linux-kernel - Re: [PATCH v1 04/11] tools/nolibc: x86-64: Use appropriate register constraints if exist

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Date:   Thu, 24 Mar 2022 16:42:10 +0100
From:   Willy Tarreau <w@....eu>
To:     Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>
Cc:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Nugraha <richiisei@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        David Laight <David.Laight@...lab.com>
Subject: Re: [PATCH v1 04/11] tools/nolibc: x86-64: Use appropriate register
 constraints if exist

On Thu, Mar 24, 2022 at 03:33:57PM +0700, Alviro Iskandar Setiawan wrote:
> On Thu, Mar 24, 2022 at 2:57 PM Willy Tarreau <w@....eu> wrote:
> > On Thu, Mar 24, 2022 at 02:30:32PM +0700, Ammar Faizi wrote:
> > > Use appropriate register constraints if exist. Don't use register
> > > variables for all inputs.
> > >
> > > Register variables with "r" constraint should be used when we need to
> > > pass data through a specific register to extended inline assembly that
> > > doesn't have a specific register constraint associated with it (anything
> > > outside %rax, %rbx, %rcx, %rdx, %rsi, %rdi).
> > >
> > > It also simplifies the macro definition.
> >
> > I'm a bit bothered by this one because I went the exact opposite route
> > in the early design precisely because I found that the current one was
> > simpler. [...]
> [...]
> > I'd say that if there is any technical benefit in doing this (occasional
> > code improvement or better support for older or exotic compilers), I'd say
> > "ok go for it", but if it's only a matter of taste, I'm not convinced at
> > all and am rather seeing this as a regression. Now if there's rough
> > consensus around this approach I'll abide, but then I'd request that other
> > archs are adapted as well so that we don't keep a different approach only
> > for these two ones.
> 
> I don't see any technical benefit for x86-64, so I don't think there
> is a need in doing this. Though I personally prefer to use register
> constraints if they exist instead of register variables for everything
> (oh yeah, matter of taste since I don't have any technical argument to
> say it's better respecting the resulting codegen). The only real issue
> is for the syscall6() implementation on i386 as we've been bitten by a
> real compiler issue. In short, I am neutral on this change.

Just to be clear, I usually only use register constraints as well but I
changed this for the syscalls since they were not sufficient, and found
that the mix of the two was really not great to deal with.

Thanks,
Willy