Re: [gentoo-dev] Re: [gentoo-commits] repo/gentoo:master commit in: media-libs/x265/

[Alexis Ballier]

On Mon, 2020-05-25 at 17:04 -0400, Mike Gilbert wrote:
> On Mon, May 25, 2020 at 3:18 PM Michał Górny <mgorny@g.o>
> wrote:
> > On Mon, 2020-05-25 at 19:49 +0200, Alexis Ballier wrote:
> > > On Mon, 25 May 2020 11:26:26 -0400
> > > Mike Gilbert <floppym@g.o> wrote:
> > >
> > > > On Mon, May 25, 2020 at 9:13 AM Alexis Ballier <
> > > > aballier@g.o>
> > > > wrote:
> > > > > On Sun, 24 May 2020 20:25:11 +0000 (UTC)
> > > > > "Thomas Deutschmann" <whissi@g.o> wrote:
> > > > >
> > > > > > commit:     6e149596cc76f1bbcee6720828c8c8c92420f2a3
> > > > > > Author:     Thomas Deutschmann <whissi <AT> gentoo <DOT>
> > > > > > org>
> > > > > > AuthorDate: Sun May 24 19:47:08 2020 +0000
> > > > > > Commit:     Thomas Deutschmann <whissi <AT> gentoo <DOT>
> > > > > > org>
> > > > > > CommitDate: Sun May 24 20:23:53 2020 +0000
> > > > > > URL:
> > > > > > https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=6e149596
> > > > > >
> > > > > > media-libs/x265: drop USE=pic
> > > > > >
> > > > > > Gentoo's toolchain uses PIC by default. Since USE=asm was
> > > > > > added,
> > > > > > we no longer need a USE flag to control that behavior.
> > > > >
> > > > > You got it wrong here it seems: USE=pic does not control
> > > > > whether
> > > > > the toolchain produces PIC or not. Shared libs always are,
> > > > > and have
> > > > > always been, built that way on Gentoo.
> > > > > In this case, USE=pic means "no matter what it costs, I do
> > > > > not want
> > > > > textrels", for the cases of hand written assembly that has to
> > > > > be
> > > > > rewritten to support PIC. And, still in this case, this costs
> > > > > a lot
> > > > > of performance, so it is enabled by default on hardened
> > > > > profiles
> > > > > and not others.
> > > > > Textrels work fine (on some architectures), they disallow W^X
> > > > > and
> > > > > force each process using the shared lib to make a "copy" at
> > > > > runtime
> > > > > in order to resolve relocations, so are not desirable but
> > > > > sometimes
> > > > > the cost outweights the gain.
> > > > >
> > > > > Plus, profiles/features/hardened enables pic by default but
> > > > > knows
> > > > > nothing about USE=asm so this is a regression for them.
> > > >
> > > > The USE flag toggles use of assembly, not use of PIC. The
> > > > default USE
> > > > value in the hardened profile should not drive decisions on
> > > > what we
> > > > name USE flags.
> > >
> > > ... but using a global well documented useflag instead of a local
> > > invention should drive such decisions.
> >
> > What 'global well documented useflag'?
>
> It's neither global, nor well-documented, but several packages do
> define it locally.
>

https://gitweb.gentoo.org/repo/gentoo/historical.git/commit/profiles/use.desc?id=103236c295aa30e5e42cfc8a7429e4eea5f0d680

https://gitweb.gentoo.org/repo/gentoo/historical.git/commit/profiles/use.desc?id=784deb7134b9d430546557a8f8a0877bf35c02ba

I guess this hasn't been really discussed back then.

It is also used in a global way in profiles (make.defaults).

> Personally, I think it should be renamed to "asm" or something
> similar
> in the majority of cases where it actually disables all use of
> assembly code.

Thankfully these days there's usually no need to disable asm to have
pic. hardened has no mention of that flag, and I think that e.g. for
openssl they would have noticed long ago.
And again, 'asm' as a useflag makes no sense: if it works and simply
replaces a C function by a faster one then it shouldn't even be an
useflag. 'pic' on the other hand conveys the tradeoff idea.