Re: [PATCH v3] c++: parser - Support for target address spaces in C++

[Georg-Johann Lay <avr@gjlay.de>]

> [PATCH v3] c++: parser - Support for target address spaces in C++

First of all, it is great news that GCC is going to implement named 
address spaces for C++.

I have some questions:

1. How is name-mangling going to work?
======================================

Clang supports address spaces in C++, and for address-space 1 it does 
generate code like the following:

#define __flash __attribute__((__address_space__(1)))

char get_p (const __flash char *p)
{
     return *p;
}


_Z5get_pPU3AS1Kc:
    ...

I.e. address-space 1 is mangled as "AS1".

(Notice that Clang's attribute actually works like a qualifier here, one 
could not get this to work with GCC attributes.)


2. Will it work with compound literals?
=======================================

Currently, the following C code works for target avr:

const __flash char *pHallo = (const __flash char[]) { "Hallo" };

This is a pointer in RAM (AS0) that holds the address of a string in 
flash (AS1) and is initialized with that address. Unfortunately, this 
does not work locally:

const __flash char* get_hallo (void)
{
     [static] const __flash char *p2 = (const __flash char[]) { "Hallo2" };
     return p2;
}

foo.c: In function 'get_hallo':
foo.c: error: compound literal qualified by address-space qualifier

Is there any way to make this work now? Would be great!


3. Will TARGET_ADDR_SPACE_DIAGNOSE_USAGE still work?
====================================================

Currently there is target hook TARGET_ADDR_SPACE_DIAGNOSE_USAGE.
I did not see it in your patches, so maybe I just missed it? See
https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gccint/Named-Address-Spaces.html#index-TARGET_005fADDR_005fSPACE_005fDIAGNOSE_005fUSAGE


4. Will it be possible to put C++ virtual tables in ASs, and how?
=================================================================

One big complaint about avr-g++ is that there is no way to put vtables 
in flash (address-space 1) and to access them accordingly.  How can this 
be achieved with C++ address spaces?

Background: The AVR architecture has non-linear address space, and you 
cannot tell from the numeric value of an address whether it's in RAM or 
flash. You will have to use different instructions depending on the 
location.

This means that .rodata must be located in RAM, because otherwise one 
would not know whether const char* pointed to RAM or flash, but to 
de-reference you's need different instructions.

One way out is named address spaces, so we could finally fix

https://gcc.gnu.org/PR43745


Regards,

Johann