[llvm] [LoopPeel] LCSSA form is destroyed by LoopPeel, preserve it (PR #78696)

[Nikita Popov via llvm-commits]

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@@ -644,6 +644,13 @@ bool SimplifyIndvar::replaceIVUserWithLoopInvariant(Instruction *I) {
 
   auto *Invariant = Rewriter.expandCodeFor(S, I->getType(), IP);
 
+  if (!LI->replacementPreservesLCSSAForm(I, Invariant)) {
+    LLVM_DEBUG(dbgs() << "INDVARS: Can not replace IV user: " << *I
+                      << " with loop invariant: " << *S
+                      << " as it breaks LCSSA form " << '\n');
+    return false;
+  }
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nikic wrote:

SCEVExpander is also supposed to preserve LCSSA form (if PreserveLCSSA is enabled, which it is by default), so I think the actual fix needs to be in SCEVExpander. It's not possible to abort a transform after using SCEVExpander.

https://github.com/llvm/llvm-project/pull/78696