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From: "acsawdey at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/103197] New: ppc inline expansion of memcpy/memmove
 should not use lxsibzx/stxsibx for a single byte
Date: Thu, 11 Nov 2021 18:27:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103197

            Bug ID: 103197
           Summary: ppc inline expansion of memcpy/memmove should not use
                    lxsibzx/stxsibx for a single byte
           Product: gcc
           Version: 10.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: acsawdey at gcc dot gnu.org
  Target Milestone: ---

This got broken sometime in gcc 10 timeframe. For this test case:

#include <string.h>
void m(char *a, char *b)
{
  memcpy(a,b,9);
}

AT13 (gcc 9.3.1) produces:

m:
.LFB0:
        .cfi_startproc
        ld 10,0(4)
        lbz 9,8(4)
        std 10,0(3)
        stb 9,8(3)
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0
        .cfi_endproc

which is the expected code to copy 9 bytes.

AT14 (gcc 10.3.1), gcc 11, and current trunk all produce:

m:
.LFB0:
        .cfi_startproc
        addi 10,4,8
        ld 9,0(4)
        lxsibzx 0,0,10
        std 9,0(3)
        addi 9,3,8
        stxsibx 0,0,9
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0
        .cfi_endproc

which is really bad, mixing gpr and vsx. The inline expansion code in
expand_block_move() does not attempt to generate vsx code at all unless the
size is at least 16 bytes.=