linux-kernel - Re: [PATCH] x86: Add optimized popcnt variants

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Date:	Mon, 22 Feb 2010 11:55:48 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...64.org>
CC:	Michal Marek <mmarek@...e.cz>,
	linux-kbuild <linux-kbuild@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jamie Lokier <jamie@...reable.org>,
	Roland Dreier <rdreier@...co.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH] x86: Add optimized popcnt variants

On 02/22/2010 10:49 AM, Borislav Petkov wrote:
>>
>> Just a note: this still means rdi is clobbered on x86-64, which is
>> probably fine, but needs to be recorded as such.  Since gcc doesn't
>> support clobbers for registers used as operands (sigh), you have to
>> create a dummy output and assign it a "=D" constraint.
>>
>> I don't know if gcc would handle -fcall-saved-rdi here... and if so, how
>> reliably.
> 
> Ok, from looking at kernel/sched.s output it looks like it saves rdi
> content over the alternative where needed. I'll do some more testing
> just to make sure.
> 

No, you can't rely on behavioral observation.  A different version of
gcc could behave differently.  We need to make sure we tell gcc what the
requirements actually are, as opposed to thinking we can just fix them.

+#define POPCNT ".byte 0xf3\n\t.byte 0x48\n\t.byte 0x0f\n\t.byte
0xb8\n\t.byte 0xc7"

BTW, this can be written:

#define POPCNT ".byte 0xf3,0x48,0x0f,0xb8,0xc7"

	-hpa
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