[llvm] [LoopPeel] Allow bidirection condition change for last iteration peeling (PR #143562)
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Tue Jun 10 08:57:05 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Philip Reames (preames)
<details>
<summary>Changes</summary>
If we're peeling the last iteration, we don't care if the (by assumption monotonic) condition transitions from true to false or false to true. It can still be removed from the main loop by peeling.
There's a possible concern here - the condition doesn't appear to be being discharged in the main loop after peeling. I can't find anything which seems specific to the inverted predicate, so I think this is an existing issue for any reverse peeling, but am I missing something?
---
Full diff: https://github.com/llvm/llvm-project/pull/143562.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/Utils/LoopPeel.cpp (+8-6)
- (modified) llvm/test/Transforms/LoopUnroll/peel-last-iteration-with-variable-trip-count.ll (+27-5)
``````````diff
diff --git a/llvm/lib/Transforms/Utils/LoopPeel.cpp b/llvm/lib/Transforms/Utils/LoopPeel.cpp
index 9149f71941db4..d5d819505226c 100644
--- a/llvm/lib/Transforms/Utils/LoopPeel.cpp
+++ b/llvm/lib/Transforms/Utils/LoopPeel.cpp
@@ -357,9 +357,9 @@ bool llvm::canPeelLastIteration(const Loop &L, ScalarEvolution &SE) {
m_scev_AffineAddRec(m_SCEV(), m_scev_One(), m_SpecificLoop(&L)));
}
-/// Returns true if the last iteration can be peeled off and the condition (Pred
-/// LeftAR, RightSCEV) is known at the last iteration and the inverse condition
-/// is known at the second-to-last.
+/// Returns true if the last iteration can be peeled off because the known
+/// monotonic condition (Pred LeftAR, RightSCEV) changes value on the last
+/// iteration.
static bool shouldPeelLastIteration(Loop &L, CmpPredicate Pred,
const SCEVAddRecExpr *LeftAR,
const SCEV *RightSCEV, ScalarEvolution &SE,
@@ -378,9 +378,11 @@ 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);
+ return (SE.isKnownPredicate(InvPred, ValAtLastIter, RightSCEV) &&
+ SE.isKnownPredicate(Pred, ValAtSecondToLastIter, RightSCEV)) ||
+ (SE.isKnownPredicate(Pred, ValAtLastIter, RightSCEV) &&
+ SE.isKnownPredicate(InvPred, ValAtSecondToLastIter, RightSCEV));
}
// Return the number of iterations to peel off from the beginning and end of the
diff --git a/llvm/test/Transforms/LoopUnroll/peel-last-iteration-with-variable-trip-count.ll b/llvm/test/Transforms/LoopUnroll/peel-last-iteration-with-variable-trip-count.ll
index 6346d7d97e826..592700eda702f 100644
--- a/llvm/test/Transforms/LoopUnroll/peel-last-iteration-with-variable-trip-count.ll
+++ b/llvm/test/Transforms/LoopUnroll/peel-last-iteration-with-variable-trip-count.ll
@@ -139,17 +139,38 @@ define i32 @peel_last_with_trip_count_check_lcssa_phi_cmp_not_invar(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]] ], [ [[IV_NEXT:%.*]], %[[LOOP]] ]
+; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, %[[ENTRY_SPLIT]] ], [ [[IV_NEXT1:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[C:%.*]] = icmp eq i32 [[IV]], [[SUB]]
; CHECK-NEXT: [[SEL:%.*]] = select i1 [[C]], i32 1, i32 2
; CHECK-NEXT: call void @foo(i32 [[SEL]])
-; CHECK-NEXT: [[IV_NEXT]] = add i32 [[IV]], 1
+; 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: [[C_PEEL:%.*]] = icmp eq 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: [[EC:%.*]] = icmp ne i32 [[IV_NEXT]], [[N]]
-; CHECK-NEXT: br i1 [[EC]], label %[[LOOP]], label %[[EXIT:.*]]
+; 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: [[EXIT]]:
-; CHECK-NEXT: [[SEL_LCSSA:%.*]] = phi i32 [ [[SEL]], %[[LOOP]] ]
; CHECK-NEXT: ret i32 [[SEL_LCSSA]]
;
entry:
@@ -214,7 +235,7 @@ 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 [[LOOP2:![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
@@ -242,4 +263,5 @@ inner.latch:
; CHECK: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]]}
; CHECK: [[META1]] = !{!"llvm.loop.peeled.count", i32 1}
; CHECK: [[LOOP2]] = distinct !{[[LOOP2]], [[META1]]}
+; CHECK: [[LOOP3]] = distinct !{[[LOOP3]], [[META1]]}
;.
``````````
</details>
https://github.com/llvm/llvm-project/pull/143562
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