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Subject: [Bug target/60879] Wrong decision in decide_alg in i386.c
Date: Fri, 18 Apr 2014 15:59:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60879

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Jakub Jelinek from comment #1)
> Does this ever matter though?  I mean, wouldn't we expand it as move by
> pieces or store by pieces for such small constant length anyway and thus
> never reach the target movmem/setmem expansion?

move by pieces or store by pieces are very efficient for
targets with unaligned move/store for integer and vector:

[hjl@gnu-6 partial]$ cat w.i
void
foo5 (const void *src, void *dest, int s)
{
  __builtin_memcpy (dest, src, 23);
}
[hjl@gnu-6 partial]$ gcc -S -O2 w.i
[hjl@gnu-6 partial]$ cat w.s

    .file    "w.i"
    .text
    .p2align 4,,15
    .globl    foo5
    .type    foo5, @function
foo5:
.LFB0:
    .cfi_startproc
    movq    (%rdi), %rax
    movq    %rax, (%rsi)
    movq    8(%rdi), %rax
    movq    %rax, 8(%rsi)
    movl    16(%rdi), %eax
    movl    %eax, 16(%rsi)
    movzwl    20(%rdi), %eax
    movw    %ax, 20(%rsi)
    movzbl    22(%rdi), %eax
    movb    %al, 22(%rsi)
    ret

I am working on a different set/mov memory strategy to generate

    movdqu    (%rdi), %xmm0
    movups    %xmm0, (%rsi)
    movq    15(%rdi), %rax
    movq    %rax, 15(%rsi)
    ret

by setting MOVE_RATIO to 1 and handling most of set/mov memory in
x86 backend.