diff options
| -rw-r--r-- | arch/riscv/usr/basics.c | 15 |
1 files changed, 13 insertions, 2 deletions
diff --git a/arch/riscv/usr/basics.c b/arch/riscv/usr/basics.c index 4333072..4fa7df3 100644 --- a/arch/riscv/usr/basics.c +++ b/arch/riscv/usr/basics.c @@ -92,6 +92,17 @@ static inline void atomic_add(uint64_t *a, uint64_t value) * result. If it fails (e.g. the address was already being used), then `t0 = * 0` and `beqz` will instruct it to try again. * + * Note that on the board I'm using the 'Zawrs' extension is not available. + * Otherwise I could have re-arranged to code to something like this: + * + * amoadd.d t0, %1, %0 + * bnez t0, .Latomic_add_done + * wrs.nto + * .Latomic_add_done: + * + * This is an extra instruction but it saves on power if the memory address + * happens to be used at access time. + * * As for the 'A' RISC-V specific constraint, it means "An address that is * held in a general-purpose register". GCC will then do the magic and * convert it to `amoadd.d t0, a1, (a0)` or something like that. Note that @@ -100,9 +111,9 @@ static inline void atomic_add(uint64_t *a, uint64_t value) * * See: https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html. */ - asm volatile(".Latomic_retry:\n\t" + asm volatile(".Latomic_add_retry:\n\t" "amoadd.d t0, %1, %0\n\t" - "beqz t0, .Latomic_retry" + "beqz t0, .Latomic_add_retry" : "+A" (*a) : "r" (value) : "memory"); |