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.
They are not implemented at all for either C or C++. Confirmed.
Note the ACLE does not require "fpsr" to be supported either only "o0:op1:CRn:CRm:op2" format is listed there ...
GCC has a builtin already for getting fpsr already too: __builtin_aarch64_get_fpsr Which of course is not documented ...
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.
(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
Thanks for the link, we'll try to use those when we detect g++.