Re: [PATCH 3/3] gimple: allow more folding of memcpy [PR102125]

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Sep 6, 2021 at 12:40 PM Richard Earnshaw <rearnsha@arm.com> wrote:
>
>
> The current restriction on folding memcpy to a single element of size
> MOVE_MAX is excessively cautious on most machines and limits some
> significant further optimizations.  So relax the restriction provided
> the copy size does not exceed MOVE_MAX * MOVE_RATIO and that a SET
> insn exists for moving the value into machine registers.
>
> Note that there were already checks in place for having misaligned
> move operations when one or more of the operands were unaligned.
>
> On Arm this now permits optimizing
>
> uint64_t bar64(const uint8_t *rData1)
> {
>     uint64_t buffer;
>     memcpy(&buffer, rData1, sizeof(buffer));
>     return buffer;
> }
>
> from
>         ldr     r2, [r0]        @ unaligned
>         sub     sp, sp, #8
>         ldr     r3, [r0, #4]    @ unaligned
>         strd    r2, [sp]
>         ldrd    r0, [sp]
>         add     sp, sp, #8
>
> to
>         mov     r3, r0
>         ldr     r0, [r0]        @ unaligned
>         ldr     r1, [r3, #4]    @ unaligned
>
> PR target/102125 - (ARM Cortex-M3 and newer) missed optimization. memcpy not needed operations

OK.

Thanks,
Richard.

> gcc/ChangeLog:
>
>         PR target/102125
>         * gimple-fold.c (gimple_fold_builtin_memory_op): Allow folding
>         memcpy if the size is not more than MOVE_MAX * MOVE_RATIO.
> ---
>  gcc/gimple-fold.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>