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

[Paul T Robinson via cfe-commits]

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@@ -1353,6 +1353,19 @@ void CodeGenFunction::EmitLifetimeEnd(llvm::Value *Size, llvm::Value *Addr) {
   C->setDoesNotThrow();
 }
 
+void CodeGenFunction::EmitFakeUse(Address Addr) {
+  // We do not emit a fake use if we want to apply optnone to this function,
+  // even if we might not apply it anyway due to minsize or similar attributes.
+  if (!CGM.getCodeGenOpts().DisableO0ImplyOptNone &&
+      CGM.getCodeGenOpts().OptimizationLevel == 0)
+    return;
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pogo59 wrote:

There's no value to enabling `-fextend-lifetimes` at O0, because we only emit stack-home locations at O0, which are valid for the entire function. However, I don't remember whether that decision is a blanket decision based on opt level or per-function based on optnone. If it's based on opt level, then adding fake-uses to an optnone function (at O1 and up) might actually be the right thing to do.

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