On Wed, Apr 14, 2021 at 5:27 AM Peng Yu via Gcc-help <gcc-help@xxxxxxxxxxx> wrote: > > https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html > > I see things like "=r", "r", "=a", "=X" and "cc". I don't quite it's constraints for asm operands, "r" means A register operand is allowed provided that it is in a general register. "=" is an operand modifier which Means that this operand is written to by this instruction: the previous value is discarded and replaced by new data. Refer to https://gcc.gnu.org/onlinedocs/gcc/Constraints.html#Constraints > understand what they mean. Could anybody help me understand them? > > #include <stdio.h> > > int main() { > int src = 1; > int dst; > > asm ("mov %1, %0\n\t" > "add $1, %0" > : "=r" (dst) > : "r" (src)); > printf("%d\n", dst); > return 0; > } > > -- > Regards, > Peng -- BR, Hongtao
