Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT)

[H. Peter Anvin]

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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