[Bug target/14552] compiled trivial vector intrinsic code is inefficient

[astrange at ithinksw dot com]

------- Comment #24 from astrange at ithinksw dot com  2008-03-19 19:21 -------
For
typedef short mmxw  __attribute__ ((mode(V4HI)));
typedef int   mmxdw __attribute__ ((mode(V2SI)));

mmxdw dw;
mmxw w;

void test(){
    w+=w;
    dw= (mmxdw)w;
}

void test2(){
        w= __builtin_ia32_paddw(w,w);
        dw= (mmxdw)w;
}

gcc SVN generates the expected code for test2(), but not test(). I don't think
using += on an MMX variable should count as autovectorization - if you're doing
either you should know where to put emms yourself.

For test() we get:
        subl    $28, %esp
        movq    _w, %mm0
        movq    %mm0, 8(%esp)
        movzwl  8(%esp), %eax
        movzwl  10(%esp), %edx
        movzwl  12(%esp), %ecx
        addl    %eax, %eax
        addl    %edx, %edx
        movw    %ax, _w
        movw    %dx, _w+2
        movzwl  14(%esp), %eax
        addl    %ecx, %ecx
        addl    %eax, %eax
        movw    %cx, _w+4
        movw    %ax, _w+6
        movq    _w, %mm0
        movq    %mm0, _dw
        addl    $28, %esp
        ret

which touches mm0 (requiring emms, I think) but not using paddw (so being slow
and silly-looking).
LLVM generates expected code for both of them.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14552