[InstCombine] Avoid infinite loop in foldSelectValueEquivalence
#142754
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This was referenced Jun 4, 2025
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No real-world impact :) |
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@llvm/pr-subscribers-llvm-transforms Author: Yingwei Zheng (dtcxzyw) ChangesBefore this patch, InstCombine hung because it replaced a value with a more complex one: This patch makes this replacement more conservative. It only performs the replacement iff the new value is one of the operands of the original value. Closes #142405. Full diff: https://github.com/llvm/llvm-project/pull/142754.diff 2 Files Affected:
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index d7d0431a5b8d0..257dc943bd5c1 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1312,7 +1312,11 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
// If NewOp is a constant and OldOp is not replace iff NewOp doesn't
// contain and undef elements.
- if (match(NewOp, m_ImmConstant()) || NewOp == V) {
+ // Make sure that V is always simpler than TrueVal, otherwise we might
+ // end up in an infinite loop.
+ if (match(NewOp, m_ImmConstant()) ||
+ (isa<Instruction>(TrueVal) &&
+ is_contained(cast<Instruction>(TrueVal)->operands(), V))) {
if (isGuaranteedNotToBeUndef(NewOp, SQ.AC, &Sel, &DT))
return replaceOperand(Sel, Swapped ? 2 : 1, V);
return nullptr;
diff --git a/llvm/test/Transforms/InstCombine/select-cmp-eq-op-fold.ll b/llvm/test/Transforms/InstCombine/select-cmp-eq-op-fold.ll
index 760b1ef055ad3..2e6bbdf5da74d 100644
--- a/llvm/test/Transforms/InstCombine/select-cmp-eq-op-fold.ll
+++ b/llvm/test/Transforms/InstCombine/select-cmp-eq-op-fold.ll
@@ -182,3 +182,23 @@ define i8 @replace_with_y_for_simple_binop_fail(i8 %x, i8 noundef %y, i8 %z, i8
%sel = select i1 %cmp, i8 %mul, i8 %z
ret i8 %sel
}
+
+; Make sure we don't run into an infinite loop.
+define i32 @pr142405(i32 noundef %x) {
+; CHECK-LABEL: @pr142405(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: [[SMAX:%.*]] = call i32 @llvm.smax.i32(i32 [[X:%.*]], i32 0)
+; CHECK-NEXT: [[MASKED:%.*]] = and i32 [[SMAX]], 65535
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], [[MASKED]]
+; CHECK-NEXT: [[TMP0:%.*]] = and i32 [[SMAX]], 1
+; CHECK-NEXT: [[AND:%.*]] = select i1 [[CMP]], i32 [[TMP0]], i32 0
+; CHECK-NEXT: ret i32 [[AND]]
+;
+entry:
+ %smax = call i32 @llvm.smax.i32(i32 %x, i32 0)
+ %masked = and i32 %smax, 65535
+ %cmp = icmp eq i32 %x, %masked
+ %sel = select i1 %cmp, i32 %smax, i32 0
+ %and = and i32 %sel, 1
+ ret i32 %and
+}
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/10/builds/6652 Here is the relevant piece of the build log for the reference |
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/169/builds/11978 Here is the relevant piece of the build log for the reference |
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Before this patch, InstCombine hung because it replaced a value with a more complex one:
This patch makes this replacement more conservative. It only performs the replacement iff the new value is one of the operands of the original value.
Closes #142405.