linux-kernel - Re: [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6 arguments

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Date:   Tue, 22 Mar 2022 12:23:59 +0100
From:   Willy Tarreau <w@....eu>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "'Ammar Faizi'" <ammarfaizi2@...weeb.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Nugraha <richiisei@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: Re: [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6
 arguments

On Tue, Mar 22, 2022 at 10:57:01AM +0000, David Laight wrote:
> From: Ammar Faizi
> > Sent: 22 March 2022 10:21
> > 
> > On i386, the 6th argument of syscall goes in %ebp. However, both Clang
> > and GCC cannot use %ebp in the clobber list and in the "r" constraint
> > without using -fomit-frame-pointer. To make it always available for
> > any kind of compilation, the below workaround is implemented.
> > 
> > For clang (the Assembly statement can't clobber %ebp):
> >   1) Push the 6-th argument.
> >   2) Push %ebp.
> >   3) Load the 6-th argument from 4(%esp) to %ebp.
> >   4) Do the syscall (int $0x80).
> >   5) Pop %ebp (restore the old value of %ebp).
> >   6) Add %esp by 4 (undo the stack pointer).
> > 
> > For GCC, fortunately it has a #pragma that can force a specific function
> > to be compiled with -fomit-frame-pointer, so it can use "r"(var) where
> > var is a variable bound to %ebp.
> 
> You need to use the 'clang' pattern for gcc.
> #pragma optimise is fundamentally broken.
> What actually happens here is the 'inline' gets lost
> (because of the implied -O0) and you get far worse code
> than you might expect.
> 
> Since you need the 'clang' version, use it all the time.

I clearly prefer it as well, it looks much cleaner!

Willy