[Mesa-dev] [00/21] Last of SSO, take 47

[Ian Romanick]

On 04/30/2014 05:13 PM, Eric Anholt wrote:
> Ian Romanick <idr at freedesktop.org> writes:
> 
>> On 04/30/2014 12:41 PM, Eric Anholt wrote:
>>> Ian Romanick <idr at freedesktop.org> writes:
>>>
>>>> A lot the patches in this series were slightly reworked to incorporate
>>>> Eric's feedback (remove ir_variable::user_location) on the previous
>>>> attempt.  Other than that change, I patch 1 is new, and patch 16 adds
>>>> previously missing display list support.
>>>
>>> Patch 2, 3, 13, 16, 17, 18, 20, 21 are:
>>>
>>> Reviewed-by: Eric Anholt <eric at anholt.net>
>>>
>>> I think my patch 15 comments were minor, and if you agree with them
>>> (meaning I actually understood what was going on), then apply the r-b
>>> there too.
>>>
>>> Patch 19 is:
>>>
>>> Acked-by: Eric Anholt <eric at anholt.net>
>>>
>>> Patch 14 I don't think is needed (since we've got explicit locations
>>> being assigned to corresponding varying slots).
>>
>> The sorting is so that a vertex shader that does
>>
>>     out vec4 a;
>>     out vec4 b;
>>
>> and a fragment shader that does
>>
>>     in vec4 b;
>>     in vec4 a;
>>
>> will assign the same locations for a and b.  If both shaders have the
>> same set of varyings, they'll all get the same locations.  Without
>> sorting, we assign the locations based on the order in which variables
>> appear in the shader text.
> 
> In SSO, we aren't assigning locations, because we have explicit
> locations for everything.  In non-SSO, we either have explicit
> locations, or we're matching ir_variables up by name and assigning the
> dynamic locations to the same name across both producer and consumer.

Ah, but we are.  Rendezvous by name still works, even with separable
programs.  Rendezvous by location works even with explicitly linked
programs.

Applications are even allowed to use explicit locations for some
varyings and rendezvous by name for others.

Relevant spec text:

    With separable program objects, interfaces between shader stages may
    involve the outputs from one program object and the inputs from a
    second program object.  For such interfaces, it is not possible to
    detect mismatches at link time, because the programs are linked
    separately.  When each such program is linked, all inputs or outputs
    interfacing with another program stage are treated as active.  The
    linker will generate an executable that assumes the presence of a
    compatible program on the other side of the interface.  If a mismatch
    between programs occurs, no GL error will be generated, but some or all
    of the inputs on the interface will be undefined.

    At an interface between program objects, the set of inputs and outputs are
    considered to match exactly if and only if:

      * The built-in input and output blocks used on the interface
        ("gl_PerVertex" or "gl_PerFragment") match, as described below.

      * Every declared input block or variable must have a matching output, as
        described above.

      * There are no output blocks or user-defined output variables
        declared without a matching input block or variable declaration.

    When the set of inputs and outputs on an interface between programs
    matches exactly, all inputs are well-defined unless the corresponding
    outputs were not written in the previous shader.  However, any mismatch
    between inputs and outputs results in all inputs being undefined except
    for cases noted below.  Even if an input has a corresponding output
    that matches exactly, mismatches on other inputs or outputs may
    adversely affect the executable code generated to read or write the
    matching variable.

The important part is "...the set of inputs and outputs on an interface
between programs matches exactly..." from the last paragraph. If the
declared inputs to the fragment shader exactly match the declared
outputs of the vertex shader, it "just works."  The implication being
that locations are assigned by the linker in a deterministic way.

This combination of use modes is a big part of the reason this has
taken so long to finish.  Gregory's initial patches had *no* support
for rendezvous by location, and it was working with his shaders that
also worked with AMD and NVIDIA.  Also, if I recall correctly, all of
the shaders in Serious Sam use rendezvous by name, but I could be
mistaken.  I believe use the exact same shader code in linked and
separable modes.


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