[Mesa-dev] [PATCH 2/2] glsl: Rewrote _mesa_glsl_process_extension to use table-driven logic.

[Ian Romanick]

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On 06/28/2011 06:49 AM, Paul Berry wrote:
> On 27 June 2011 18:30, Ian Romanick <idr at freedesktop.org> wrote:
>> I like this a lot.  It's a really good clean up of a rotting cesspool.
> 
> Thanks!
> 
>>>   that are avaiable in geometry shaders.
>>
>>             available
> 
> *smacks forehead*  Oops.
> 
>>> +   /* Name of the extension when referred to in a GLSL extension
>>> +    * statement */
>>
>> Field comments should get the doxygen
>>
>>    /**
>>     * Short description
>>     *
>>     * Detailed description
>>     */
>>
>> or
>>
>>    /** Description */
>>
>> treatment.
>>
>> Also, we generally prefer the closing */ of a multiline comment to be on
>> its own line.
> 
> Ok, I can do doxygen-style.  BTW, is there a web site where the
> doxygen-extracted documentation for mesa is automatically uploaded?
> It would be convenient to be able to browse the docs without having to
> build them locally.
> 
>>> +   /* Flag in the gl_extensions struct indicating whether this
>>> +    * extension is supported by the driver, or
>>> +    * &gl_extensions::dummy_true if supported by all drivers */
>>> +   const GLboolean gl_extensions::* supported_flag;
>>
>> WTF?  Seriously.  What does this do?  I was expecting to see this code
>> use the offsetof macro (like src/mesa/main/extensions.c does), but I'm
>> now suspecting that C++ has some built-in magic for this.  Is that true?
> 
> Yes.  The feature is called "pointer to data member" and I'm surprised
> it doesn't get more press, considering how frequently people reinvent
> this particular wheel.  The syntax is pretty straightforward once you
> get the hang of it:
> 
> - foo bar::* p declares p to be an "offset" to a field of type foo
> that exists within struct bar
> - &bar::baz computes the "offset" of field baz within struct bar
> - x.*p accesses the field of x that exists at "offset" p
> - x->*p is equivalent to (*x).*p
> 
> I hope my use of this C++ feature doesn't come across as too
> newfangled.  IMHO it's superior to the offsetof macro because (a) it
> can represent null pointers unambiguously, and (b) the compiler
> detects mistakes like referring to a data member of the wrong type, or
> referring to a member of the wrong class (both of which would be
> uncaught by offsetof).
> 
> But it's not always an easy call.  Offsetof, for instance, handles
> structures-within-structures and arrays-within-structures more
> gracefully than pointers to data members do.
> 
> BTW, if you're worried about whether all C++ compilers support this
> feature, rest assured that it has been part of the language since the
> old CFront days.

Wow.  I'm surprised I didn't know about that.  Unless Eric objects, I
approve.

>>> +const _mesa_glsl_extension _mesa_glsl_supported_extensions[] = {
>>
>> This should probably be static.
> 
> Good point.  I'll fix this.
> 
>>> +   case vertex_shader:   if (!this->avail_in_VS) return false; break;
>>> +   case geometry_shader: if (!this->avail_in_GS) return false; break;
>>> +   case fragment_shader: if (!this->avail_in_FS) return false; break;
>>
>> Delete the spurious breaks.
> 
> Geez, what was I smoking?
> 
>>> +   for (unsigned int i = 0; i < Elements(_mesa_glsl_supported_extensions);
>>> +        ++i) {
>>
>> Just 'unsigned'.
> 
> Really?  I'm surprised you care about this detail, esp. considering
> that there are many instances of "unsigned int" already in Mesa.  But
> I'll acquiesce, especially since I'm trying to talk you into letting
> me use a little-known C++ feature above :)

Usually it doesn't matter, but in this case 'unsigned int' causes an
ugly line wrap.  If it weren't for that, it would have been too small a
nit for me to pick.

>>> +            if (extension->compatible_with_state(state)) {
>>> +               _mesa_glsl_supported_extensions[i].set_flags(state, behavior);
>>
>>                  extension->set_flags(state, behavior);
> 
> Yes, you're right, of course.
> 
> Thanks for your comments, Ian.  I'll post a revised patch later this morning.
> 

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