1427344 - Adjust GCC to reduce differences when landing bug 1399679

Status: RESOLVED FIXED

(Firefox Build System :: General, enhancement)

Description

[Mike Hommey [:glandium]]

This is, hopefully, the final step before actually going forward with bug 1399679.

This is going to generate massive differences in the binaries, which is why they are a separate preparatory step, but mostly because of offset differences.

Comment 1

[Mike Hommey [:glandium]]

Attachment: Bug 1427344 - Build GCC without --disable-initfini-array.

When building on Debian (which we now are), this means we enable
.init_array/.fini_array.

When building on CentOS, this means no change. Which implies we could
roll back to CentOS-based toolchains by just switching back the
toolchain jobs to use the desktop-build docker image again.

This change causes massive differences in the resulting binaries because
of the offset differences, but practically speaking, there is no
difference. .init_array/.fini_array have been supported in glibc for 18
years.

Review commit: https://reviewboard.mozilla.org/r/211230/diff/#index_header
See other reviews: https://reviewboard.mozilla.org/r/211230/

Comment 2

[Mike Hommey [:glandium]]

Attachment: Bug 1427344 - Add Debian CRT objects to GCC.

One of the last remaining differences when building Firefox on Debian
with all the changes we've done so far is that linking against the
system libc statically links some CRT objects. This causes massive
differences in the resulting binaries because of slight differences
in those objects (because they weren't compiled with the same compiler
and because they're not for the exact same glibc version)

In practice, their content difference don't cause any problem. If they
did, we wouldn't be able to run our builds on newer systems than those
we build them on. The only hypothetical risk would be to run on systems
with a glibc older than Debian 7's, but those already can't run Firefox
anyways (those systems don't have Gtk+3, which is a system requirement).

AFAICT, this is only an hypothetical problem anyways, even such systems
with Gtk+3 should be able to run those builds. Plus, this is a change
that will happen anyways when switching to Debian-based build images,
since they would be using the CRT objects from there. We're merely
making it happen earlier so that the differences from switching to
Debian-based build images are more tractables.

Note we only do this when building GCC on Debian, allowing to roll back
to CentOS-based toolchains by just switching back the toolchain jobs to
use the desktop-build docker image again.

Also note this is meant to be backed out after bug 1399679 lands.

Review commit: https://reviewboard.mozilla.org/r/211232/diff/#index_header
See other reviews: https://reviewboard.mozilla.org/r/211232/

Comment 3

[Nathan Froyd [:froydnj]]

Comment on attachment 8940948 [details]
Bug 1427344 - Build GCC without --disable-initfini-array.

https://reviewboard.mozilla.org/r/211230/#review217124

Yay modern options!

Comment 4

[Nathan Froyd [:froydnj]]

Comment on attachment 8940949 [details]
Bug 1427344 - Add Debian CRT objects to GCC.

https://reviewboard.mozilla.org/r/211232/#review217130

Looks reasonable.  One thing to fix, and assuming that I have correctly understood the purpose of this code in the comment below, r=me.  The question about not removing this code after bug 13799679 lands does not have to be resolved before landing this patch.

::: commit-message-6c964:21
(Diff revision 1)
> +AFAICT, this is only an hypothetical problem anyways, even such systems
> +with Gtk+3 should be able to run those builds. Plus, this is a change
> +that will happen anyways when switching to Debian-based build images,
> +since they would be using the CRT objects from there. We're merely
> +making it happen earlier so that the differences from switching to
> +Debian-based build images are more tractables.

Nit: "...more tractable."

::: commit-message-6c964:23
(Diff revision 1)
> +Note we only do this when building GCC on Debian, allowing to roll back
> +to CentOS-based toolchains by just switching back the toolchain jobs to
> +use the desktop-build docker image again.
> +
> +Also note this is meant to be backed out after bug 1399679 lands.

Just to make sure I understand here: we have a lot of differences in the generated binaries when building Firefox on Debian vs. building Firefox on CentOS.  In order to minimize those differences for the purposes of debugging *actual* differences, we are going to ensure that our GCC toolchain is using a consistent set of CRT objects.  So whether we're using that GCC toolchain on Debian or CentOS, we will be using a consistent set of CRT objects--and libc files, it looks like.  And this will make a number of issues go away.

But once we switch to Debian everywhere, we don't care about the differences (because we'll be building on a single platform again), so we're going to nuke this code at that point.

Is that a correct summary of what we're doing here and why?  Do we want to *always* copy in the CRT objects to minimize differences for future upgrades?

Comment 5

[Mike Hommey [:glandium]]

(In reply to Nathan Froyd [:froydnj] from comment #4)
> Is that a correct summary of what we're doing here and why?

Yes

> Do we want to *always* copy in the CRT objects to minimize differences for future upgrades?

Mmmm that's a good question. Maybe decide next time we need this?

Comment 6

[Mike Hommey [:glandium]]

Comment on attachment 8940948 [details]
Bug 1427344 - Build GCC without --disable-initfini-array.

Review request updated; see interdiff: https://reviewboard.mozilla.org/r/211230/diff/1-2/

Comment 7

[Mike Hommey [:glandium]]

Comment on attachment 8940949 [details]
Bug 1427344 - Add Debian CRT objects to GCC.

Review request updated; see interdiff: https://reviewboard.mozilla.org/r/211232/diff/1-2/

Comment 8

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/6e4c1669b6ba
Build GCC without --disable-initfini-array. r=nfroyd
https://hg.mozilla.org/integration/mozilla-inbound/rev/7c92680e18db
Add Debian CRT objects to GCC. r=nfroyd

Comment 9

[Natalia Csoregi [:nataliaCs]]

https://hg.mozilla.org/mozilla-central/rev/6e4c1669b6ba
https://hg.mozilla.org/mozilla-central/rev/7c92680e18db