linux-kernel - Re: [PATCH 1/1] X86: use explicit register name for get/put

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Date:	Mon, 07 Dec 2009 10:35:56 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jiri Slaby <jirislaby@...il.com>
CC:	mingo@...e.hu, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org
Subject: Re: [PATCH 1/1] X86: use explicit register name for get/put_user

On 12/07/2009 04:37 AM, Jiri Slaby wrote:
> Is this documented somewhere? Or do we rely on an undocumented feature?
> I mean it doesn't refer only to the constraint but also to a concrete
> register allocation. As far as I understand it (from the gcc 4.4
> documentation), if one does
>  "insn %0" : "=r" (out) : "0" (in)
> the "0" constraint corresponds to the concrete register allocated for
> out, not to any register (which is the constraint "r").

Yes, but it only corresponds to the information that is conveyed in the
register selection.

> In the document they write only about the "same location" occupied by in
> and out, nothing is said about size (and hence I think we cannot
> mismatch size of operands). And I couldn't find any other
> restrictions/documentation about inline assembly, hence the patch,
> because nothing assured me this cannot change in the future.

There is almost no documentation at all; some of the little
documentation there is is in comments in the source code.  To a first
order of approximation, asm() is defined by behavior, not by a written
spec.  Trying to play language lawyer with the little bit that is
written down is pointless -- the gcc people have been more than happy to
break asm() between releases regardless of what is and is not written down.

> Now I tried different compilers (clang, llvm-gcc) and they choke on that:
> $ cat c.c
> void x(void)
> {
>         unsigned long in;
>         int out;
>         asm("insn %0" : "=r" (out) : "0" (in));
> }
> $ clang c.c -S -o -
> c.c:5:36: error: unsupported inline asm: input with type 'unsigned long'
>       matching output with type 'int'
>         asm("insn %0" : "=r" (out) : "0" (in));
>                               ~~~         ^~
> 1 diagnostic generated.
> $ llvm-gcc c.c -S -o -
> c.c: In function 'x':
> c.c:5: error: unsupported inline asm: input constraint with a matching
> output constraint of incompatible type!
> 
> thanks for the review,

gcc is the standard for gcc-style asm()... if they don't comply, that a
bug...

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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