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Re: [PATCH 00/12] S390: Implement __fentry__

On 08/02/2018 12:43 PM, Ilya Leoshkevich wrote:

Am 02.08.2018 um 11:17 schrieb Florian Weimer <>:

On 08/02/2018 09:57 AM, Ilya Leoshkevich wrote:
This patch series adds the runtime support in glibc for the -mfentry
gcc feature introduced in [1] and [2].
Patches 1-9 deal with avoiding clobbering %r0 when calling lazily bound
functions, so that the new __fentry__ symbol could be called with return
address in that register.

Should GCC arrange for suitable ABI markup if -mfentry is used?  I'm worried that unexpected clobbers of r0 could be quite difficult to figure out otherwise.

I don’t think we need to do anything ABI-related at this point.

__fentry__ itself is being newly introduced, so there is no old ABI to
comply with. Instrumented functions’ ABI does not change, particularly,
%r0, being a volatile register, used to be clobbered anyway.

Are there other cases that I’m missing?

Agreed, with the __fentry__ addition in the same release as the trampoline change, this should be safe and avoid the issue I was worried about.


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