[Mesa-dev] [PATCH 2/3] i965/fs: Fix comparisions with uint negation.

[Kenneth Graunke]

On 10/03/2011 03:41 PM, Eric Anholt wrote:
> The condmod instruction ends up generating garbage condition codes,
> because apparently the comparison happens on the accumulator value (33
> bits for UD), not the truncated value that would be written.
> 
> Fixes fs-op-neg-*

I am uneasy about these two patches for a number of reasons.

1. fs-op-neg-* and vs-op-neg-* fail on AMD (Catalyst 11.7),
   which makes me wonder if the tests are correct.

AMD does the expected behavior if you change the line:
  uint result = (- arg0);
to this:
  uint result = uint(-int(arg0));
or this:
  uint result = 0 - arg0;

While the GLSL spec isn't terribly clear about how unary negate works on
unsigned, the fact -arg0 != 0 - arg0 seems horrendously broken.  So
maybe AMD's driver is just buggy.

2. I can't seem to find documentation to explain this.

Ivybridge uses explicit CMP instructions rather than the embedded
conditional modifier on the IF instruction, and these tests still fail
there.  So CMP must be using the accumulator as well?  But I see no
documentation that actually states anything of the sort.

I do, however, see some rambling text in the BSpec (sadly not the PRM)
about accumulators, 33-bit precision, and unsigned negates being
unpredictable.

I guess, in lieu of anything else, these patches are probably okay.
I'm still not pleased with the situation, but that's not your fault.

By the way, I mentioned that accumulators had been reworked on
Ivybridge, so I was nervous about these patches.  That text has been
removed from the BSpec (grr!) and your patches work fine on IVB.  So, no
concerns on that front.