Can I use Intel syntax of x86 assembly with GCC?

Question

I want to write a small low level program. For some parts of it I will need to use assembly language, but the rest of the code will be written on C/C++.

So, if I will use GCC to mix C/C++ with assembly code, do I need to use AT&T syntax or can I use Intel syntax? Or how do you mix C/C++ and asm (intel syntax) in some other way?

I realize that maybe I don't have a choice and must use AT&T syntax, but I want to be sure..

And if there turns out to be no choice, where I can find full/official documentation about the AT&T syntax?

Thanks!

Answer 1

If you are using separate assembly files, gas has a directive to support Intel syntax:

.intel_syntax noprefix

which uses Intel syntax and doesn't need the % prefix before register names.

(You can also run as with -msyntax=intel -mnaked-reg to have that as the default instead of att, in case you don't want to put .intel_syntax noprefix at the top of your files.)

---

If you are using inline assembly, you can compile your C/C++ sources with gcc -masm=intel (See How to set gcc to use intel syntax permanently? for details.) This may not work with clang.

Using .intel_syntax noprefix at the start of inline asm, and switching back with .att_syntax can work, but will break if you use any m constraints. The memory reference will still be generated in AT&T syntax.

Answer 2

You can use inline assembly with -masm=intel as ninjalj wrote, but it may cause errors when you include C/C++ headers using inline assembly. This is code to reproduce the errors on Cygwin.

sample.cpp:
#include <cstdint>
#include <iostream>
#include <boost/thread/future.hpp>

int main(int argc, char* argv[]) {
    using Value = uint32_t;
    Value value = 0;
    asm volatile (
        "mov  %0, 1\n\t"   // Intel syntax
//      "movl $1, %0\n\t"  // AT&T  syntax
        :"=r"(value)::);

    auto expr = [](void) -> Value { return 20; };
    boost::unique_future<Value> func { boost::async(boost::launch::async, expr) };
    std::cout << (value + func.get());
    return 0;
}

When I built this code, I got error messages below.

g++ -E -std=c++11 -Wall -o sample.s sample.cpp
g++ -std=c++11 -Wall -masm=intel -o sample sample.cpp -lboost_system -lboost_thread
/tmp/ccuw1Qz5.s: Assembler messages:
/tmp/ccuw1Qz5.s:1022: Error: operand size mismatch for `xadd'
/tmp/ccuw1Qz5.s:1049: Error: no such instruction: `incl DWORD PTR [rax]'
/tmp/ccuw1Qz5.s:1075: Error: no such instruction: `movl DWORD PTR [rcx],%eax'
/tmp/ccuw1Qz5.s:1079: Error: no such instruction: `movl %eax,edx'
/tmp/ccuw1Qz5.s:1080: Error: no such instruction: `incl edx'
/tmp/ccuw1Qz5.s:1082: Error: no such instruction: `cmpxchgl edx,DWORD PTR [rcx]'

To avoid these errors, it needs to separate inline assembly (the upper half of the code) from C/C++ code which requires boost::future and the like (the lower half). The -masm=intel option is used to compile .cpp files that contain Intel syntax inline assembly, not to other .cpp files.

sample.hpp:
#include <cstdint>
using Value = uint32_t;
extern Value GetValue(void);

sample1.cpp: compile with -masm=intel
#include <iostream>
#include "sample.hpp"
int main(int argc, char* argv[]) {
    Value value = 0;
    asm volatile (
        "mov  %0, 1\n\t"   // Intel syntax
        :"=r"(value)::);
    std::cout << (value + GetValue());
    return 0;
}

sample2.cpp: compile without -masm=intel
#include <boost/thread/future.hpp>
#include "sample.hpp"
Value GetValue(void) {
    auto expr = [](void) -> Value { return 20; };
    boost::unique_future<Value> func { boost::async(boost::launch::async, expr) };
    return func.get();
}