[llvm] [DFAJumpThreading] Only unfold select coming from directly where it is defined (PR #70966)

[Nikita Popov via llvm-commits]

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@@ -460,6 +461,11 @@ struct MainSwitch {
 
     BasicBlock *SIBB = SI->getParent();
 
+    // Only fold the select coming from directly where it is defined.
+    PHINode *PHIUser = dyn_cast<PHINode>(SIUse);
+    if (PHIUser && PHIUser->getIncomingBlock(*SI->use_begin()) != SIBB)
+      return false;
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nikic wrote:

There is a `SIBB->getSingleSuccessor() != cast<Instruction>(SIUse)->getParent()` check a few lines below -- can it be dropped now? I think it had the same purpose, but did not account for the loop entry case.

https://github.com/llvm/llvm-project/pull/70966