Re: wishlist: support for shorter pointers

[David Brown <david@westcontrol.com>]

On 03/07/2023 18:42, Rafał Pietrak via Gcc wrote:
> Hi Ian,
> 
> W dniu 3.07.2023 o 17:07, Ian Lance Taylor pisze:
>> On Wed, Jun 28, 2023 at 11:21 PM Rafał Pietrak via Gcc 
>> <gcc@gcc.gnu.org> wrote:
> [--------]
>>> I was thinking about that, and it doesn't look as requiring that deep
>>> rewrites. ABI spec, that  could accomodate the functionality could be as
>>> little as one additional attribute to linker segments.
>>
>> If I understand correctly, you are looking for something like the x32
>> mode that was available for a while on x86_64 processors:
>> https://en.wikipedia.org/wiki/X32_ABI .  That was a substantial amount
>> of work including changes to the compiler, assembler, linker, standard
>> library, and kernel.  And at least to me it's never seemed
>> particularly popular.
> 
> Yes.
> 
> And WiKi reporting up to 40% performance improvements in some corner 
> cases is impressive and encouraging. I believe, that the reported 
> average of 5-8% improvement would be significantly better within MCU 
> tiny resources environment. In MCU world, such improvement could mean 
> fit-nofit of a project into a particular device.
> 
> -R
> 

A key difference is that using 32-bit pointers on an x86 is enough 
address space for a large majority of use-cases, while even on the 
smallest small ARM microcontroller, 16-bit is not enough.  (It's not 
even enough to access all memory on larger AVR microcontrollers - the 
only 8-bit device supported by mainline gcc.)  So while 16 bits would 
cover the address space of the RAM on a small ARM microcontroller, it 
would not cover access to code/flash space (including read-only data), 
IO registers, or other areas of memory-mapped memory and peripherals. 
Generic low-level pointers really have to be able to access everything.

So an equivalent of x32 mode would not work at all.  Really, what you 
want is a 16-bit "small pointer" that is added to 0x20000000 (the base 
address for RAM in small ARM devices, in case anyone following this 
thread is unfamiliar with the details) to get a real data pointer.  And 
you'd like these small pointers to have convenient syntax and efficient use.

I think a C++ class (or rather, class template) with inline functions is 
the way to go here.  gcc's optimiser will give good code, and the C++ 
class will let you get nice syntax to hide the messy details.

There is no good way to do this in C.  Named address spaces would be a 
possibility, but require quite a bit of effort and change to the 
compiler to implement, and they don't give you anything that you would 
not get from a C++ class.

(That's not quite true - named address spaces can, I believe, also 
influence the section name used for allocation of data defined in these 
spaces, which cannot be done by a C++ class.)

David