[InstCombine] Avoid infinite loop in foldSelectValueEquivalence
#142754
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This was referenced Jun 4, 2025
Member
Author
|
No real-world impact :) |
Member
Author
llvmbot
added
llvm:instcombine
Member
|
@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
+}
|
Collaborator
|
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 |
Collaborator
|
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 |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
4 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
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.