On Tue, 04 Oct 2011 23:55:21 -0700, Kenneth Graunke <kenn...@whitecape.org> wrote: > 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.
Yeah, that seems just plain broken, based on the "0 - arg0" result. > 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 think it's using the result-that-goes-to-the-accumulator-if-you've-enabled-that, which is something that does seem likely to have been unmentioned because this is the condmod is the only piece that would be using it. > 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. OK. I realized that I wanted to check and be sure that this issue has existed across generations, so I'll wait to push until then.
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