Re: Build reproducibility of gcc @ NixOS

[Arthur Gautier <gcc.gnu.org@superbaloo.net>]

Hi Tadeus,

On Fri, Apr 2, 2021 at 9:07 AM Tadeus Prastowo <tadeus.prastowo@unitn.it> wrote:
>
> Hi Arthur,
>
> On Fri, Apr 2, 2021 at 5:56 AM Arthur Gautier
> <gcc.gnu.org@superbaloo.net> wrote:
> >
> > Dear GCC development team,
> >
> > We've been trying to build reproducibly the minimal NixOS image, and
> > gcc was one of the last issues we had.
> > We found that disabling profiled bootstrap compilation of GCC allowed
> > us to get a reproducible build of gcc.
> > Our efforts can be followed here: https://github.com/NixOS/nixpkgs/pull/112928
> >
> > But I measured disabling this optimization to cost around 7-12%
> > depending on the build.
>
> That is expected as mentioned in the manual:
>
> And, I have reproduced it as well as reported here:
> https://gcc.gnu.org/ml/gcc-help/2019-05/msg00118.html, the last
> questions that remain being here:
> https://gcc.gnu.org/legacy-ml/gcc-help/2019-07/msg00053.html
>
> > Because of this performance regression, we're trying to find a middle
> > ground. Ideally we'd like to keep the performance of gcc as untouched
> > as possible (even if that costs us on compilation time of gcc itself).
> >
> > Compiling gcc twice on the same machine gets us the same output, but
> > compiling on a different architecture gets us a different result.
> > Reading the documentation, it would seem that autoprofiledback
> > bootstrap would use machine metrics and injects them in the build (and
> > we don't use autoprofiledback), But I would not expect the stagetrain
> > of profiledbootstrap to do that.
> > I tried disabling concurrency of the stagetrain without luck.
> >
> > It feels like I'm missing something.
> > Would anyone have any idea what could inject the host's behavior here?
>
> Since an optimized build is likely to be machine-dependent regardless
> of any intended injection (e.g., different instructions used in GCC
> binaries depending on /proc/cpuinfo), I don't understand why an
> optimized build should be reproducible on different machines, unless
> of course every channel that GCC uses to find out about the machine
> (e.g., /proc/cpuinfo) is under your total control.  So, do you mean to
> ask a list of all channels that GCC uses to find out about the
> machine?

This is where I'm getting confused. According to the manual,
stagetrain only record branch statistics. And I would expect, given
the same input provided in the same order, two different architectures
to take the same branch, and not observe any difference. I understand
that with autoprofiled builds, the local architecture behavior is
injected in the build, but I don't use that.
I'm not using any -march in the build either (as far as I can
understand/tell). So I do not expect the build to change its
instruction set either.

Is that normal that two different architectures would issue two
different "execution counts of instruction and branch probabilities"?
Or is there something more?

Thank you for your reply!

-- 
Arthur