[llvm] [SCEV] Check whether the start is non-zero in `ScalarEvolution::howFarToZero` (PR #131522)
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Sun Mar 16 07:47:31 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-analysis
Author: Yingwei Zheng (dtcxzyw)
<details>
<summary>Changes</summary>
https://github.com/llvm/llvm-project/pull/94525 assumes that the loop will be infinite when the stride is zero. However, it doesn't hold when the start value of addrec is also zero.
Closes https://github.com/llvm/llvm-project/issues/131465.
---
Full diff: https://github.com/llvm/llvm-project/pull/131522.diff
3 Files Affected:
- (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+5-4)
- (modified) llvm/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll (+26-8)
- (added) llvm/test/Transforms/LoopUnroll/pr131465.ll (+43)
``````````diff
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 8f74c1c398ced..314baa7c7aee1 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10635,10 +10635,11 @@ ScalarEvolution::ExitLimit ScalarEvolution::howFarToZero(const SCEV *V,
if (ControlsOnlyExit && AddRec->hasNoSelfWrap() &&
loopHasNoAbnormalExits(AddRec->getLoop())) {
- // If the stride is zero, the loop must be infinite. In C++, most loops
- // are finite by assumption, in which case the step being zero implies
- // UB must execute if the loop is entered.
- if (!loopIsFiniteByAssumption(L) && !isKnownNonZero(StepWLG))
+ // If the stride is zero and the start is non-zero, the loop must be
+ // infinite. In C++, most loops are finite by assumption, in which case the
+ // step being zero implies UB must execute if the loop is entered.
+ if (!(loopIsFiniteByAssumption(L) && isKnownNonZero(Start)) &&
+ !isKnownNonZero(StepWLG))
return getCouldNotCompute();
const SCEV *Exact =
diff --git a/llvm/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll b/llvm/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll
index 2d02cb6194f4c..1f08a620b2e15 100644
--- a/llvm/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll
+++ b/llvm/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll
@@ -329,10 +329,9 @@ define void @ne_nsw_nonneg_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nsw_nonneg_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nsw_nonneg_step
-; CHECK-NEXT: Loop %for.body: backedge-taken count is (((-1 * %s) + %n) /u %s)
-; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -1
-; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %s) + %n) /u %s)
-; CHECK-NEXT: Loop %for.body: Trip multiple is 1
+; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
+; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
+; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
;
entry:
%nonneg_step = icmp sge i32 %s, 0
@@ -442,10 +441,9 @@ define void @ne_nuw_nonneg_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nuw_nonneg_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nuw_nonneg_step
-; CHECK-NEXT: Loop %for.body: backedge-taken count is (((-1 * %s) + %n) /u %s)
-; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -1
-; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %s) + %n) /u %s)
-; CHECK-NEXT: Loop %for.body: Trip multiple is 1
+; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
+; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
+; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
;
entry:
%nonneg_step = icmp sge i32 %s, 0
@@ -493,6 +491,26 @@ for.end: ; preds = %for.body, %entry
ret void
}
+define i32 @pr131465(i1 %x) mustprogress {
+; CHECK-LABEL: 'pr131465'
+; CHECK-NEXT: Determining loop execution counts for: @pr131465
+; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
+; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
+; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
+;
+entry:
+ %inc = zext i1 %x to i32
+ br label %for.body
+
+for.body:
+ %indvar = phi i32 [ 2, %entry ], [ %next, %for.body ]
+ %next = add nsw i32 %indvar, %inc
+ %exitcond = icmp eq i32 %next, 2
+ br i1 %exitcond, label %for.end, label %for.body
+
+for.end:
+ ret i32 0
+}
declare void @llvm.assume(i1)
diff --git a/llvm/test/Transforms/LoopUnroll/pr131465.ll b/llvm/test/Transforms/LoopUnroll/pr131465.ll
new file mode 100644
index 0000000000000..643b020c6c110
--- /dev/null
+++ b/llvm/test/Transforms/LoopUnroll/pr131465.ll
@@ -0,0 +1,43 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=loop-unroll -unroll-runtime %s | FileCheck %s
+
+define i32 @pr131465(i1 %x) mustprogress {
+; CHECK-LABEL: define i32 @pr131465(
+; CHECK-SAME: i1 [[X:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT: [[ENTRY:.*]]:
+; CHECK-NEXT: [[INC:%.*]] = zext i1 [[X]] to i32
+; CHECK-NEXT: br label %[[FOR_BODY:.*]]
+; CHECK: [[FOR_BODY]]:
+; CHECK-NEXT: [[INDVAR:%.*]] = phi i32 [ 2, %[[ENTRY]] ], [ [[NEXT_1:%.*]], %[[FOR_BODY_1:.*]] ]
+; CHECK-NEXT: [[NEXT:%.*]] = add nsw i32 [[INDVAR]], [[INC]]
+; CHECK-NEXT: [[EXITCOND:%.*]] = icmp eq i32 [[NEXT]], 2
+; CHECK-NEXT: br i1 [[EXITCOND]], label %[[FOR_END:.*]], label %[[FOR_BODY_1]], !llvm.loop [[LOOP0:![0-9]+]]
+; CHECK: [[FOR_BODY_1]]:
+; CHECK-NEXT: [[NEXT_1]] = add nsw i32 [[NEXT]], [[INC]]
+; CHECK-NEXT: [[EXITCOND_1:%.*]] = icmp eq i32 [[NEXT_1]], 2
+; CHECK-NEXT: br i1 [[EXITCOND_1]], label %[[FOR_END]], label %[[FOR_BODY]], !llvm.loop [[LOOP2:![0-9]+]]
+; CHECK: [[FOR_END]]:
+; CHECK-NEXT: ret i32 0
+;
+entry:
+ %inc = zext i1 %x to i32
+ br label %for.body
+
+for.body:
+ %indvar = phi i32 [ 2, %entry ], [ %next, %for.body ]
+ %next = add nsw i32 %indvar, %inc
+ %exitcond = icmp eq i32 %next, 2
+ br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !0
+
+for.end:
+ ret i32 0
+}
+
+; Force runtime unrolling.
+!0 = !{!0, !{!"llvm.loop.unroll.count", i32 2}}
+;.
+; CHECK: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]]}
+; CHECK: [[META1]] = !{!"llvm.loop.unroll.count", i32 2}
+; CHECK: [[LOOP2]] = distinct !{[[LOOP2]], [[META3:![0-9]+]]}
+; CHECK: [[META3]] = !{!"llvm.loop.unroll.disable"}
+;.
``````````
</details>
https://github.com/llvm/llvm-project/pull/131522
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