[Bug c++/108642] New: ACLE function __arm_wsr missing when compiling in C++ mode for AArch64

["david.spickett at linaro dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108642

            Bug ID: 108642
           Summary: ACLE function __arm_wsr missing when compiling in C++
                    mode for AArch64
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: david.spickett at linaro dot org
  Target Milestone: ---

This was found by a user trying to compile llvm's libc (which is largely
written in c++) using g++ 8.5.0 for AArch64. I then tried versions up to 12.2.0
and a recent trunk build.

https://godbolt.org/z/qxaYxdTcv, and I've copied the details below.

The following source:
```
#include <stdint.h>
#include <arm_acle.h>

void writeStatusWord(uint32_t fpsr) { __arm_wsr("fpsr", fpsr); }
```

When compiled with g++ gives the error:
```
<source>:4:39: error: '__arm_wsr' was not declared in this scope
    4 | void writeStatusWord(uint32_t fpsr) { __arm_wsr("fpsr", fpsr); }
      |                                       ^~~~~~~~~
```

When compiled as C, there is instead a warning and it does compile:
```
<source>:4:39: warning: implicit declaration of function '__arm_wsr'
[-Wimplicit-function-declaration]
    4 | void writeStatusWord(uint32_t fpsr) { __arm_wsr("fpsr", fpsr); }
      |                                       ^~~~~~~~~
```

The expected result would be that in either mode, we can compile without errors
or warnings.

This __arm_wsr function should be present after including arm_acle.h according
to https://github.com/ARM-software/acle/releases/tag/r2022Q4 (acle-2022Q4.pdf
). See "11.1 Special register intrinsics".

I looked in the arm_acle.h gcc/g++ is including and see no reference to it.
Which makes me wonder if it's a compiler builtin that somehow isn't present for
C++.

Over in clang there is a builtin for it, that is then #defined into the
__arm_wsr name in the header.