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

[Jeff Law <law@redhat.com>]

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.

jeff