[PATCH] D20352: Add XRay flags to Clang

[Aaron Ballman via cfe-commits]

aaron.ballman accepted this revision.
aaron.ballman added a comment.
This revision is now accepted and ready to land.

In http://reviews.llvm.org/D20352#477203, @rnk wrote:

> In http://reviews.llvm.org/D20352#477084, @aaron.ballman wrote:
>
> > No, I'm not. I am worried about how this conflicts with another in-flight patch for supporting MS hotpatchable functions since it seems these two attributes do roughly the same thing. I'd like to understand how these two user-facing attributes will not be confusing to users, what should happen if both attributes wind up on a function, etc. I am hoping that we can wind up with only one attribute that covers both cases, if possible.
>
>
> From a user perspective, I would definitely want to keep these attributes separate. It's only possible to completely replace an MS hotpatchable function, whereas XRay allows you to instrument function entry and exit with low overhead. You can achieve the same effect as XRay with MS hotpatchable prologues and a full trampoline with a stack frame, but probably not at the same runtime cost. They really are suited for different tasks.


Okay, that was the key piece I was missing -- I was under the impression that this was two different ways to achieve the same task. Thank you for the explanation (and sorry if I was being dense). :-)

When the MS hot patching attribute lands, that will be the time to discuss and document conflicts between the two (I am assuming you can't enable XRay and MS hot patching for the same function, which may be an incorrect assumption). So I think this LGTM now.


http://reviews.llvm.org/D20352