[RFC PATCH] gcc: Improve reproducability when building cc1/cc1plus

[Richard Biener]

On Sat, Nov 20, 2021 at 9:43 AM Jacob Kroon via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On 11/20/21 09:32, Jakub Jelinek wrote:
> > On Sat, Nov 20, 2021 at 12:24:21AM -0800, Andrew Pinski via Gcc-patches wrote:
> >> On Sat, Nov 20, 2021 at 12:18 AM Jacob Kroon via Gcc-patches
> >> <gcc-patches@gcc.gnu.org> wrote:
> >>>
> >>> cc1/cc1plus both include a checksum of the object files, archives and
> >>> options used during linking. Unless the host binutils has been built
> >>> with --enable-deterministic-archives, the archives will have different
> >>> checksums each build due to changes in timestamps of the containing
> >>> object files, and thus the checksum that is embedded in cc1/cc1plus will
> >>> also change.
> >>>
> >>> Fix this by passing "D" to ar/ranlib when creating the archives.
> >>
> >> How portable is the D option? That does it work with Mac OS X's ar;
> >> what about AIX's ar; what about Solaris's ar, etc?
> >> GNU binutils is not the only ar which is supported for the host.
> >
> > It isn't portable and even if ar/ranlib do support that option, not all
> > users will want it, so forcing it upon them unconditionally is IMO not a
> > good idea.  If anything, configure needs to check if those options are
> > supported and there needs to be preferrably non-default gcc configure
> > option to request it.
> >
>
> Aha, I see. Then perhaps an easier and more portable solution for
> improving the reproducibility would be to make it optional whether or
> not to include the archives when calculating the checksums.
>
> Or perhaps by checksumming the containing object files instead of the
> archives themselves ?
>
> Would something like that be acceptable ?

At SUSE we are carrying patches to not include the checksum at all
but instead use the linker generated build-id as checksum for PCH.
I suppose when we add the configure option to disable PCH support
we could also elide checksum processing (or as in the patch simply
always use all-zeros).

Attached for reference.

Richard.

>
> Jacob
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