Re: wishlist: support for shorter pointers

["Rafał Pietrak" <embedded@ztk-rp.eu>]

Hi,

W dniu 5.07.2023 o 11:11, David Brown pisze:
> On 05/07/2023 10:05, Rafał Pietrak via Gcc wrote:
[-----------]
>>> type) would then be smaller.  At least, this is my understanding
>>> of how it could work.
> 
> Note that this only applies to pointers declared to be of the address 
> space specific type.  If you have "__smalldata int x;" using a 
> hypothetical new address space, then "&x" is of type "__smalldata int *" 
> and you need to specify the address space specific pointer type to get 
> the size advantages.  (Since the __smalldata address space is a subset 
> of the generic space, conversions between pointer types are required to 
> work correctly.)

I see.

[--------]
>> thing like "#pragma" at the top of a file would do a better job), 
>> better something then nothing. Then again, should you happen to fall 
>> onto an actual documentation of syntax to use this feature with, I'd 
>> appreciate you sharing it :)
>>
> 
> I am not sure if you are clear about this, but the address space 
> definition macros here are for use in the source code for the compiler, 
> not in user code.  There is (AFAIK) no way for user code to create 
> address spaces - you need to check out the source code for GCC, modify 
> it to support your new address space, and build your own compiler.  This 
> is perfectly possible (it's all free and open source, after all), but it 
> is not a minor undertaking - especially if you don't like C++ !

Hmmm.

Wouldn't it be easier and more natural to make the "named spaces" a 
synonym to specific linker sections (like section names, or section name 
prefix when instead of ".data.array.*" one gets ".mynamespace.array.*")?

[------]
> I realise that learning at least some C++ is a significant step beyond 
> learning C - but /using/ C++ classes or templates is no harder than C 
> coding.  And it is far easier, faster and less disruptive to make a C++ 
> header library implementing such features than adding new named address 
> spaces into the compiler itself.
> 
> The one key feature that is missing is that named address spaces can 
> affect the allocation details of data, which cannot be done with C++ 
> classes.  You could make a "small_data" class template, but variables 
> would still need to be marked __attribute__((section(".smalldata"))) 
> when used.  I think this could be handled very neatly with one single 
> additional feature in GCC - allow arbitrary GCC variable attributes to 
> be specified for types, which would then be applied to any variables 
> declared for that type.

OK. I see your point.

But let's have look at it. You say, that "names spaces affect allocation 
details, which cannot be done with C++". Pls consider:
1. for small embedded devices C++ is not a particularly "seller". We 
even turn to assembler occasionally.
2. affecting allocation details is usually the hole point of engineering 
skills when dealing with small embedded devices - the hole point is to 
have tools to do that.

So your current objections to named spaces ... are in fact in favor of 
them. Isn't it so?

-R