[llvm-dev] XRay: Demo on x86_64/Linux almost done; some questions.

Tim Northover via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 29 11:00:53 PDT 2016

On 28 July 2016 at 16:14, Serge Rogatch via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Can I ask you why you chose to patch both function entrances and exits,
> rather than just patching the entrances and (in the patches) pushing on the
> stack the address of __xray_FunctionExit , so that the user function returns
> normally (with RETQ or POP RIP or whatever else instruction) rather than
> jumping into __xray_FunctionExit?

> This approach should also be faster because smaller code better fits in CPU
> cache, and patching itself should run faster (because there is less code to
> modify).

It may well be slower. Larger CPUs tend to track the call stack in
hardware and returning to an address pushed manually is an inevitable
branch mispredict in those cases.



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