On 8/9/23 00:11, Tsukasa OI via Gcc-patches wrote:
Hello,
I found that a built-in function "__builtin_riscv_pause" is not usable
unless we enable the 'Zihintpause' extension explicitly (still, this
built-in exists EVEN IF the 'Zihintpause' extension is disabled).
Contents of a.c:
void sample(void)
{
__builtin_riscv_pause();
}
Compiling with the 'Zihintpause' extension is fine.
$ riscv64-unknown-elf-gcc -O2 -march=rv64i_zihintpause -mabi=lp64 -c a.c
However, compiling without the 'Zihintpause' causes an assembler error
(tested with GNU Binutils 2.41):
$ riscv64-unknown-elf-gcc -O2 -march=rv64i -mabi=lp64 -c a.c
/tmp/ccFjacAz.s: Assembler messages:
/tmp/ccFjacAz.s:11: Error: unrecognized opcode `pause', extension `zihintpause'
required
This is because:
1. GCC does not handle the 'Zihintpause' extension and
2. "riscv_pause" (insn) unconditionally emits "pause" even if the
assembler does not accept it (when the extension is disabled).
This patch set (PATCH 1/2) resolves this issue by:
1. Handling the 'Zihintpause' extension and
2. Splitting the "__builtin_riscv_pause" implementation
depending on the existence of the 'Zihintpause' extension.
Because a released version of GCC defines "__builtin_riscv_pause"
unconditionally, I chose to define no-'Zihintpause' version.
There is another option to unconditionally emit ".insn 0x0100000f"
(the machine code of "pause") but I didn't because I wanted to improve the
diagnostics (e.g. *.s output).
I also fixed the description of this built-in function (in PATCH 2/2).
I'm not sure whether this is a good method to split the implementation
depending on the 'Zihintpause' extension. Other than that, I believe that
this is okay and approval is appreciated.
Note that because I assigned copyright of GCC contribution to FSF, I didn't
attach "Signed-off-by" tag. Tell me if I need it.
I'd tend to think we do not want to expose the intrinsic unless the
right extensions are enabled -- even though the encoding is a no-op and
we could emit it as a .insn.
If others think otherwise, I'll go with the consensus opinion. So let's
hold off a bit until others have chimed in.
Thanks,
jeff
