Can I use Intel syntax of x86 assembly with GCC?
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!
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.
If you are using inline assembly, you can compile with -masm=intel
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.
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, 1nt" // Intel syntax
// "movl $1, %0nt" // 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, 1nt" // 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();
}
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