Re: [AArch64, PATCH] Improve Neon store of zero

[Jackson Woodruff <jackson dot woodruff at foss dot arm dot com>]

Hi all,

I've attached a new patch that addresses some of the issues raised with 
my original patch.

On 08/23/2017 03:35 PM, Wilco Dijkstra wrote:
> Richard Sandiford wrote:
> > Sorry for only noticing now, but the call to aarch64_legitimate_address_p
> > is asking whether the MEM itself is a legitimate LDP/STP address.  Also,
> > it might be better to pass false for strict_p, since this can be called
> > before RA.  So maybe:
> >
> >     if (GET_CODE (operands[0]) == MEM
> > 	&& !(aarch64_simd_imm_zero (operands[1], <MODE>mode)
> > 	     && aarch64_mem_pair_operand (operands[0], <MODE>mode)))

There were also some issues with the choice of mode for the call the 
aarch64_mem_pair_operand.

For a 128-bit wide mode, we want to check `aarch64_mem_pair_operand 
(operands[0], DImode)` since that's what the stp will be.

For a 64-bit wide mode, we don't need to do that check because a normal
`str` can be issued.

I've updated the condition as such.

> Is there any reason for doing this check at all (or at least this early during
> expand)?

Not doing this check means that the zero is forced into a register, so 
we then carry around a bit more RTL and rely on combine to merge things.

> There is a similar issue with this part:
>
>   (define_insn "*aarch64_simd_mov<mode>"
>     [(set (match_operand:VQ 0 "nonimmediate_operand"
> -		"=w, m,  w, ?r, ?w, ?r, w")
> +		"=w, Ump,  m,  w, ?r, ?w, ?r, w")
>
> The Ump causes the instruction to always split off the address offset. Ump
> cannot be used in patterns that are generated before register allocation as it
> also calls laarch64_legitimate_address_p with strict_p set to true.

I've changed the constraint to a new constraint 'Umq', that acts the 
same as Ump, but calls aarch64_legitimate_address_p with strict_p set to 
false and uses DImode for the mode to pass.


OK for trunk?

Jackson

> Wilco

ChangeLog:

gcc/

2017-08-29  Jackson Woodruff  <jackson.woodruff@arm.com>

	* config/aarch64/constraints.md (Umq): New constraint.
	* config/aarch64/aarch64-simd.md (*aarch64_simd_mov<mode>):
	Change to use Umq.
	(mov<mode>): Update condition.

gcc/testsuite

2017-08-29  Jackson Woodruff  <jackson.woodruff@arm.com>

	* gcc.target/aarch64/simd/vect_str_zero.c:
	Update testcase.

Attachment: patchfile
Description: Text document