[llvm] [IndVarSimplify] Allow predicateLoopExit on some loops with thread-local writes (PR #155901)

[Nikita Popov via llvm-commits]

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@@ -1840,6 +1880,23 @@ bool IndVarSimplify::predicateLoopExits(Loop *L, SCEVExpander &Rewriter) {
     const SCEV *ExitCount = SE->getExitCount(L, ExitingBB);
 
     auto *BI = cast<BranchInst>(ExitingBB->getTerminator());
+    if (HasThreadLocalSideEffects) {
+      const BasicBlock *Unreachable = nullptr;
+      const BasicBlock *InLoop = nullptr;
+      for (const BasicBlock *Succ : BI->successors()) {
+        if (isa<UnreachableInst>(Succ->getTerminator()))
+          Unreachable = Succ;
+        else if (L->contains(Succ))
+          InLoop = Succ;
+      }
+      // Exit BB which have one branch back into the loop and another one to
+      // a trap can still be optimized, because local side effects cannot
+      // be observed in the exit case (the trap). We could be smarter about
+      // this, but for now lets pattern match common cases that directly trap.
+      if (Unreachable == nullptr || InLoop == nullptr ||
+          !crashingBBWithoutEffect(*Unreachable))
+        return Changed;
+    }
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nikic wrote:

Can you please add tests to check this does the right thing when there are multiple exits (some of which trap and some of which don't)?

https://github.com/llvm/llvm-project/pull/155901