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

[Nikita Popov via llvm-commits]

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@@ -1704,6 +1709,27 @@ bool IndVarSimplify::optimizeLoopExits(Loop *L, SCEVExpander &Rewriter) {
   return Changed;
 }
 
+static bool crashingBBWithoutEffect(const BasicBlock &BB) {
+  return llvm::all_of(BB, [](const Instruction &I) {
+    // TODO: for now this is overly restrictive, to make sure nothing in this
+    // BB can depend on the loop body.
+    // It's not enough to check for !I.mayHaveSideEffects(), because e.g. a
+    // load does not have a side effect, but we could have
+    // %a = load ptr, ptr %ptr
+    // %b = load i32, ptr %a
+    // Now if the loop stored a non-nullptr to %a, we could cause a nullptr
+    // dereference by skipping over loop iterations.
+    if (const auto *CB = dyn_cast<CallBase>(&I)) {
+      if (CB->onlyAccessesInaccessibleMemory() &&
+          llvm::all_of(CB->args(), [](const llvm::Use &U) {
+            return isa<Constant>(U.get());
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nikic wrote:

Why do we care about constant arguments?

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