[clang] [Clang] Add fake use emission to Clang with -fextend-lifetimes (PR #110102)

[Stephen Tozer via cfe-commits]

================
@@ -3595,15 +3595,26 @@ static llvm::StoreInst *findDominatingStoreToReturnValue(CodeGenFunction &CGF) {
     llvm::BasicBlock *IP = CGF.Builder.GetInsertBlock();
     if (IP->empty()) return nullptr;
 
-    // Look at directly preceding instruction, skipping bitcasts and lifetime
-    // markers.
+    // Look at directly preceding instruction, skipping bitcasts, lifetime
+    // markers, and fake uses and their operands.
+    const llvm::Instruction *LoadIntoFakeUse = nullptr;
     for (llvm::Instruction &I : make_range(IP->rbegin(), IP->rend())) {
+      // Ignore instructions that are just loads for fake uses; the load should
+      // immediately precede the fake use, so we only need to remember the
+      // operand for the last fake use seen.
+      if (LoadIntoFakeUse == &I)
+        continue;
       if (isa<llvm::BitCastInst>(&I))
         continue;
-      if (auto *II = dyn_cast<llvm::IntrinsicInst>(&I))
+      if (auto *II = dyn_cast<llvm::IntrinsicInst>(&I)) {
         if (II->getIntrinsicID() == llvm::Intrinsic::lifetime_end)
           continue;
 
+        if (II->getIntrinsicID() == llvm::Intrinsic::fake_use) {
+          LoadIntoFakeUse = dyn_cast<llvm::Instruction>(II->getArgOperand(0));
----------------
SLTozer wrote:

All fake uses at this stage have a load (which should be the immediately preceding instruction) as their operand, since fake uses are implemented as cleanups for stack variables. A check that it's a load could be added here, but if anything I'd say an assert would be preferred, since if the above fact ever changes this will need to be revisited.

https://github.com/llvm/llvm-project/pull/110102