[RFC] Adding a new attribute to function param to mark it as constant

[Richard Earnshaw]

On 04/08/2021 18:59, Segher Boessenkool wrote:
> On Wed, Aug 04, 2021 at 07:08:08PM +0200, Florian Weimer wrote:
>> * Segher Boessenkool:
>>
>>> On Wed, Aug 04, 2021 at 03:27:00PM +0100, Richard Earnshaw wrote:
>>>> On 04/08/2021 14:40, Segher Boessenkool wrote:
>>>>> On Wed, Aug 04, 2021 at 02:00:42PM +0100, Richard Earnshaw wrote:
>>>>>> We don't want to have to resort to macros.  Not least because at some
>>>>>> point we want to replace the content of arm_neon.h with a single #pragma
>>>>>> directive to remove all the parsing of the header that's needed.  What's
>>>>>> more, if we had a suitable pragma we'd stand a fighting chance of being
>>>>>> able to extend support to other languages as well that don't use the
>>>>>> pre-processor, such as Fortran or Ada (not that that is on the cards
>>>>>> right now).
>>>>>
>>>>> So how do you want to handle constants-that-are-not-yet-constant, say
>>>>> before inlining?  And how do you want to deal with those possibly not
>>>>> ever becoming constant, perhaps because you used a too low "n" in -On
>>>>> (but there are very many random other causes)?  And, what *is* a
>>>>> constant, anyway?  This is even more fuzzy if you consider those
>>>>> other languages as well.
>>>>>
>>>>> (Does skipping parsing of some trivial header save so much time?  Huh!)
>>>>
>>>> Trivial?  arm_neon.h is currently 20k lines of source.  What's more, it 
>>>> has to support inline functions that might not be available when the 
>>>> header is parsed, but might become available if the user subsequently 
>>>> compiles a function with different attributes enabled.  It is very 
>>>> definitely *NOT* trivial.
>>>
>>> Ha yes :-)  I just assumed without looking that it would be like other
>>> architectures' intrinsics headers.  Whoops.
>>
>> But isn't it?
>>
>> $ echo '#include <x86intrin.h>' | gcc -E - | wc -l
>> 41045
> 
> $ echo '#include <altivec.h>' | gcc -E - -maltivec | wc -l
> 9
> 
> Most of this file (774 lines) is #define's, which take essentially no
> time at all.  And none of the other archs I have looked at have big
> headers either!
> 
> 
> Segher
> 

arm_sve.h isn't large either, but that's because all it contains (other
than a couple of typedefs is

#pragma GCC aarch64 "arm_sve.h"

:)

R.