[Bug target/113618] New: [14 Regression] AArch64: memmove idiom regression

["wilco at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113618

            Bug ID: 113618
           Summary: [14 Regression] AArch64: memmove idiom regression
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wilco at gcc dot gnu.org
  Target Milestone: ---

The following is often used as an idiom for memmove since GCC mid-end and most
back-ends have no support for inlining memmove:

void move64 (char *a, char *b)
{
    char temp[64];
    memcpy (temp, a, 64);
    memcpy (b, temp, 64);
}

On trunk this generates:

        ldp     q30, q31, [x0]
        sub     sp, sp, #64
        ldp     q28, q29, [x0, 32]
        stp     q30, q31, [sp]
        ldp     q30, q31, [sp]
        stp     q28, q29, [sp, 32]
        ldp     q28, q29, [sp, 32]
        stp     q30, q31, [x1]
        stp     q28, q29, [x1, 32]
        add     sp, sp, 64
        ret

This is a significant regression from GCC13 which has redundant stores but
avoids load-after-store forwarding penalties:

        ldp     q2, q3, [x0]
        sub     sp, sp, #64
        ldp     q0, q1, [x0, 32]
        stp     q2, q3, [sp]
        stp     q2, q3, [x1]
        stp     q0, q1, [sp, 32]
        stp     q0, q1, [x1, 32]
        add     sp, sp, 64
        ret

LLVM avoids writing to the temporary and removes the stackframe altogether:

        ldp     q1, q0, [x0, #32]
        ldp     q2, q3, [x0]
        stp     q1, q0, [x1, #32]
        stp     q2, q3, [x1]
        ret

The reason for the regression appears to be the changed RTL representation of
LDP/STP. The RTL optimizer does not understand LDP/STP, so emitting LDP/STP
early in memcpy expansion means it cannot remove the redundant stack stores.

A possible fix would be to avoid emitting LDP/STP in memcpy/memmove/memset
expansions.