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

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

[david.spickett at linaro dot 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.

[Bug target/108642] ACLE function __arm_wsr missing for AArch64

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2023-02-02
             Target|                            |aarch64-linux-gnu
            Summary|ACLE function __arm_wsr     |ACLE function __arm_wsr
                   |missing when compiling in   |missing for AArch64
                   |C++ mode for AArch64        |
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
They are not implemented at all for either C or C++.

Confirmed.

[Bug target/108642] ACLE function __arm_wsr missing for AArch64

[pinskia at gcc dot gnu.org]

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

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note the ACLE does not require "fpsr" to be supported either only
"o0:op1:CRn:CRm:op2" format is listed there ...

[Bug target/108642] ACLE function __arm_wsr missing for AArch64

[pinskia at gcc dot gnu.org]

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
GCC has a builtin already for getting fpsr already too:
__builtin_aarch64_get_fpsr

Which of course is not documented ...

[Bug target/108642] ACLE function __arm_wsr missing for AArch64

[david.spickett at linaro dot org]

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

--- Comment #4 from David Spickett <david.spickett at linaro dot org> ---
Of course, I was just looking at at assembly output in compiler explorer and
then locally I didn't link the object. That's why it seemed to work.

Compiling and linking I get:
$ ./bin/aarch64-none-linux-gnu-gcc /tmp/test.c -o /tmp/test.o
/tmp/test.c: In function ‘writeStatusWord’:
/tmp/test.c:4:39: warning: implicit declaration of function ‘__arm_wsr’
[-Wimplicit-function-declaration]
    4 | void writeStatusWord(uint32_t fpsr) { __arm_wsr("fpsr", fpsr); }
      |                                       ^~~~~~~~~
<...>aarch64-none-linux-gnu/bin/ld: /tmp/ccLie8Sh.o: in function
`writeStatusWord':
test.c:(.text+0x18): undefined reference to `__arm_wsr'
collect2: error: ld returned 1 exit status

Which makes more sense.

[Bug target/108642] ACLE function __arm_wsr missing for AArch64

[pinskia at gcc dot gnu.org]

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

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> Which of course is not documented ...

They are documented but not in a decent way:
https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/AArch64-Built-in-Functions.html#AArch64-Built-in-Functions

[Bug target/108642] ACLE function __arm_wsr missing for AArch64

[david.spickett at linaro dot org]

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

--- Comment #6 from David Spickett <david.spickett at linaro dot org> ---
Thanks for the link, we'll try to use those when we detect g++.