421761 - Use multiplication instead of division for gfxRGBA (gfxColor.h)

Status: RESOLVED FIXED

Description

[Michael Moy]

The first set of divides below gets called moderately (5K times logging into
gmail and reading a few emails) and changing the [/ 255.0] to [* (1.0 / 255.0)] results in a multiply instruction instead of a divide instruction on Win32 at -O1 optimization. Floating point divides typically take several times as long as floating point multiplies.

    gfxRGBA(PRUint32 c, PackedColorType colorType = PACKED_ABGR) {
        if (colorType == PACKED_ABGR ||
            colorType == PACKED_XBGR ||
            colorType == PACKED_ABGR_PREMULTIPLIED)
        {
            r = ((c >> 0) & 0xff) / 255.0;
            g = ((c >> 8) & 0xff) / 255.0;
            b = ((c >> 16) & 0xff) / 255.0;
            a = ((c >> 24) & 0xff) / 255.0;
        } else if (colorType == PACKED_ARGB ||
                   colorType == PACKED_XRGB ||
                   colorType == PACKED_ARGB_PREMULTIPLIED)
        {
            b = ((c >> 0) & 0xff) / 255.0;
            g = ((c >> 8) & 0xff) / 255.0;
            r = ((c >> 16) & 0xff) / 255.0;
            a = ((c >> 24) & 0xff) / 255.0;
        }

Comment 1

[Michael Moy]

Attachment: Change divisions to multiply

Comment 2

[Joe Drew (not getting mail)]

Comment on attachment 308253 [details] [diff] [review]
Change divisions to multiply

This patch doesn't really need sr.

I can't think of any fp precision issues that can result from this, so looks good on my end!

Comment 3

[Joe Drew (not getting mail)]

Comment on attachment 308253 [details] [diff] [review]
Change divisions to multiply

Should've mentioned that I still want Jeff to look at it, because he is very smart.

Comment 4

[Jeff Muizelaar [:jrmuizel]]

Comment on attachment 308253 [details] [diff] [review]
Change divisions to multiply

I don't know that the performance of this code really matters and the new code is less clear. Still it doesn't really hurt either and does save a couple of cycles so why not.

Comment 5

[Reed Loden [:reed] (use needinfo?)]

http://hg.mozilla.org/mozilla-central/rev/69bcd1261933