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? ...
> 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.
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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