I'm trying to push an xmm register onto the stack in x86_64 C code using GCC-style inline assembly. I looked at the answer to this question and am using this code
int main(void) {asm volatile("subq 16, %rsp");
asm volatile("movdqu xmm0, xmmword ptr (%rsp)");
}
and when I compile it on OS X 10.10.2 with clang 6.0, I get the error error: unexpected token in argument list, and a green arrow pointing to the ptr in the second asm line.
I change the code to
int main(void) {asm volatile("subq 16, %rsp");
asm volatile("movdqu xmm0, xmmword (%rsp)");
}
and it gives me error: invalid operand for instruction. I've tried changing xmmword to dqword, to no avail, and I'm not sure what I'm doing wrong.
Thanks in advance.
There are (at least) two dialects of assembler for the x86: intel format and at&t format. It looks like you are trying to write code using intel format, but compiling using at&t.
Your code will compile with:
int main(void) {
asm volatile("subq 16, %rsp");
asm volatile("movdqu %xmm0, (%rsp)");
}
If you use the -masm=intel compile switch, you can also use this (which may look more familiar to you):
int main(void) {
asm volatile("sub rsp, 16");
asm volatile("movdqu xmmword ptr [rsp], xmm0");
}
That said, writing an asm block using multiple asm statements like this is a bad idea. gcc docs explicitly state:
Do not expect a sequence of asm statements to remain perfectly consecutive after compilation. If certain instructions need to remain consecutive in the output, put them in a single multi-instruction asm statement.
So perhaps something more like:
int main(void) {
asm volatile("subq 16, %rsp\n"
"movdqu %xmm0, (%rsp)");
}
Also, if you are going to be reading or updating variables, you should not be using basic asm, but instead use Extended. The docs there are very detailed and there are a number of samples.