C++ compiler warning: compound assignment with 'volatile'-qualified left operand is deprecated

[jonathangjertsen]

Including hardware/pio.h from a C++ file generates warnings like the following. My compiler version is arm-none-eabi-g++ (GNU Toolchain for the Arm Architecture 11.2-2022.02 (arm-11.16)) 11.2.1 20220111.

In file included from <snip>/ws2812.cpp:1:
<snip>/pio.h: In function 'void pio_sm_restart(PIO, uint)':
<snip>/pio.h:583:15: error: compound assignment with 'volatile'-qualified left operand is deprecated [-Werror=volatile]
  583 |     pio->ctrl |= 1u << (PIO_CTRL_SM_RESTART_LSB + sm);
      |     ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<snip>/pio.h: In function 'void pio_restart_sm_mask(PIO, uint32_t)':
<snip>/pio.h:598:15: error: compound assignment with 'volatile'-qualified left operand is deprecated [-Werror=volatile]
  598 |     pio->ctrl |= (mask << PIO_CTRL_SM_RESTART_LSB) & PIO_CTRL_SM_RESTART_BITS;
<snip>/pio.h: In function 'void pio_sm_clkdiv_restart(PIO, uint)':

... and so on.

This is because operations such as |= and &= on volatile variables is deprecated in C++20. Here is a summary of the change: https://blog.feabhas.com/2021/05/modern-embedded-c-deprecation-of-volatile/

What the C++ committee wants you to do is to rewrite each such statement as follows:

- pio->ctrl |= 1u << (PIO_CTRL_SM_RESTART_LSB + sm);
+ pio->ctrl = pio->ctrl | (1u << (PIO_CTRL_SM_RESTART_LSB + sm));

Sample of responses from other projects:

• Zephyr silenced the warning by adding -Wno-volatile, pointing to the fact that they do not control the CMSIS header files.
• CMSIS will not fix the issue in their files as they do not officially support C++. The maintainer's suggestion that it should work with extern "C" is not correct since that's just a linkage directive.
• esp-idf updated some macros which generated this warning.
• STMCubeWB has not done anything

[jonathangjertsen]

I tried compiling a C++ version of kitchen_sink.c and found that only pio.h and interp.h are affected, as those are the only files which perform that kind of operation. I will make a PR to make them C++20 compliant

[geurtv]

This has been de-deprecated in C++23, so in my opinion the code should be left unchanged. Instead add -Wno-volatile to suppress this warning (until GCC13 is available). See P2327R1: De-deprecating volatile compound operations.

[jonathangjertsen]

This has been de-deprecated in C++23, so in my opinion the code should be left unchanged. Instead add -Wno-volatile to suppress this warning (until GCC13 is available). See P2327R1: De-deprecating volatile compound operations.

Ah, that's great, I thought it was a strange decision. maintainers please feel free to decline #1018 and add the flag in the appropriate place

[kilograham]

merged into develop