[Mesa-dev] ARB_sso layout() + other qualifiers

[Chris Forbes]

Hi Ian,

When I was writing the `precise` support I found some error cases in
the GLSL parser where we reject combinations of layout() with
invariant / interpolation / etc qualifiers.

This seems to be consistent with the GLSL 1.50 grammar (or, at least,
admits all the examples that were given in various GLSL specs and
extension specs), but I don't think it works any more with SSO, since
you'd want to be able to do rendezvous-by-location on qualified
variables. The body of the ARB_sso spec doesn't clearly make the
changes required to allow this, but various parts of the spec hint at
it being possible, with the most obvious being in the resolution of
issue 13:

    13. How are interpolation modifiers handled for separate shader
        programs?

        RESOLVED:  GLSL only provides interpolation modifiers for user-
        defined varyings. These modifiers can be used in conjunction
        with the layout location qualifiers specified in this extension.
        Such modifiers must match.

I propose relaxing the rules in type_qualifier as follows:

* If neither GLSL 4.20 nor ARB_shading_language_420pack is supported,
then require layout qualifiers to precede any other qualifiers;
continue to disallow multiple layout qualifiers.

* Remove all other error generation for combining layout with
invariant / interpolation / (with my other patches) precise.

I think this retains all the useful current behavior, and will accept
all the examples I've seen.

-- Chris