Re: [AArch64] Implement movmem for the benefit of inline memcpy

[Andrew Pinski <pinskia at gmail dot com>]

On Fri, Jun 6, 2014 at 1:50 AM, James Greenhalgh
<james.greenhalgh@arm.com> wrote:
>
> Hi,
>
> The move_by_pieces infrastructure performs a copy by repeatedly trying
> the largest safe copy it can make. So for a 15-byte copy we might see:
>
> offset   amount  bytes copied
> 0        8       0-7
> 8        4       8-11
> 12       2       12-13
> 14       1       14
>
> However, we can implement a 15-byte copy as so:
>
> offset   amount  bytes copied
> 0        8       0-7
> 7        8       7-14
>
> Which can prove more efficient for both space and speed.
>
> In this patch we set MOVE_RATIO low to avoid using move_by_pieces, and
> implement the movmem pattern name to expand small block copy cases. Note, this
> optimization does not apply for -mstrict-align targets, which must continue
> copying byte-by-byte.
>
> Setting MOVE_RATIO low in this way causes a few tests to begin failing,
> both of these are documented in the test-case as expected to fail for
> low MOVE_RATIO targets, which do not allow certain Tree-Level optimizations.


I think you should reevaluate setting MOVE_RATIO this low.  It is used
for SRA and IPA-SRA which both are very useful; more useful than
memmove optimizations can do.

In fact this optimization is not even valid for volatile variables.
Here is a testcase for the volatile issue:
struct __attribute__((packed)) t15{
  long long t8;
  int t4;
  short t2;
  unsigned char t1;
};
volatile struct t15 t15;
int f(struct t15 *a)
{
  t15 = *a;
}

Notice how we are writing to byte 7 twice to t15 in the outputted code.

Thanks,
Andrew Pinski

>
> Bootstrapped on aarch64-unknown-linux-gnu with no issues.
>
> OK for trunk?
>
> Thanks,
> James
>
> ---
> gcc/
>
> 2014-06-06  James Greenhalgh  <james.greenhalgh@arm.com>
>
>         * config/aarch64/aarch64-protos.h (aarch64_expand_movmem): New.
>         * config/aarch64/aarch64.c (aarch64_move_pointer): New.
>         (aarch64_progress_pointer): Likewise.
>         (aarch64_copy_one_part_and_move_pointers): Likewise.
>         (aarch64_expand_movmen): Likewise.
>         * config/aarch64/aarch64.h (MOVE_RATIO): Set low.
>         * config/aarch64/aarch64.md (movmem<mode>): New.
>
> gcc/testsuite/
>
> 2014-06-06  James Greenhalgh  <james.greenhalgh@arm.com>
>
>         * gcc.dg/tree-ssa/pr42585.c: Skip for AArch64.
>         * gcc.dg/tree-ssa/sra-12.c: Likewise.