Question

There is a code fragment that GCC produce the result I didn’t expect:

(I am using gcc version 4.6.1 Ubuntu/Linaro 4.6.1-9ubuntu3 for target i686-linux-gnu)

[test.c]

#include <stdio.h>

int *ptr;

int f(void)
{
    (*ptr)++;

    return 1;
}

int main()
{
    int a = 1, b = 2;

    ptr = &b;

    a = b++ + f() + f() ? b : a;

    printf ("b = %d\n", b);

    return a;
}

In my understanding, there is a sequence point at function call.
The post-increment should be taken place before f().

see C99 5.1.2.3:
“… called sequence points, all side effects of previous evaluations
shall be complete and no side effects of subsequent evaluations shall
have taken place.”

For this test case, perhaps the order of evaluation is unspecified,
but the final result should be the same. So I expect b’s final result is 5.
However, after compiling this case with ‘gcc test.c -std=c99’, the output shows b = 3.

Then I use “gcc test.c -std=c99 -S” to see what happened:

        movl    $1, 28(%esp)
        movl    $2, 24(%esp)
        leal    24(%esp), %eax
        movl    %eax, ptr
        movl    24(%esp), %ebx
        call    f
        leal    (%ebx,%eax), %esi
        call    f
        addl    %esi, %eax
        testl   %eax, %eax
        setne   %al
        leal    1(%ebx), %edx
        movl    %edx, 24(%esp)
        testb   %al, %al
        je      .L3
        movl    24(%esp), %eax
        jmp     .L4
.L3:
        movl    28(%esp), %eax
.L4:
        movl    %eax, 28(%esp)

It seems that GCC uses evaluated value before f() and perform ‘++’
operation after two f() calls.

I also use llvm-clang to compile this case,
and the result shows b = 5, which is what I expect.

Is my understanding incorrect on post-increment and sequence point behavior ??
Or this is a known issue of GCC461 ??

Answer 1

I reported this GCC bug some time ago and it was fixed earlier this year. See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48814