[PATCH] D133860: StackProtector: enable tail call optimization even without musttail

[Liqiang Tao via Phabricator via llvm-commits]

taolq added a comment.

In D133860#3796726 <https://reviews.llvm.org/D133860#3796726>, @efriedma wrote:

> Do we care that the backend might not choose to tail-call a function even if it's in tail position?  The "tail" marker is a hint; depending on attributes, calling convention, etc., the backend might not actually generate a tail call.

I am not sure about this. Is that possible that a musttail-call is handled but a tail-call is not generated with all the same conditions?



================
Comment at: llvm/lib/CodeGen/StackProtector.cpp:485
       Prev = Prev->getPrevNonDebugInstruction();
-      if (Prev && isa<CallInst>(Prev) && cast<CallInst>(Prev)->isMustTailCall())
+      if (Prev && isa<CallInst>(Prev) && cast<CallInst>(Prev)->isTailCall())
         CheckLoc = Prev;
----------------
efriedma wrote:
> Are you sure this actually does the right thing?  Consider something like:
> 
> ```
> tail call void f()
> call void g()
> ret void
> ```
> 
> If I'm following correctly, we'll insert the guard before the call to f(), which seems wrong.
> 
> (The old code worked because a musttail call is always in tail position; a "tail" call is not.)
Thanks for your comment. I didn't notice that.
So we should remove the latter if condition.
Or let the verifier guarantee a tail call is always in the tail position.
Which one could be better?


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https://reviews.llvm.org/D133860