Why Is GCC Using Mov Instead Of Push In Function Calls?

Question

So I've got this example C program.

int worship(long john)
{
    return 0 * john;
}

int main()
{
    return worship(666);
}

The assembly looks (essentially) like this:

worship(long):
    pushq   %rbp
    movq    %rsp, %rbp
    movq    %rdi, -8(%rbp)
    movl    $0, %eax
    popq    %rbp
    ret
main:
    pushq   %rbp
    movq    %rsp, %rbp
    movl    $666, %edi
    call    worship(long)
    popq    %rbp
    ret

I ran into this while reading about stack smashing. In the assembly worship(long): section where it says movq %rdi, -8(%rbp) I would expect it to be using pushq based on everything I've read so far. Is this the new way that GCC is pushing arguments onto the stack and if so is there a compiler flag I could be using to toggle this?

Answer 1

GCC manual says,

-mpush-args

Push instructions will be used to pass outgoing arguments when functions are called. Enabled by default.

-mno-push-args   

Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies.

-maccumulate-outgoing-args

If enabled, the maximum amount of space required for outgoing arguments will be computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage when preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This switch implies -mno-push-args.

Even -mpush-args enabled by default it is override by -maccumulate-outgoing-args which is enabled by default. Compiling passing option -mno-accumulate-outgoing-args explicitly could change the instructions to push.

Answer 2

Compilers like GCC are written by people who very carefully consider how to make often used code snippets (like function call/return) as efficient as possible. Sure, their solutions target the general case, in special cases there might be better options.