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

[Florian Mayer via llvm-commits]

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@@ -1840,6 +1865,34 @@ bool IndVarSimplify::predicateLoopExits(Loop *L, SCEVExpander &Rewriter) {
     const SCEV *ExitCount = SE->getExitCount(L, ExitingBB);
 
     auto *BI = cast<BranchInst>(ExitingBB->getTerminator());
+    if (HasLocalSideEffects) {
+      BasicBlock *Unreachable = nullptr;
+      BasicBlock *InLoop = nullptr;
+      for (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)
+        return Changed;
+      if (llvm::any_of(*Unreachable, [](Instruction &I) {
+            if (auto *II = dyn_cast<IntrinsicInst>(&I)) {
+              if (II->getIntrinsicID() != Intrinsic::trap &&
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fmayer wrote:

> The general idea here is useful also for libcxx hardening, rust bounds checks, etc.

AFAICT, libcxx hardening just calls `llvm.trap()` in the failure path for non-debug mode, so this patch should work as is for that.

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