support removal of register storage class specifier in C++17

[pabigot]

Prior to C++17 the register storage class specifier was deprecated; in C++17 it has been removed, making attempts to compile with CMSIS 5.3 with gcc --std=c++17 fail. Something like the following is needed in every header that uses that feature, along with the corresponding modifications at all use-points.

#if (__cplusplus - 0) >= 201703L
  #define __REGISTER
#else
  #define __REGISTER                             register
#endif

See: http://en.cppreference.com/w/cpp/language/storage_duration

[JonatanAntoni]

May I ask you to be slightly more specific, please?
Where exactly do you have this issue?

Please be aware that CMSIS itself it intended to be ANSI-C (C99) and not C++. Simply removing the storage class register from the code is not guaranteed to work. Depending on the implementation it might be required that a value is kept as a register value only.

Using CMSIS from C++ should be possible when compiling all .c files in C-Mode. Inline functions defined in header files should be compiled as extern C. Please let me know if that does not work for you.

[pabigot]

As of C++17 the existing solution does not work.

CMSIS Core headers like core_cm0.h can be included into C++ source files; this is enabled through the use of the __cplusplus macro to add the required extern "C" block around the declarations. This was sufficient to allow core inline functions like NVIC_SetEnableIRQ() and structures like SCB_Type to be available to and used in C++ applications through C++14.

The change to the register storage class specifier in C++17 means this isn't possible anymore because simply including core_cm0.h into a C++ application pulls in cmsis_compiler.h which pulls in cmsis_gcc.h which declares a local register uint32_t result; which is a syntax error. The modification I suggest is required if the existing capability is to be maintained beyond C++14.

The change can probably be conditional on using C++ at an affected version (as suggested) and isolated to compiler-specific headers like cmsis_gcc.h which should instead declare __REGISTER uint32 result;. For implementation files that use register declarations in C99-compiled code I agree it's not relevant and undesirable.

Some information on the reason for the change is here. As that material indicates gcc completely ignores the register keyword unless all optimizations are explicitly disabled (the __ASM statements in CMSIS include a constraint that ensures a register is used). In practice I've had no problems with gcc-arm-none-eabi-7-2017-q4-major and --std=c++17 with this modification.

[JonatanAntoni]

Thanks for the details. I can know comprehend the issues you face. Its all about the inline functions using raw assembly instructions for special function register accesses like

__STATIC_FORCEINLINE uint32_t __get_PSP(void)
{
    register uint32_t result;
    __ASM volatile ("MRS %0, psp"  : "=r" (result) );
    return(result);
}

In other function we already skipped the register keyword. To be consistent we could remove the register keyword from cmsis_gcc.h completely. The same change should apply to cmsis_armclang.h.

Is it feasible for you to provide the changes in terms of a pull request?

Thanks again for your contribution.

[pabigot]

Yes, I can do a PR sometime in the next week or so. Not sure whether I'll be able to test cmsis_armclang but if not will note that in the PR.

[FreddieChopin]

Alternative pull request which removes the keyword wherever possible - #365

[JonatanAntoni]

Thanks @FreddieChopin for your contribution.
May I ask you all to double check the result, please?

[pabigot]

@JonatanAntoni @FreddieChopin The merged changes work for my application. Thanks.