[llvm-branch-commits] [llvm] [SimplifyCFG] Avoid threading loop-header branches in convergent functions (PR #204958)

[Hongyu Chen via llvm-branch-commits]

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@@ -3713,7 +3713,10 @@ bool SimplifyCFGOpt::foldCondBranchOnValueKnownInPredecessor(CondBrInst *BI) {
   // Note: If BB is a loop header then there is a risk that threading introduces
   // a non-canonical loop by moving a back edge. So we avoid this optimization
   // for loop headers if NeedCanonicalLoop is set.
-  if (Options.NeedCanonicalLoop && is_contained(LoopHeaders, BI->getParent()))
+  // Also avoid threading loop headers in convergent functions, since changing
+  // the branch structure can change the dynamic instances of convergent ops.
+  if ((Options.NeedCanonicalLoop || BI->getFunction()->isConvergent()) &&
+      is_contained(LoopHeaders, BI->getParent()))
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XChy wrote:

AFAIK, JumpThreading performs similar transformations and uses `LoopHeaders` too. But it is just for heuristics.

https://github.com/llvm/llvm-project/pull/204958