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 inte

Answer 1

Note that -masm= also affects the default inline assembler syntax:

Output assembly instructions using selected dialect. Also affects which dialect is used for basic "asm" and extended "asm". Supported choices (in dialect order) are att or intel. The default is att. Darwin does not support intel.

That means that your first .intel_syntax directive is superfluous and the final .att_syntax is wrong because your GCC call compiles C to Intel assembler code.

IOW, either stick to -masm=intel or sandwich your inline Intel assembler code sections between .intel_syntax noprefix and .att_syntax prefix directives - but don't do both.

Note that the sandwich method isn't compatible with all inline assembler constraints - e.g. a constraint that involves m (i.e. memory operand) would insert an operand in ATT syntax which would yield an error like 'Error: junk (%rbp) after expression'. In those cases you have to use -masm=intel.

Answer 2

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.

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-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)

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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.)

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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.

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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.

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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.