[LOOP3:![0-9]+]]
; CHECK-NEXT: br i1 [[EC]], label %[[LOOP]], label %[[EXIT_PEEL_BEGIN_LOOPEXIT:.*]], !llvm.loop [[LOOP2:![0-9]+]]
; CHECK: [[EXIT_PEEL_BEGIN_LOOPEXIT]]:
; CHECK-NEXT: [[DOTPH:%.*]] = phi i32 [ [[IV_NEXT]], %[[LOOP]] ]
; CHECK-NEXT: br label %[[EXIT_PEEL_BEGIN]]
Expand Down Expand Up @@ -363,7 +342,7 @@ define void @peel_last_with_trip_count_check_nested_loop(i32 %n) {
; CHECK-NEXT: [[IV_NEXT1]] = add nuw i32 [[IV1]], 1
; CHECK-NEXT: [[TMP2:%.*]] = sub i32 [[N]], 1
; CHECK-NEXT: [[EXITCOND_NOT1:%.*]] = icmp eq i32 [[IV_NEXT1]], [[TMP2]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT1]], label %[[OUTER_HEADER_LOOPEXIT_PEEL_BEGIN_LOOPEXIT]], label %[[INNER_HEADER]], !llvm.loop [[LOOP4:![0-9]+]]
; CHECK-NEXT: br i1 [[EXITCOND_NOT1]], label %[[OUTER_HEADER_LOOPEXIT_PEEL_BEGIN_LOOPEXIT]], label %[[INNER_HEADER]], !llvm.loop [[LOOP3:![0-9]+]]
;
entry:
%sub = add i32 %n, -1
Expand Down Expand Up @@ -392,5 +371,4 @@ inner.latch:
; CHECK: [[META1]] = !{!"llvm.loop.peeled.count", i32 1}
; CHECK: [[LOOP2]] = distinct !{[[LOOP2]], [[META1]]}
; CHECK: [[LOOP3]] = distinct !{[[LOOP3]], [[META1]]}
; CHECK: [[LOOP4]] = distinct !{[[LOOP4]], [[META1]]}
;.
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