Re: Fwd: [PATCH, GCC, Vect] Fix costing for vector shifts

[Sudakshina Das <Sudi.Das@arm.com>]

Hi Jeff

On 07/12/2019 17:44, Jeff Law wrote:
> On Fri, 2019-12-06 at 14:05 +0000, Sudakshina Das wrote:
>> Hi
>>
>> While looking at the vectorization for following example, we
>> realized
>> that even though vectorizable_shift function was distinguishing
>> vector
>> shifted by vector from vector shifted by scalar, while modeling the
>> cost
>> it would always add the cost of building a vector constant despite
>> not
>> needing it for vector shifted by scalar.
>>
>> This patch fixes this by using scalar_shift_arg to determine whether
>> we
>> need to build a vector for the second operand or not. This reduces
>> prologue cost as shown in the test.
>>
>> Build and regression tests pass on aarch64-none-elf and
>> x86_64-pc-linux-gnu-gcc. This gives a 3.42% boost to 525.x264_r in
>> Spec2017 for AArch64.
>>
>> gcc/ChangeLog:
>>
>> 2019-xx-xx  Sudakshina Das  <sudi.das@arm.com>
>> 	    Richard Sandiford  <richard.sandiford@arm.com>
>>
>> 	* tree-vect-stmt.c (vectorizable_shift): Condition ndts for
>> 	vect_model_simple_cost call on scalar_shift_arg.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 2019-xx-xx  Sudakshina Das  <sudi.das@arm.com>
>>
>> 	* gcc.dg/vect/vect-shift-5.c: New test.
> It's a bit borderline, but it's really just twiddling a cost, so OK.

Thanks :) Committed as r279114.

Sudi

> 
> jeff
>