On 01/28/2015 02:04 PM, Matt Turner wrote: > 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/
But... it will produce the same "wrong" result on all GPUs. My guess is that's the actual goal. The GLSL spec says round() does "something" with x.5, and I suspect the HLSL documentation says something similar. On older version of the language that didn't have roundEven(), developers probably just wanted something consistent. I don't think the original code handles -0 correctly either. I wouldn't expect myRound(-0.0) to give +0. That's an awfully fine hair to split, I guess. >> I think 'round(x + (intToFloatBits(floatToIntBits(.5) | >> (floatToIntBits(x) & 0x80000000))))' should produce the same result. >> That should be 4 instructions, I think. My code above incorrectly says round instead of trunc. > 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? Something like trunc(x + mix(.5, -.5, sign(x) < 0.0)). I guess that would probably be 5 instructions due to loading .5 into a register. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
