linux-kernel - Re: [PATCH 2/5] bitops: compile time optimization for hweight

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Date:	Sat, 06 Feb 2010 17:55:47 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...64.org>
CC:	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 2/5] bitops: compile time optimization for hweight_long(CONSTANT)

On 02/06/2010 01:36 AM, Borislav Petkov wrote:
> 
>> c) You call a C function, but you don't clobber the set of registers
>> that a C function would clobber.  You either need to put the function in
>> an assembly wrapper (which is better in the long run), or clobber the
>> full set of registers that is clobbered by a C function (which is better
>> in the short term) -- which is eax, edx, ecx on 32 bits, but rax, rdi,
>> esi, rdx, rcx, r8, r9, r10, r11 on 64 bits.
> 
> I think you mean rsi instead of esi here.
> 
> Well, the example Brian pointed me to - __mutex_fastpath_lock - lists
> the full set of clobbered registers. Please elaborate on the assembly
> wrapper for the function, wouldn't I need to list all the clobbered
> registers there too or am I missing something?
> 

The notion there would be that you do push/pop in the assembly wrapper.

>> d) On the other hand, you do *not* need a "memory" clobber.
> 
> Right, in this case we have all non-barrier like inlines so no memory
> clobber, according to the comment above alternative() macro.

OK, I'm missing something here.

A few more notions:

a. This is exactly the kind of code where you don't want to put
"volatile" on your asm statement, because it's a pure compute.

b. It is really rather pointless to go through the whole alternatives
work if you are then going to put it inside a function which isn't an
inline ...

	-hpa

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

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