On Wed, Jan 28, 2015 at 1:53 PM, Ian Romanick <i...@freedesktop.org> wrote: > On 01/28/2015 11:51 AM, Matt Turner wrote: >> What I'm asking is whether we suspect that they specifically want >> half-up behavior (speculation, so not likely insightful), or if >> there's a way we can emulate round-half-up behavior using round-even >> in fewer than four instructions. > > They may or may not want half-up or round-even or something else. If we > change it, someone will see different pixels, and they will probably > report a bug. It seems better to play it safe.
It's interesting to note [0] that floor(x + 0.5) fails to give the proper result (0.0) for the largest value smaller than 0.5 (in doubles and floats). So what they've given us isn't even half-up technically. [0] http://ericlippert.com/2013/05/16/spot-the-defect-rounding-part-two/ > I think 'round(x + (intToFloatBits(floatToIntBits(.5) | > (floatToIntBits(x) & 0x80000000))))' should produce the same result. > That should be 4 instructions, I think. Yeah, that's the four instruction sequence I was thinking of. > Were you thinking of using CSEL > for your 4 instruction version? I'm not sure. You must be a step ahead of me. Where could we use CSEL? _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
