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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/52252] An opportunity for x86 gcc vectorizer (gain up to 3 times)
Date: Wed, 15 Feb 2012 11:55:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52252

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-02-15
          Component|target                      |tree-optimization
            Version|unknown                     |4.7.0
     Ever Confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-02-15 11:53:58 UTC ---
We fail to SLP vectorize this because of

6: Build SLP failed: different operation in stmt k_15 = MIN_EXPR <tmp_14,
y_13>;

thus,

        out[0] = c - k;
        out[1] = m - k;
        out[2] = y - k;
        out[3] = k;

isn't detected as equivalent to

        out[0] = c - k;
        out[1] = m - k;
        out[2] = y - k;
        out[3] = <magic> - k;

or

        out[3] = k - 0;

whatever would be more suitable (the latter would fail to be detected as
induction I guess, the former would fail with a similar issue for the
definition of <magic>).

With

        out[3] = y - k;

we fail with

6: Load permutation 0 1 2 2 1 1 1 1 0 0 0 0 2 2 2 2
6: Build SLP failed: unsupported load permutation *out_37 = D.1721_16;

we can vectorize

void convert_image(byte *in, byte *out, int size) {
    int i;
    for(i = 0; i < size; i++) {
        byte r = in[0];
        byte g = in[1];
        byte b = in[2];
        byte a = in[3];
        byte c, m, y, k, z, tmp;
        c = 255 - r;
        m = 255 - g;
        y = 255 - b;
        z = 255 - a;
        tmp = MIN(m, y);
        k = MIN(c, tmp);
        out[0] = c - k;
        out[1] = m - k;
        out[2] = y - k;
        out[3] = z - k;
        in += 4;
        out += 4;
    }
}

though.