[Mesa-dev] [PATCH] radeonsi: enable 32-bit denormals on VI+

[Marek Olšák]

On Thu, Jan 12, 2017 at 12:33 AM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> On Wed, Jan 11, 2017 at 4:00 PM, Roland Scheidegger <sroland at vmware.com> wrote:
>> Am 11.01.2017 um 21:08 schrieb Samuel Pitoiset:
>>>
>>>
>>> On 01/11/2017 07:00 PM, Roland Scheidegger wrote:
>>>> I don't think there's any glsl, es or otherwise, specification which
>>>> would require denorms (since obviously lots of hw can't do it, d3d10
>>>> forbids them), with any precision qualifier. Hence these look like bugs
>>>> of the test suite to me?
>>>> (Irrespective if it's a good idea or not to enable denormals, which I
>>>> don't realy know.)
>>>
>>> That test works on NVIDIA hw (both with blob and nouveau) and IIRC it
>>> also works on Intel hw. I don't think it's buggy there.
>> The question then is why it needs denorms on radeons...
>
> I spent some time with Samuel looking at this. So, this is pretty
> funny... (or at least feels that way after staring at floating point
> for a while)
>
> dEQP is, in fact, feeding denorms to the min/max functions. But it's
> smart enough to know that flushing denorms to 0 is OK, and so it
> treats a 0 as a pass. (And obviously it treats the "right answer" as a
> pass.) So that's why enabling denorm processing fixes it - that causes
> the hw to return the proper correct answer and all is well.
>
> However the issue is that without denorm processing, the hw is
> returning the *wrong* answer. At first I thought that max was being
> lowered into something like
>
> if (a > b) { x = a; } else { x = b; }
>
> which would end up with potentially wrong results if a and b are being
> flushed as inputs into the comparison but not into the assignments.
> But that's not (explicitly) what's happening - the v_max_f32_e32
> instruction is being used. Perhaps that's what it does internally? If
> so, that means that results of affected float functions in LLVM need
> explicit flushing before being stored into results.
>
> FWIW the specific values triggering the issue are:
>
> in0=-0x0.000002p-126, in1=-0x0.fffffep-126, out0=-0x0.fffffep-126 -> FAIL
>
> With denorm processing, it correctly reports out0=-0x0.000002p-126,
> while nouveau with denorm flushing enabled reports out0=0.0 which also
> passes.

The denorm configuration has 2 bits:
- flush (0) or allow (1) input denorms
- flush (0) or allow (1) output denorms

In the case of v_max, it looks like output denorms are not flushed and
it behaves almost like you said:

if (a >= b) { x = a; } else { x = b; }

Marek