Help on merging two patches

[Yao Qi]

On 11/06/2010 01:38 AM, Richard Earnshaw wrote:
>
> On Wed, 2010-11-03 at 17:39 +0800, Yao Qi wrote:
>> Hi,
>> I am backporint some patches from FSF mainline, which may improve Linaro
>> 4.5 gcc on thumb2 speed.
>>
>> The first one is done by Richard E. "Improve optimization to transform
>> TST into LSLS"
>> http://gcc.gnu.org/ml/gcc-patches/2010-06/msg02518.html
>> After it applied to Linaro 4.5 tree, EEMBC speed number downgrades,
>> while code size is reduced to some extent.  The code difference is like
>> this,
>>
>>    6801      	ldr	r1, [r0, #0]
>>    f831 3013 	ldrh.w	r3, [r1, r3, lsl #1]
>> -f413 6f00 	tst.w	r3, #2048	; 0x800
>> -f43f af41 	beq.w	cc<t_run_test+0xcc>
>> +0518      	lsls	r0, r3, #20
>> +f57f af44 	bpl.w	cc<t_run_test+0xcc>
>>    4610      	mov	r0, r2
>>
>> After reading cortex-a8 TRM, I can't find exact timing cycles of lsls.
>> Under Chung-Lin's help, we feel that lsls should be slower than tst, but
>> don't have any evidence to prove.  If any people is familiar with arm
>> microarch, help is welcome.  If our assumption is correct, we may can
>> change this patch to an optimization specific to size only.
>>
>> The second patch is Bernd's "Fix an if statement in arm_rtx_costs_1"
>> http://gcc.gnu.org/ml/gcc-patches/2010-07/msg02096.html
>> After this patch applied, EEMBC benchmark number is not changed.  Shall
>> we merge this patch to linaro 4.5 tree?  I am inclined to merge it, but
>> if you have concerns on this patch, let us discuss here.
>
> So I have no reason to expect lsls to ever take longer to execute than
> tst.  I suspect what you are seeing here is some unfortunate side effect
> that can't be explained from the small code snippet.  An example would
> include BTAC aliasing, but there could be other reasons for this
> happening.
>
> So overall, I'd expect the change to be a Good Thing (tm), but there's
> always the chance that individual blocks of code may run more slowly.
>

Richard,
We feel that "TST to LSLS transformation" is a sort of size 
optimization.  Can we change this "TST to LSLS transformation" to a -Os 
specific optimization?  If you agree on this, we can submit a patch for 
this change.

-- 
Yao Qi
CodeSourcery
yao at codesourcery.com
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