[Mesa-dev] [RFC] glsl: Recognize pattern for roundEven().

[Matt Turner]

On Wed, Jan 28, 2015 at 1:53 PM, Ian Romanick <idr at 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?