How to manipulating C variable directly (not via register) via inline assembly "incl" instruction in gcc?

Question

I have the following code

#include <stdio.h>

volatile int global_counter = 0;

void increment_counter() {
    for (int i = 0; i < 100000; ++i) {
        //global_counter++;
        asm ("incl %0"
            :"+a"(global_counter)
        );
    }
}

int main() {

    increment_counter();
    printf("Final value of global_counter: %d\n", global_counter);

    return 0;
}

However, gcc with -S -O2 options compiles to the following assembly code

_increment_counter:
LFB1:
    movl    $100000, %edx
    .p2align 4,,10
    .p2align 3
L2:
    movl    _global_counter(%rip), %eax
# 8 "x_test.c" 1
    incl %eax
# 0 "" 2
    subl    $1, %edx
    movl    %eax, _global_counter(%rip)
    jne L2
    ret

It means that the assembly code is compiled to:

    movl    _global_counter(%rip), %eax
    incl %eax
    movl    %eax, _global_counter(%rip)

How can I make it compile to

    incl _global_counter(%rip)

directly without the movl instruction?

Is it possible to make the gcc inline assembly to use the incl instruction directly on the memory, without using the register eax?

You explicitly used the a constraint which forces the use of eax. Not sure why you are surprised. If you want memory, use m constraint. — Jester
– Jester, Commented Jan 14 at 0:03
Not quite a duplicate of GNU C inline asm "m" constraint with a pointer: address vs. pointed-to value? . Also somewhat related: GCC inline assembly: "g" constraint and parameter size uses a few different constraint types and shows what they do, including "=m" and "g" but not "+m". GCC inline assembly: constraints mentions "+m". Can I tell the compiler that I need to earlyclobber a memory operand? is about & not the +m. — Peter Cordes
– Peter Cordes, Commented Jan 14 at 0:53
Just for reference, the m constraint is doced here, as it applies to all platforms. The a constraint is machine specific, so search for 'i386' on this page. — David Wohlferd
– David Wohlferd, Commented Jan 14 at 0:56
@Jester Oh, I see. There should be "+m" instead of "+a" here. Sorry for my fault. Problem solved. — Victor
– Victor, Commented Jan 14 at 5:25

Victor · Accepted Answer · 2025-01-14 05:28:00Z

2

I am the OP. Sorry for my fault. It should be "+m" instead of "+a" for using the memory instead of register.

I modified the code to:

        asm ("incl %0":"+m"(global_counter));

Problem solved.

answered Jan 14 at 5:28

Victor

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