linux-kernel - [PATCH v2 00/10] x86: use gcc 6+ asm flag output feature

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Message-Id: <1465414726-197858-1-git-send-email-hpa@linux.intel.com>
Date:	Wed,  8 Jun 2016 12:38:36 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>
Subject: [PATCH v2 00/10] x86: use gcc 6+ asm flag output feature

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

v2: fix a conflict between <linux/random.h> and <asm/archrandom.h>
    discovered by Ingo Molnar.  There are a few places in x86-specific
    code where we need all of <arch/archrandom.h> even when
    xCONFIG_ARCH_RANDOM is disabled, so <linux/random.h> does not
    suffice.  Only the <asm/archrandom.h> patch has been changed;
    since it is a little bit different from the others I have moved it
    to the end.