wishlist: support for shorter pointers
Rafał Pietrak
embedded@ztk-rp.eu
Wed Jul 5 09:42:21 GMT 2023
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
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