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

[Andrew Pinski]

On Mon, Oct 24, 2016 at 7:27 AM, Kyrill Tkachov
<kyrylo.tkachov@foss.arm.com> 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.


Doing STP might be worse on ThunderX 1 than the mov/movk.  Or this
might cause an ICE with -mcpu=thunderx; I can't remember if the check
for slow unaligned store pair word is with the pattern or not.
Basically the rule is
1) if 4 byte aligned, then it is better to do two str.
2) If 8 byte aligned, then doing stp is good
3) Otherwise it is better to do two str.

Thanks,
Andrew


>
> Bootstrapped and tested on aarch64-none-linux-gnu.
>
> Ok for trunk?
>
> 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.