When should I omit the frame pointer?

Question

Is there any substantial optimization when omitting the frame pointer? If I have understood correctly by reading this page, -fomit-frame-pointer is used when we want to avoid saving, setting up and restoring frame pointers.

Is this done only for each function call and if so, is it really worth to avoid a few instructions for every function? Isn't it trivial for an optimization. What are the actual implications of using this option apart from the debugging limitations?

I compiled the following C code with and without this option

int main(void) {         int i;          i = myf(1, 2); }  int myf(int a, int b) {         return a + b; } 

,

# gcc -S -fomit-frame-pointer code.c -o withoutfp.s # gcc -S code.c -o withfp.s 

.

diff -u 'ing the two files revealed the following assembly code:

 --- withfp.s    2009-12-22 00:03:59.000000000 +0000 +++ withoutfp.s 2009-12-22 00:04:17.000000000 +0000 @@ -7,17 +7,14 @@         leal    4(%esp), %ecx         andl    $-16, %esp         pushl   -4(%ecx) -       pushl   %ebp -       movl    %esp, %ebp         pushl   %ecx -       subl    $36, %esp +       subl    $24, %esp         movl    $2, 4(%esp)         movl    $1, (%esp)         call    myf -       movl    %eax, -8(%ebp) -       addl    $36, %esp +       movl    %eax, 20(%esp) +       addl    $24, %esp         popl    %ecx -       popl    %ebp         leal    -4(%ecx), %esp         ret         .size   main, .-main @@ -25,11 +22,8 @@  .globl myf         .type   myf, @function  myf: -       pushl   %ebp -       movl    %esp, %ebp -       movl    12(%ebp), %eax -       addl    8(%ebp), %eax -       popl    %ebp +       movl    8(%esp), %eax +       addl    4(%esp), %eax         ret         .size   myf, .-myf         .ident  "GCC: (GNU) 4.2.1 20070719  

Could someone please shed light on the key points of the above code where -fomit-frame-pointer did actually make the difference?

Edit: objdump's output replaced with gcc -S's

Answer 1

-fomit-frame-pointer allows one extra register to be available for general-purpose use. I would assume this is really only a big deal on 32-bit x86, which is a bit starved for registers.*

One would expect to see EBP no longer saved and adjusted on every function call, and probably some additional use of EBP in normal code, and fewer stack operations on occasions where EBP gets used as a general-purpose register.

Your code is far too simple to see any benefit from this sort of optimization-- you're not using enough registers. Also, you haven't turned on the optimizer, which might be necessary to see some of these effects.

* ISA registers, not micro-architecture registers.

Answer 2

The only downside of omitting it is that debugging is much more difficult.

The major upside is that there is one extra general purpose register which can make a big difference on performance. Obviously this extra register is used only when needed (probably in your very simple function it isn't); in some functions it makes more difference than in others.