linux-kernel - Re: [PATCH 00/10] x86: use gcc 6+ asm flag output feature

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Message-ID: <20160608082629.GA9645@gmail.com>
Date:	Wed, 8 Jun 2016 10:26:29 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 00/10] x86: use gcc 6+ asm flag output feature


* H. Peter Anvin <hpa@...ux.intel.com> wrote:

> From: "H. Peter Anvin" <hpa@...or.com>
> 
> gcc 6+ has the ability to let flags (actually, conditions, which are
> specific combinations of flags) to be used directly as asm() outputs.
> The syntax for that is "=@cc<cc>" where <cc> is the same set of
> letters that would be used in a j<cc> or set<cc> instruction
> (e.g. "=@ccz" to test the ZF flag.)
> 
> This patchset by itself reduces the size of the x86-64 kernel by
> 0.12%, from a baseline of 4.7-rc2 built with gcc 6.1 (first line is
> with the patchset, the second one is without):
> 
>      text       data       bss        dec       hex filename
>      
>  68245656   41004339  20533248  129783243   7bc55cb o.i386-allconfig/vmlinux
>  68355716   41008499  20533248  129897463   7be13f7 o.i386-allconfig/vmlinux
> 
> 127384005  129742359  38150144  295276508  11998fdc o.x86_64-allconfig/vmlinux
> 127538765  129742295  38150144  295431204  119bec24 o.x86_64-allconfig/vmlinux

The series introduces a new build error:

./arch/x86/include/asm/archrandom.h:95:20: error: redefinition of ‘arch_get_random_long’
./arch/x86/include/asm/archrandom.h:100:20: error: redefinition of ‘arch_get_random_int’
./arch/x86/include/asm/archrandom.h:105:20: error: redefinition of ‘arch_get_random_seed_long’
./arch/x86/include/asm/archrandom.h:110:20: error: redefinition of ‘arch_get_random_seed_int’

(config attached)

Thanks,

	Ingo

View attachment "config" of type "text/plain" (113780 bytes)