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:
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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
