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

[Segher Boessenkool <segher@kernel.crashing.org>]

On Thu, Aug 05, 2021 at 02:31:02PM +0530, Prathamesh Kulkarni wrote:
> On Wed, 4 Aug 2021 at 18:30, Richard Earnshaw
> <Richard.Earnshaw@foss.arm.com> 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).
> Hi,
> IIUC, a more general issue here, is that the intrinsics require
> special type-checking of arguments, beyond what is dictated by the
> Standard.
> An argument needing to be an ICE could be seen as one instance.
> 
> So perhaps, should there be some mechanism to tell the FE to let the
> target do additional checking for a particular function call, say by

An integer constant expression can be checked by the frontend itself, it
does not depend on optimisation etc.  That is the beauty of it: it is a)
more local, and b) a more reliable / less surprising thing to use.

But it *is* less powerful than "it is a constant integer after a travel
through the bowels of the compiler".  Which of course is less reliable
and more surprising (think what happens if you use -O0 or -O1 or -Og or
-Os or any -fno- etc.)  So it will be a lot more maintenance work
(answering PRs about it is only the start).


Segher