[clang] [Clang][CodeGen] Emit fake uses before musttail calls (PR #136867)

[John McCall via cfe-commits]

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@@ -6001,8 +6001,18 @@ RValue CodeGenFunction::EmitCall(const CGFunctionInfo &CallInfo,
     for (auto it = EHStack.find(CurrentCleanupScopeDepth); it != EHStack.end();
          ++it) {
       EHCleanupScope *Cleanup = dyn_cast<EHCleanupScope>(&*it);
-      if (!(Cleanup && Cleanup->getCleanup()->isRedundantBeforeReturn()))
+      // Fake uses can be safely emitted immediately prior to the tail call; we
+      // choose to emit the fake use before the call rather than after, to avoid
+      // forcing variable values from every call on the "stack" to be preserved
+      // simultaneously.
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rjmccall wrote:

The second half of this comment is odd. There isn't really a choice about this; the fake uses must be emitted prior to the tail call because there cannot be anything after it.

https://github.com/llvm/llvm-project/pull/136867