How to set gcc to use intel syntax permanently?

Question

I have the following code which compiles fine with the gcc command gcc ./example.c. The program itself calls the function "add_two" which simply adds two integers. To use the intel syntax within the extended assembly instructions I need to switch at first to intel and than back to AT&T. According to the gcc documentation it is possible to switch to intel syntax entirely by using gcc -masm=intel ./exmaple.

Whenever I try to compile it with the switch -masm=intel it won't compile and I don't understand why? I already tried to delete the instruction .intel_syntax but it still don't compile.

#include <stdio.h>

int add_two(int, int);

int main(){
     int src = 3;
     int dst = 5;
     printf("summe = %d \n", add_two(src, dst));
     return 0;
}

int add_two(int src, int dst){

    int sum;

    asm (
        ".intel_syntax;"  //switch to intel syntax
        "mov %0, %1;"
        "add %0, %2;"

        ".att_syntax;"  //switch to at&t syntax
        : "=r" (sum) //output
        : "r" (src), "r" (dst) //input
    );

    return sum;
}

The error message by compiling the above mentioned program with gcc -masm=intel ./example.c is:

tmp/ccEQGI4U.s: Assembler messages:
/tmp/ccEQGI4U.s:55: Error: junk `PTR [rbp-4]' after expression
/tmp/ccEQGI4U.s:55: Error: too many memory references for `mov'
/tmp/ccEQGI4U.s:56: Error: too many memory references for `mov'

How do you use gcc to generate assembly code in Intel syntax?, Can I use Intel syntax of x86 assembly with GCC? what does it mean by "it won't compile"? What's the error? — phuclv
The most obvious thing in your example is that you use -masm=intel to tell C to generate Intel assembler syntax, but before you end your extended assembler template you tell it to switch to at&t syntax with .att_syntax. The code generator has no idea you have done this in the template so is still emitting Intel . When passed to GNU assembler everything after .att_syntax will be Intel but you've told the assembler to treat it at AT&T. Remove .att_syntax. If you are always using -masm=intel you need not bother switching to Intel syntax at the beginning of the assembler template. — Michael Petch
You are right. Removing .intel_syntax and .att_syntax by using gcc -masm=intel ./example works. — Dennis
A quirk: You might expect the compiler to use 3 registers here (2 in, 1 out). And yes, it might. But it might also decide to use the same register for both %0 and %2 (resulting in the wrong answer). Why would it? The compiler thinks of the asm as a single instruction that consumes the inputs and produces the outputs. If it's a single instruction, why "waste" one of the very few registers it has that it could be using for something else? To tell the compiler not to do reuse any regs, use "=&r" (aka earlyclobber) for the output constraint. — David Wohlferd

Peter Cordes Peter Cordes · Accepted Answer · 2019-09-29T11:55:49

Use -masm=intel and don't use any .att_syntax directives in your inline asm. This works with GCC and I think ICC, and with any constraints you use. Other methods don't.

I don't think Clang supports Intel-syntax GNU C inline asm. Clang's options to use Intel-syntax asm (-masm=intel or the equivalent -mllvm --x86-asm-syntax=intel) only control how it prints asm, not how it assembles inputs inline-asm. e.g. https://godbolt.org/z/8BCzp- shows clang -masm=intel interpreting add %0, 1 as add dword ptr [1], eax.

Clang does support Intel-syntax inside MSVC-style asm-blocks, but that's terrible (no constraints so inputs / outputs have to go through memory.

If you were hard-coding register names with clang, -masm=intel would be usable. But it chokes on mov %eax, 5 in Intel-syntax mode so you can't let %0 expand to an AT&T-syntax register name.

-masm=intel makes the compiler use .intel_syntax noprefix at the top of its asm output file, and use Intel-syntax when generating asm from C outside your inline-asm statement. Using .att_syntax at the bottom of your asm template breaks the compiler's asm, hence the error messages like PTR [rbp-4] looking like junk to the assembler (which is expecting AT&T syntax).

The "too many operands for mov" is because in AT&T syntax, mov eax, ebx is a mov from a memory operand (with symbol name eax) to a memory operand (with symbol name ebx)

Some people suggest using .intel_syntax noprefix and .att_syntax prefix around your asm template. That can sometimes work but it's problematic. And incompatible with the preferred method of -masm=intel.

Problems with the "sandwich" method:

When the compiler substitutes operands into your asm template, it will do so according to -masm=. This will always break for memory operands (the addressing-mode syntax is completely different).

It will also break with clang even for registers. Clang's built-in assembler does not accept %eax as a register name in Intel-syntax mode, and doesn't accept .intel_syntax prefix (as opposed to the noprefix that's usually used with Intel-syntax).

Consider this function:

int foo(int x) {
    asm(".intel_syntax noprefix \n\t"
        "add  %0, 1  \n\t"
        ".att_syntax"
         : "+r"(x)
        );
    return x;
}

It assembles as follows with GCC (Godbolt):

        movl    %edi, %eax
        .intel_syntax noprefix 
         add %eax, 1                    # AT&T register name in Intel syntax
        .att_syntax

The sandwich method depends on GAS accepting %eax as a register name even in Intel-syntax mode. GAS from GNU Binutils does, but clang's built-in assembler doesn't.

On a Mac, even using real GCC the asm output has to assemble with an as that's based on clang, not GNU Binutils.

Using clang on that source code complains:

<source>:2:35: error: unknown token in expression
    asm(".intel_syntax noprefix \n\t"
                                  ^
<inline asm>:2:6: note: instantiated into assembly here
        add %eax, 1
            ^

(The first line of the error message didn't handle the multi-line string literal very well. If you use ; instead of \n\t and put everything on one line the clang error message works better but the source is a mess.)

I didn't check what happens with "ri" constraints when the compiler picks an immediate; it will still decorate it with $ but IDK if GAS silently ignores that, too, in Intel syntax mode.

PS: your asm statement has a bug: you forgot an early-clobber on your output operand so nothing is stopping the compiler from picking the same register for the %0 output and the %2 input that you don't read until the 2nd instruction. Then mov will destroy an input.

But using mov as the first or last instruction of an asm-template is usually also a missed-optimization bug. In this case you can and should just use lea %0, [%1 + %2] to let the compiler add with the result written to a 3rd register, non-destructively. Or just wrap the add instruction (using a "+r" operand and an "r", and let the compiler worry about data movement.) If it had to load the value from memory anyway, it can put it in the right register so no mov is needed.

PS: it's possible to write inline asm that works with -masm=intel or att, using GNU C inline asm dialect alternatives. e.g.

void atomic_inc(int *p) {
    asm( "lock add{l $1, %0 | %0, 1}"
       : "+m" (*p)
       :: "memory"
    );
}

compiles with gcc -O2 (-masm=att is the default) to

atomic_inc(int*):
    lock addl $1, (%rdi) 
    ret

Or with -masm=intel to:

atomic_inc(int*):
    lock add DWORD PTR [rdi], 1
    ret

Notice that the l suffix is required for AT&T, and the dword ptr is required for intel, because memory, immediate doesn't imply an operand-size. And that the compiler filled in valid addressing-mode syntax for both cases.

This works with clang, but only the AT&T version ever gets used.

How to set gcc to use intel syntax permanently?

2 Answers

Problems with the "sandwich" method: