linux-kernel - Re: [RFC PATCH] x86/hweight: Get rid of the special calling convention

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Date:	Thu, 05 May 2016 15:02:37 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>
CC:	Brian Gerst <brgerst@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Andi Kleen <andi@...stfloor.org>, zengzhaoxiu@....com,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Zhaoxiu Zeng <zhaoxiu.zeng@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH] x86/hweight: Get rid of the special calling convention

On 05/04/2016 10:22 PM, Borislav Petkov wrote:
> On Wed, May 04, 2016 at 12:49:17PM -0700, H. Peter Anvin wrote:
>> Sigh.  Doesn't look like -Wa is going to help due to the lack of the
>> equivalent of an -include option in gas.
> 
> So much for the register "freedom" - I'll resurrect the hardcoded insn
> bytes. :-\
> 
> Unless my gcc friends have some other ideas...
> 
> sarge:~# gcc --version
> gcc (GCC) 3.3.5 (Debian 1:3.3.5-13)
> Copyright (C) 2003 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> 
> sarge:~# cat popcnt.c
> int main(void)
> {
>         int a, b;
> 
>         asm volatile("popcnt %0, %1" : "=r" (a) : "r" (b));
> 
>         return 0;
> }
> sarge:~# gcc -Wall -o popcnt{,.c}
> /tmp/ccHmmgjH.s: Assembler messages:
> /tmp/ccHmmgjH.s:14: Error: no such instruction: `popcnt %eax,%eax'
> sarge:~#

You are looking at the version of a wrong program.
gcc doesn't process assembly, it generates it.
as is part of binutils, not gcc. "as --version".