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

[H. Peter Anvin]

On 02/04/2010 07:10 AM, Borislav Petkov wrote:
>>
>> Arguably the "best" option is to have the alternative being a jump to an
>> out-of-line stub which does the necessary parameter marshalling before
>> calling a stub.  This technique is already used in a few other places.
> 
> Ok, here's a first alpha prototype and completely untested. The asm
> output looks ok though. I've added separate 32-bit and 64-bit helpers in
> order to dispense with the if-else tests. The hw-popcnt versions are the
> opcodes for "popcnt %eax, %eax" and "popcnt %rax, %rax", respectively,
> so %rAX has to be preloaded with the bitmask and the computed value
> has to be retrieved from there afterwards. And yes, it looks not that
> elegant so I'm open for suggestions.
> 
> The good thing is, this should work on any toolchain since we don't rely
> on the compiler to know about popcnt and we're protected by CPUID flag
> so that the hw-popcnt version is used only on processors which support
> it.
> 
> Please take a good look and let me know what do you guys think.
> 

I think you misunderstood me.  The idea was to use the alternatives
mechanism to put the popcnt instruction inline instead of the call.

	-hpa

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

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