Question

I wanted to look into how certain C/C++ features were translated into assembly and I created the following file:

struct foo {
    int x;
    char y[0];
};

char *bar(struct foo *f)
{
    return f->y;
}

I then compiled this with gcc -S (and also tried with g++ -S) but when I looked at the assembly code, I was disappointed to find a trivial redundancy in the bar function that I thought gcc should be able to optimize away:

_bar:
Leh_func_begin1:
        pushq   %rbp
Ltmp0:
        movq    %rsp, %rbp
Ltmp1:
        movq    %rdi, -8(%rbp)
        movq    -8(%rbp), %rax
        movabsq $4, %rcx
        addq    %rcx, %rax
        movq    %rax, -24(%rbp)
        movq    -24(%rbp), %rax
        movq    %rax, -16(%rbp)
        movq    -16(%rbp), %rax
        popq    %rbp
        ret
Leh_func_end1:

Among other things, the lines

        movq    %rax, -24(%rbp)
        movq    -24(%rbp), %rax
        movq    %rax, -16(%rbp)
        movq    -16(%rbp), %rax

seem pointlessly redundant. Is there any reason gcc (and possibly other compilers) cannot/does not optimize this away?

Answer 1

I thought gcc should be able to optimize away.

From the gcc manual:

Without any optimization option, the compiler’s goal is to reduce the cost of compilation and to make debugging produce the expected results.

In other words, it doesn’t optimize unless you ask it to. When I turn on optimizations using the -O3 flag, gcc 4.4.6 produces much more efficient code:

bar:
.LFB0:
        .cfi_startproc
        leaq    4(%rdi), %rax
        ret
        .cfi_endproc

For more details, see Options That Control Optimization in the manual.