Re: .../asm-i386/bitops.h performance improvements

["Richard B. Johnson" <linux-os@analogic.com>]

LEA was designed for address calculation on ix86 processors.
If it is used to ready the value of an index register for the
next memory access, it can run in parallel with the next operations.
However, if it is just used to put a value into a register, where
the CPU can't proceed until that value is finalized, it does
nothing more useful than shifts and adds.

In other words, don't substitute LEA for INC or ADD just because
you can.

 	leal	0x04(%ebx), %ebx
... and
 	addl	$0x04, %ebx

... are functionally the same if the CPU needs the value in ebx
immediately. In the code sequence....

 	movl	(%ebx), %eax
 	leal	0x04(%ebx), %ebx	# Next address
 	xorl	%ecx, %eax
 	movl	%eax, (%ebx)

... the address calculation for the marked next address can proceed
in parallel with the xorl operation that follows. This makes LEA
helpful. However, in the following...

>> leal (%%eax,%%edi,8),%%eax

... the CPU needs to complete the whole operation before proceeding.
If you measure this, LEA with two index registers, you will find
that the shift and add is faster, guaranteed.

On Wed, 15 Jun 2005, Gene Heskett wrote:

> On Wednesday 15 June 2005 04:53, cutaway@bellsouth.net wrote:
>> In find_first_bit() there exists this the sequence:
>>
>> shll $3,%%edi
>> addl %%edi,%%eax
>>
>> LEA knows how to multiply by small powers of 2 and add all in one
>> shot very efficiently:
>>
>> leal (%%eax,%%edi,8),%%eax
>>
>>
>> In find_first_zero_bit() the sequence:
>>
>> shll $3,%%edi
>> addl %%edi,%%edx
>>
>> could similarly become:
>>
>> leal (%%edx,%%edi,8),%%edx
>>
> To what cpu families does this apply?  eg, this may be true for intel,
> but what about amd, via etc?
>>
>>
>> -
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>
> -- 
> Cheers, Gene
> "There are four boxes to be used in defense of liberty:
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Cheers,
Dick Johnson
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