[Bug other/46847] Missed optimization for variant of Duff's device

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46847

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-12-08 12:02:09 UTC ---
GCC 4.5 doesn't move end computation inside the loop.  What do you expect
to be "good" code?  I get at -O[23]:

foo:
.LFB0:
        mov     %esi, %ecx
        subl    %edi, %esi
        mov     %edi, %eax
        andl    $7, %esi
        leaq    (%rdx,%rax,8), %rax
        leaq    (%rdx,%rcx,8), %rdx
        jmp     *.L10(,%rsi,8)
        .section        .rodata
        .align 8
        .align 4
.L10:
        .quad   .L2
        .quad   .L3
        .quad   .L4
        .quad   .L5
        .quad   .L6
        .quad   .L7
        .quad   .L8
        .quad   .L9
        .text
        .p2align 4,,10
        .p2align 3
.L9:
        movq    $0, (%rax)
        addq    $8, %rax
.L8:
        movq    $0, (%rax)
        addq    $8, %rax
.L7:
        movq    $0, (%rax)
        addq    $8, %rax
.L6:
        movq    $0, (%rax)
        addq    $8, %rax
.L5:
        movq    $0, (%rax)
        addq    $8, %rax
.L4:
        movq    $0, (%rax)
        addq    $8, %rax
.L3:
        movq    $0, (%rax)
        addq    $8, %rax
.L2:
        movq    $0, (%rax)
        addq    $8, %rax
        cmpq    %rax, %rdx
        jae     .L9
        rep
        ret

Similar code is generated by ICC 11.1.

Duffs device may be a fun thing from a C language perspective, but it
is a bad thing in general because you defy most
loop optimizations as it is a loop with multiple entries (which means
the loop isn't recognized as a loop by GCC).