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

[Jeremy Morse via cfe-commits]

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@@ -1664,6 +1710,17 @@ CodeGenFunction::EmitAutoVarAlloca(const VarDecl &D) {
                                          emission.getOriginalAllocatedAddress(),
                                          emission.getSizeForLifetimeMarkers());
 
+  // Analogous to lifetime markers, we use a 'cleanup' to emit fake.use
+  // calls for local variables. We are exempting volatile variables and
+  // non-scalars larger than 4 times the size of an unsigned int (32 bytes).
+  // Larger non-scalars are often allocated in memory and may create unnecessary
+  // overhead.
+  if (CGM.getCodeGenOpts().ExtendLifetimes) {
+    if (extendLifetime(getContext(), CurCodeDecl, D, CXXABIThisDecl))
+      EHStack.pushCleanup<FakeUse>(NormalFakeUse,
+                                   emission.getAllocatedAddress());
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jmorse wrote:

I see the nearby EHStack cleanup code uses getOriginalAllocatedAddress -- should we be using that instead? Related comments talk about there being a difference the address and the object; dunno what to make of that, but it's worth checking.

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