[PATCH][AArch64] Expand DImode constant stores to two SImode stores when profitable

[James Greenhalgh]

On Mon, Oct 24, 2016 at 03:27:10PM +0100, Kyrill Tkachov wrote:
> Hi all,
> 
> When storing a 64-bit immediate that has equal bottom and top halves we currently
> synthesize the repeating 32-bit pattern twice and perform a single X-store.
> With this patch we synthesize the 32-bit pattern once into a W register and store
> that twice using an STP. This reduces codesize bloat from synthesising the same
> constant multiple times at the expense of converting a store to a store-pair.
> It will only trigger if we can save two or more instructions, so it will only transform:
>          mov     x1, 49370
>          movk    x1, 0xc0da, lsl 32
>          str     x1, [x0]
> 
> into:
> 
>          mov     w1, 49370
>          stp     w1, w1, [x0]
> 
> when optimising for -Os, whereas it will always transform a 4-insn synthesis
> sequence into a two-insn sequence + STP (see comments in the patch).
> 
> This patch triggers already but will trigger more with the store merging pass
> that I'm working on since that will generate more of these repeating 64-bit constants.
> This helps improve codegen on 456.hmmer where store merging can sometimes create very
> complex repeating constants and target-specific expand needs to break them down.
> 
> Bootstrapped and tested on aarch64-none-linux-gnu.
> 
> Ok for trunk?

Hi Kyrill,

Does this do the right thing for:

  void bar(u64 *x)
  {
	*(volatile u64 *)x = 0xabcdef10abcdef10;
  }

C.f. https://lore.kernel.org/lkml/20190821103200.kpufwtviqhpbuv2n@willie-the-truck/

i.e. is this optimization still valid for volatile?

Thanks,
James

> 
> Thanks,
> Kyrill
> 
> 2016-10-24  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
> 
>      * config/aarch64/aarch64.md (mov<mode>): Call
>      aarch64_split_dimode_const_store on DImode constant stores.
>      * config/aarch64/aarch64-protos.h (aarch64_split_dimode_const_store):
>      New prototype.
>      * config/aarch64/aarch64.c (aarch64_split_dimode_const_store): New
>      function.
> 
> 2016-10-24  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
> 
>      * gcc.target/aarch64/store_repeating_constant_1.c: New test.
>      * gcc.target/aarch64/store_repeating_constant_2.c: Likewise.