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

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Message-ID: <20100206093659.GA28326@aftab>
Date:	Sat, 6 Feb 2010 10:36:59 +0100
From:	Borislav Petkov <bp@...64.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Fri, Feb 05, 2010 at 01:54:42PM -0800, H. Peter Anvin wrote:
> On 02/05/2010 04:11 AM, Borislav Petkov wrote:
> > +
> > +unsigned int __arch_hweight16(unsigned int w)
> > +{
> > +	unsigned int res = 0;
> > +
> > +	asm volatile("xor %%dh, %%dh\n\t"
> > +		     __arch_hweight_alt(32)
> > +			: "=di" (res)
> > +			: "di" (w)
> > +			: "ecx", "memory");
> > +
> 
> This is wrong in more ways than I can shake a stick at.

Thanks for reviewing it though - how else would I learn :).

> a) "di" doesn't mean the DI register - it means the DX register (d) or
> an immediate (i).  Since you don't have any reference to either %0 or %1
> in your code, you have no way of knowing which one it is.  The
> constraint for the di register is "D".

right.

> b) On 32 bits, the first argument register is in %eax (with %edx used
> for the upper half of a 32-bit argument), but on 64 bits, the first
> argument is in %rdi, with the return still in %rax.

Sure, it is right there in arch/x86/include/asm/calling.h. Shame on me.

> 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?

> 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.

Thanks.

-- 
Regards/Gruss,
Boris.

-
Advanced Micro Devices, Inc.
Operating Systems Research Center
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