[llvm] [InstCombine] Prevent crash when folding struct-returning ops into vector selects (PR #173208)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: 陈子昂 (Michael-Chen-NJU)

<details>
<summary>Changes</summary>

This patch adds a safety check to` FoldOpIntoSelect `to prevent an invalid transformation where an operation returning a struct type is folded into a vector select.

LLVM's SelectInst does not support selecting between two struct operands when the condition is a vector. This logic error is currently latent on main because no struct-returning intrinsics are marked as trivially vectorizable.

This was verified using the existing test `llvm/test/Transforms/InstCombine/select_frexp.ll`.

By temporarily marking llvm.frexp as vectorizable (as proposed in #<!-- -->112408), this existing test triggers an assertion failure in SelectInst::Create.

This patch ensures the test passes (by skipping the invalid fold) even when frexp vectorization is enabled.

---
Full diff: https://github.com/llvm/llvm-project/pull/173208.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+3) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 14244236d75d5..a339ada97c7e8 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1782,6 +1782,9 @@ static Value *foldOperationIntoSelectOperand(Instruction &I, SelectInst *SI,
 Instruction *InstCombinerImpl::FoldOpIntoSelect(Instruction &Op, SelectInst *SI,
                                                 bool FoldWithMultiUse,
                                                 bool SimplifyBothArms) {
+  if (SI->getCondition()->getType()->isVectorTy() &&
+      !Op.getType()->isVectorTy())
+    return nullptr;
   // Don't modify shared select instructions unless set FoldWithMultiUse
   if (!SI->hasOneUse() && !FoldWithMultiUse)
     return nullptr;

``````````

</details>


https://github.com/llvm/llvm-project/pull/173208