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

[Richard Earnshaw <Richard.Earnshaw@foss.arm.com>]

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.

R.

> 
> Segher
>