ARM内联汇编-输入操作数约束包含'='

Question

This is my current code:

void int32hex(u32 val, char *out) {
    asm("rev %[dst], %[src]" :: [dst]"=r"(val), [src]"r"(val));

    binhex((u8*)&val, 4, out);
}

My idea is to take the argument val , flip it (endianness) using the rev instruction, and then pass it on.

From what I've read, the above code seems to be correct, the destination register has the =r flag, which implies that the register can be written to. However, when run though GCC, I get the error: input operand constraint contains '='

If I change the flag to simply r then it will compile fine, but the value of val does not change.

Answer 1

The error is telling you what is going wrong -- the = constraint applies only to outputs, not inputs, and your asm pattern has two inputs (one confusingly called 'dst') and no outputs. You probably meant to have 'dst' be an output:

asm("rev %[dst], %[src]" : [dst]"=r"(val) : [src]"r"(val));