[Mesa-dev] [PATCH] glsl: Add a citation for uniform precision matching.

[Tapani Pälli]

On 09/07/2016 12:37 PM, Kenneth Graunke wrote:
> On Wednesday, September 7, 2016 10:42:06 AM PDT Tapani Pälli wrote:
>> On 09/07/2016 09:56 AM, Ian Romanick wrote:
>>> On 09/06/2016 03:24 PM, Kenneth Graunke wrote:
>>>> On Tuesday, September 6, 2016 1:04:43 PM PDT Eric Anholt wrote:
>>>>> Kenneth Graunke <kenneth at whitecape.org> writes:
>>>>>
>>>>>> Ian added this check in commit 259fc505454ea6a67aeacf6cdebf1398d9947759.
>>>>>> While reviewing the rules, I found a citation which spells this out
>>>>>> clearly, so I figured I'd send a patch to add it as a comment.
>>>>>>
>>>>>> Cc: idr at freedesktop.org
>>>>>> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
>>>>> Reviewed-by: Eric Anholt <eric at anholt.net>
>>>>>
>>>>> I was wondering about this change, because glmark2 is failing to compile
>>>>> its terrain shaders now.
>>> Out of curiosity... is the uniform used in all the stages?
>>>
>>>> Really?  GLBenchmark 2.7 also fails to compile:
>>>>
>>>> https://bugs.freedesktop.org/show_bug.cgi?id=97532
>>>>
>>>> I'm beginning to doubt whether any other vendor implements this part of
>>>> the spec, or if they have some variation of it.
>>> I added this specifically for a dEQP test.  Presumably people are
>>> passing that.
>>>
>>> dEQP-GLES31.functional.shaders.linkage.geometry.uniform.rules.type_mismatch_1.
>>>
>>> I wonder if this is only enforced in GLSL ES 3.2 shaders?  I'll do some
>>> spec archaeology to see if the language changed at all over the years.  Ugh.
>> As mentioned in the bug 97532 GLSL ES 1.x spec seems a bit vague in this
>> matter and talks about 'warning' (section 10 Issues).
> It seems pretty clear to me...
>
> GLSL ES 1.0.17, Section 10 Issues:
>
>     "Do precision qualifiers for uniforms need to match?
>
>      Option 1: Yes.
>      Uniforms are defined to behave as if they are using the same storage
>      in the vertex and fragment processors and may be implemented this way.
>
>      If uniforms are used in both the vertex and fragment shaders,
>      developers should be warned if the precisions are different.
>      Conversion of precision should never be implicit.
>
>      Option 2: No.
>      Uniforms may be used by both shaders but the same precision may not
>      be available in both so there is a justification for allowing them
>      to be different.
>
>      Using the same uniform in the vertex and fragment shaders will
>      always require the precision to be specified in the vertex shader
>      (since the default precision is highp). This is an unnecessary
>      burden on developers.
>
>      RESOLUTION: Yes, precision qualifiers for uniforms must match."
>
> The "options" are just suggestions that were discussed when drafting the
> spec.  I believe the "developers should be warned" comment in option 1
> is expressing the sentiment that silently handling mismatched precision
> behind the developers' back is doing them a disservice.  Plus, the only
> way to issue a warning but handle it would be to implicitly convert one
> to a different precision...and they say it "should never be implicit."
>
> Regardless, the important part is the resolution: they must match.
> Even better, the actual body of the spec says they must match:
>
> GLSL ES 1.0.17, Section 4.3.4 Uniform:
>
>     "When the vertex and fragment shaders are linked together, then they
>      will share a single global uniform name space. Hence, types and
>      precisions of uniforms with the same name must match across all
>      shaders that are linked into a single executable."

Yes agreed, this is true and is definitively stronger argument than 
'Issues section'.

> I noticed that there's several years of discussion in the Khronos
> bugzilla, though, which may offer some insight (or a lot of confusion).

:)

> --Ken


// Tapani