[llvm] [VectorCombine] Fix trunc generated between PHINodes (PR #108228)

[Nikita Popov via llvm-commits]

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@@ -2652,8 +2652,12 @@ bool VectorCombine::shrinkType(llvm::Instruction &I) {
     return false;
 
   Value *Op0 = ZExted;
-  if (auto *OI = dyn_cast<Instruction>(OtherOperand))
-    Builder.SetInsertPoint(OI->getNextNode());
+  if (auto *OI = dyn_cast<Instruction>(OtherOperand)) {
+    if (isa<PHINode>(OI))
+      Builder.SetInsertPoint(OI->getParent()->getFirstInsertionPt());
+    else
+      Builder.SetInsertPoint(OI->getNextNode());
+  }
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nikic wrote:

You need to use getInsertionPointAfterDef() to handle all the edge cases correctly (I'd expect you will handle invoke and catchswitch incorrectly right now). Or even better yet, just don't do this and create all the instructions at `&I`. From a cursory look I don't see a need to create this instruction in a different place.

https://github.com/llvm/llvm-project/pull/108228