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Re: Skipping assembler when producing slim LTO files
- From: Jan Hubicka <hubicka at ucw dot cz>
- To: Steven Bosscher <stevenb dot gcc at gmail dot com>
- Cc: Ian Lance Taylor <iant at google dot com>, Jan Hubicka <hubicka at ucw dot cz>, Andi Kleen <andi at firstfloor dot org>, GCC Development <gcc at gcc dot gnu dot org>
- Date: Thu, 25 Sep 2014 02:49:41 +0200
- Subject: Re: Skipping assembler when producing slim LTO files
- Authentication-results: sourceware.org; auth=none
- References: <20140924054651 dot GB5371 at kam dot mff dot cuni dot cz> <878ul96pnx dot fsf at tassilo dot jf dot intel dot com> <CAKOQZ8zCs8ZjGP3JWw6HVNXGS0junJn1BuvnMXmK8Ou8WHEnvg at mail dot gmail dot com> <20140924161906 dot GA26922 at atrey dot karlin dot mff dot cuni dot cz> <20140924163227 dot GB26922 at atrey dot karlin dot mff dot cuni dot cz> <CABu31nO248rEg2xmj1MYvt-p3Y7+EbNu6U-UV6aTeMzxbiZ_Pw at mail dot gmail dot com> <CAKOQZ8ytc5nevvq2AAX4TwztWopenhF5Ah9k_+zubb+-Z1JaYQ at mail dot gmail dot com> <CABu31nOYHv1X+LD8Vzs5mfkKT=SPjTRq4cEeY74dxoBnY7zGWQ at mail dot gmail dot com>
> On Wed, Sep 24, 2014 at 11:47 PM, Ian Lance Taylor wrote:
> > On Wed, Sep 24, 2014 at 10:04 AM, Steven Bosscher wrote:
> >> Are you using -pipe? AFAIR this still isn't the default, even on
> >> GNU/Linux, but it is typically a lot faster than without.
> >
> > Is that true even when TMPDIR is on a ram disk? There's no obvious
> > reason that it should be true in a parallel build. Using -pipe
> > effectively constrains communication between the compiler and the
> > assembler to work in PIPE_BUF blocks. Using TMPDIR introduces no such
> > constraints, and in a big program a parallel build should obscure the
> > fact that the compiler and assembler are serialized for each
> > individual compilation unit.
>
> I've done my most recent timings on a machine that has /dev/md3
> mounted on /tmp. That's gcc110 on the compile farm. With/without -pipe
> made a significant difference.
>
> If TMPDIR is a tmpfs or other kind of ram disk, I suppose the benefits
> would be less (to the point of vanishing). Unfortunately I can't test
> it...
OK, I tried it on my hello world benchmark with tmpfs and -fpipe really seems
like a small loss. I wonder if we can work out better defaults that works for
most of people. I use tmpfs as I am worried about my notebook ssd still
being alive and well in 3 years, but it is still far from mainstream.
Honza