commit 5e2dcfe42cd4af14d6e6155314aa5d4167710b65
author Yingwei Zheng <dtcxzyw2333@gmail.com> Wed Jun 04 19:42:56 2025 +0800
committer GitHub <noreply@github.com> Wed Jun 04 19:42:56 2025 +0800
tree 4ee649940450287fd670db0092cf2fd8e23f8f9c
parent 20d70196c9a4da344d0944f3c78447c3bd7079c7

[InstCombine] Avoid infinite loop in `foldSelectValueEquivalence` (#142754)

Before this patch, InstCombine hung because it replaced a value with a
more complex one:
```
%sel = select i1 %cmp, i32 %smax, i32 0 ->
%sel = select i1 %cmp, i32 %masked, i32 0 ->
%sel = select i1 %cmp, i32 %smax, i32 0 ->
...
```
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 https://github.com/llvm/llvm-project/issues/142405.

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index d7d0431..257dc94 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1312,7 +1312,11 @@
 
       // 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;

llvm/test/Transforms/InstCombine/select-cmp-eq-op-fold.ll

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 760b1ef..2e6bbdf 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 @@
   %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
+}