Re: wishlist: support for shorter pointers

[David Brown <david@westcontrol.com>]

On 05/07/2023 10:05, Rafał Pietrak via Gcc wrote:
> Hi,
> 
> W dniu 5.07.2023 o 09:29, Martin Uecker pisze:
>> Am Mittwoch, dem 05.07.2023 um 07:26 +0200 schrieb Rafał Pietrak:
> [-------]
>>> And if it's so ... there is no mention of how does it show up for
>>> "simple user" of the GCC (instead of the use of that "machinery" by
>>> creators of particular GCC port). In other words: how the sources should
>>> look like for the compiler to do "the thing"?
>>>
>>
>> Not sure I understand the question.  You would add a name space
>> to an object as a qualifier and then the object would be allocated
>> in a special (small) region of memory.  Pointers known to point
>> into that special region of memory (which is encoded into the
>> 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.)

> 
> Apparently you do understand my question.
> 
> Then again ... apparently you are guessing the answer. Incidentally, 
> that would be my guess, too. And while such "syntax" is not really 
> desirable (since such attribution at every declaration of every "short 
> pointer" variable would significantly obfuscate the sources and a 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++ !

In my personal opinion (which you are all free to disregard), named 
address spaces were an interesting idea that failed.  I was enthusiastic 
about a number of the extensions in TR 18307 "C Extensions to support 
embedded processors" when the paper was first published.  As I learned 
more, however, I saw it was a dead-end.  The features are too 
under-specified to be useful or portable, gave very little of use to 
embedded programmers, and fit badly with C.  It was an attempt to 
standardise and generalise some of the mess of different extensions that 
proprietary toolchain developers had for a variety of 8-bit CISC 
microcontrollers that could not use standard C very effectively.  But it 
was all too little, too late - and AFAIK none of these proprietary 
toolchains support it.  GCC supports some of the features to some extent 
- a few named address spaces on a few devices, for "gnuc" only (not 
standard C, and not C++), and has some fixed point support for some 
targets (with inefficient generated code - it appears to be little more 
than an initial "proof of concept" implementation).

I do not think named address spaces have a future - in GCC or anywhere 
else.  The only real use of them at the moment is for the AVR for 
accessing data in flash, and even then it is of limited success since it 
does not work in C++.


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.

David