Messages in this thread | | | Subject | Re: [PATCH v6 1/9] ppc64 (le): prepare for -mprofile-kernel | From | AKASHI Takahiro <> | Date | Thu, 4 Feb 2016 18:31:40 +0900 |
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Jiri, Torsten
Thank you for your explanation.
On 02/03/2016 08:24 PM, Torsten Duwe wrote: > On Wed, Feb 03, 2016 at 09:55:11AM +0100, Jiri Kosina wrote: >> On Wed, 3 Feb 2016, AKASHI Takahiro wrote: >>> those efforts, we are proposing[1] a new *generic* gcc option, -fprolog-add=N. >>> This option will insert N nop instructions at the beginning of each function. > >> The interesting part of the story with ppc64 is that you indeed want to >> create the callsite before the *most* of the prologue, but not really :) > > I was silently assuming that GCC would do this right on ppc64le; add the NOPs > right after the TOC load. Or after TOC load and LR save? ...
On arm/arm64, link register must be saved before any function call. So anyhow we will have to add something, 3 instructions at the minimum, like: save lr branch _mcount restore lr <prologue> ... <body> ...
>> The part of the prologue where TOC pointer is saved needs to happen before >> the fentry/profiling call. > > Yes, any call, to any profiler/tracer/live patcher is potentially global > and needs the _new_ TOC value.
I don't want to bother you, but for my better understandings, could you show me an example of asm instructions for a function prologue under -mprofile-kernel, please?
-Takahiro AKASHI
> This proposal, if implemented in a too naive fashion, will worsen the problem > we currently discuss: a few NOPs _never_ cause any global reference. GCC might > be even more inclined to not load a new TOC value. That change would need to be > fairly smart on ppc64le. > > Torsten >
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