Re: [PATCH] aarch64: Improve size heuristic for cpymem expansion

[Christophe LYON <christophe.lyon@foss.st.com>]

On 29/09/2021 12:20, Kyrylo Tkachov via Gcc-patches wrote:
> Hi all,
>
> Similar to my previous patch for setmem this one does the same for the cpymem expansion.
> We count the number of ops emitted and compare it against the alternative of just calling
> the library function when optimising for size.
> For the code:
> void
> cpy_127 (char *out, char *in)
> {
>    __builtin_memcpy (out, in, 127);
> }
>
> void
> cpy_128 (char *out, char *in)
> {
>    __builtin_memcpy (out, in, 128);
> }
>
> we now emit a call to memcpy (with an extra MOV-immediate instruction for the size) instead of:
> cpy_127(char*, char*):
>          ldp     q0, q1, [x1]
>          stp     q0, q1, [x0]
>          ldp     q0, q1, [x1, 32]
>          stp     q0, q1, [x0, 32]
>          ldp     q0, q1, [x1, 64]
>          stp     q0, q1, [x0, 64]
>          ldr     q0, [x1, 96]
>          str     q0, [x0, 96]
>          ldr     q0, [x1, 111]
>          str     q0, [x0, 111]
>          ret
> cpy_128(char*, char*):
>          ldp     q0, q1, [x1]
>          stp     q0, q1, [x0]
>          ldp     q0, q1, [x1, 32]
>          stp     q0, q1, [x0, 32]
>          ldp     q0, q1, [x1, 64]
>          stp     q0, q1, [x0, 64]
>          ldp     q0, q1, [x1, 96]
>          stp     q0, q1, [x0, 96]
>          ret
>
> which is a clear code size win. Speed optimisation heuristics remain unchanged.
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Pushing to trunk.
>
> Thanks,
> Kyrill
>
> 2021-09-29  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
> 	* config/aarch64/aarch64.c (aarch64_expand_cpymem): Count number of
> 	emitted operations and adjust heuristic for code size.
>      
> 2021-09-29  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
> 	* gcc.target/aarch64/cpymem-size.c: New test.


Hi Kyrill,

Just to mention that the new test fails with -mabi=ilp32...


Thanks,


Christophe