clang, -O2

	.text
	.file	"fib.cc"
	.globl	_Z3fibi
	.align	16, 0x90
	.type	_Z3fibi,@function
_Z3fibi:                                # @_Z3fibi
	.cfi_startproc
# BB#0:
	pushq	%rbp
.Ltmp0:
	.cfi_def_cfa_offset 16
	pushq	%rbx
.Ltmp1:
	.cfi_def_cfa_offset 24
	pushq	%rax
.Ltmp2:
	.cfi_def_cfa_offset 32
.Ltmp3:
	.cfi_offset %rbx, -24
.Ltmp4:
	.cfi_offset %rbp, -16
	movl	%edi, %ebx
	cmpl	$2, %ebx
	jge	.LBB0_1
# BB#2:
	movl	%ebx, %eax
	jmp	.LBB0_3
.LBB0_1:
	leal	-1(%rbx), %edi
	callq	_Z3fibi
	movl	%eax, %ebp
	addl	$-2, %ebx
	movl	%ebx, %edi
	callq	_Z3fibi
	addl	%ebp, %eax
.LBB0_3:
	addq	$8, %rsp
	popq	%rbx
	popq	%rbp
	retq
.Ltmp5:
	.size	_Z3fibi, .Ltmp5-_Z3fibi
	.cfi_endproc

	.globl	main
	.align	16, 0x90
	.type	main,@function
main:                                   # @main
	.cfi_startproc
# BB#0:
	pushq	%rax
.Ltmp6:
	.cfi_def_cfa_offset 16
	movl	$10, %edi
	callq	_Z3fibi
	movl	%eax, %ecx
	movl	$.L.str, %edi
	xorl	%eax, %eax
	movl	%ecx, %esi
	callq	printf
	xorl	%eax, %eax
	popq	%rdx
	retq
.Ltmp7:
	.size	main, .Ltmp7-main
	.cfi_endproc

	.type	.L.str,@object          # @.str
	.section	.rodata.str1.1,"aMS",@progbits,1
.L.str:
	.asciz	"%d\n"
	.size	.L.str, 4


	.ident	"clang version 3.6.1 (tags/RELEASE_361/final 238189)"
	.section	".note.GNU-stack","",@progbits

fib.c

#include <stdio.h>

int fib(int n) {
  if (n < 2) {
    return n;
  } else {
    return fib(n - 1) + fib(n - 2);
  }
}

int main() {
  printf("%d\n", fib(10));
  return 0;
}

gcc, -O2

	.file	"fib.cc"
	.text
	.p2align 4,,15
	.globl	_Z3fibi
	.type	_Z3fibi, @function
_Z3fibi:
.LFB12:
	.cfi_startproc
	pushq	%r12
	.cfi_def_cfa_offset 16
	.cfi_offset 12, -16
	cmpl	$1, %edi
	movl	%edi, %r12d
	pushq	%rbp
	.cfi_def_cfa_offset 24
	.cfi_offset 6, -24
	pushq	%rbx
	.cfi_def_cfa_offset 32
	.cfi_offset 3, -32
	jle	.L5
	movl	%edi, %ebx
	xorl	%ebp, %ebp
	.p2align 4,,10
	.p2align 3
.L4:
	leal	-1(%rbx), %edi
	subl	$2, %ebx
	call	_Z3fibi
	addl	%eax, %ebp
	cmpl	$1, %ebx
	jg	.L4
	andl	$1, %r12d
.L2:
	popq	%rbx
	.cfi_remember_state
	.cfi_def_cfa_offset 24
	leal	(%r12,%rbp), %eax
	popq	%rbp
	.cfi_def_cfa_offset 16
	popq	%r12
	.cfi_def_cfa_offset 8
	ret
.L5:
	.cfi_restore_state
	xorl	%ebp, %ebp
	jmp	.L2
	.cfi_endproc
.LFE12:
	.size	_Z3fibi, .-_Z3fibi
	.section	.rodata.str1.1,"aMS",@progbits,1
.LC0:
	.string	"%d\n"
	.section	.text.startup,"ax",@progbits
	.p2align 4,,15
	.globl	main
	.type	main, @function
main:
.LFB13:
	.cfi_startproc
	subq	$8, %rsp
	.cfi_def_cfa_offset 16
	movl	$10, %edi
	call	_Z3fibi
	movl	$.LC0, %edi
	movl	%eax, %esi
	xorl	%eax, %eax
	call	printf
	xorl	%eax, %eax
	addq	$8, %rsp
	.cfi_def_cfa_offset 8
	ret
	.cfi_endproc
.LFE13:
	.size	main, .-main
	.ident	"GCC: (GNU) 4.8.4"
	.section	.note.GNU-stack,"",@progbits