riscv-llvm does not support inline ASM constraints "A" or "rK"

[markww]

I have been running into the following complaints while attempting to use riscv-clang in a number of RISCV repositories:

"error: invalid output constraint '+A' in asm"
"error: invalid input constraint 'rK' in asm"

These constraints seem to be recognized and handled just fine by riscv-gcc. These constraints are used in both the riscv-linux project ( see https://github.com/riscv/riscv-linux/blob/riscv-next/arch/riscv/include/asm/bitops.h ) and in the riscv-musl project ( see https://github.com/lluixhi/musl-riscv/blob/master/arch/riscv64/atomic_a.h )

[markww]

Note that these are recognized as RISCV machine-specific constraints by GCC here:

https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html

[kishigo]

Looking at the https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html, it seems that 'K' is a supported constraint, a 5 bit value for CSR. The rK would seem to mean register constraint + 5 bit constraint