[Mesa-dev] [PATCH 1/2] mesa: Detect and provide macros for function attributes pure and const.

[Jose Fonseca]

On 22/07/15 21:01, Jose Fonseca wrote:
> On 22/07/15 17:13, Jose Fonseca wrote:
>> On 21/07/15 15:57, Emil Velikov wrote:
>>> On 18 July 2015 at 08:13, Jose Fonseca <jfonseca at vmware.com> wrote:
>>>> On 18/07/15 01:38, Eric Anholt wrote:
>>>>>
>>>>> Emil Velikov <emil.l.velikov at gmail.com> writes:
>>>>>
>>>>>> On 14/07/15 19:45, Eric Anholt wrote:
>>>>>>>
>>>>>>> These are really useful hints to the compiler in the absence of
>>>>>>> link-time
>>>>>>> optimization, and I'm going to use them in VC4.
>>>>>>>
>>>>>>> I've made the const attribute be ATTRIBUTE_CONST unlike other
>>>>>>> function
>>>>>>> attributes, because we have other things in the tree #defining
>>>>>>> CONST for
>>>>>>> their own unrelated purposes.
>>>>>>
>>>>>> Mindly related: how people feel about making these macros less
>>>>>> screamy,
>>>>>> by following the approach used in the kernel: PURE -> __pure and so
>>>>>> on ?
>>>>>
>>>>>
>>>>> I'd love it.
>>>>
>>>>
>>>> Less screamy is fine, but beware prefixing double underscore: the C
>>>> standard
>>>> stipulates that its use is reserved for for C/C++ runtime. [1]
>>>>
>>> I though about it before posting although I've seen others define
>>> those, even do so in their public headers.
>>> Now that I have some examples from my current /usr/include
>>>
>>> Searching for __pure
>>> dwarves/dutil.h:#define __pure __attribute__ ((pure))
>>>
>>> Searching for __attribute_const__
>>> sys/cdefs.h:# define __attribute_const__ __attribute__ ((__const__))
>>> sys/cdefs.h:# define __attribute_const__ /* Ignore */
>>>
>>> Searching for __printf
>>>
>>> Searching for __always_unused
>>>
>>> Searching for __noreturn
>>>
>>> Searching for __packed
>>> libvisual-0.4/libvisual/lv_defines.h:# define __packed __attribute__
>>> ((packed))
>>> libvisual-0.4/libvisual/lv_defines.h:# define __packed /* no packed */
>>> bsd/sys/cdefs.h:#  define __packed __attribute__((__packed__))
>>> bsd/sys/cdefs.h:#  define __packed
>>>
>>> Searching for __deprecated
>>> pciaccess.h:#define __deprecated __attribute__((deprecated))
>>> pciaccess.h:#define __deprecated
>>>
>>> Searching for __weak
>>>
>>> Searching for __alias
>>>
>>> With a handful of other headers defining more double underscore
>>> prefixed macros.
>>>
>>>> Look at stdlibc++ implementation: every internal variable has a double
>>>> underscore prefix.
>>>>
>>> Unless we're talking about STL/other template library we don't care
>>> what library foo uses in it's internal implementation do we ? After
>>> all these will be resolved at compile time.
>>>
>>>> Maybe kernel gets away on GLIBC (and because it doesn't use C++), but
>>>> there's no guarantee it will work on other C runtimes, and even if it
>>>> does,
>>>> it could start failing anytime.
>>>>
>>> True, it's not the best of ideas. Just worth pointing out that "the
>>> cat is already out", for other projects.
>>
>>
>>  >  There are already more than 12K "#define __foo" cases on my system.
>>
>> These defines are reserved for system headers, so it's natural to be
>> lots of them in /usr/include.
>>
>>
>> MacOSX also defines some of these on its sys/cdefs.h:
>>
>>
>> http://www.opensource.apple.com/source/xnu/xnu-1456.1.26/bsd/sys/cdefs.h
>>
>> The question is: can we expect that most systems will define these
>> __foo, or at least not use them for other purposes.
>>
>> I don't know the answer.  At a glance MSVC doesn't seem to rely on them
>> for anything.  So it might work.  I don't oppose if you want to give it
>> a shot.
>>
>>
>> Jose
>
> Ironically, Windows headers already define PURE (its used in COM
> interfaces).  Just realized this now after this was merged.
>
> So we really need a different name...

Question is which name: "ATTRIBUTE_PURE", "__pure", something else?

As I said, I don't oppose __pure, but some logic will be necessary to 
avoid redefition when system headers (like sys/cdefs.h) already define 
it, which is not trivial.  It will need something like


    #if HAVE_SYS_CDEFS_H
    #include <sys/cdefs.h>  // for __pure
    #endif
    #ifndef __pure
    ....
    #endif


Jose