我有C代码：
long fib(long n) {
if (n < 2) return 1;
return fib(n-1) + fib(n-2);
}
int main(int argc, char** argv) {
return 0;
}
which I compiled by running gcc -O0 -fno-optimize-sibling-calls -S file.c yielding assembly code that has not been optimized:
.file "long.c"
.text
.globl fib
.type fib, @function
fib:
.LFB5:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
pushq %rbx
subq $24, %rsp
.cfi_offset 3, -24
movq %rdi, -24(%rbp)
cmpq $1, -24(%rbp)
jg .L2
movl $1, %eax
jmp .L3
.L2:
movq -24(%rbp), %rax
subq $1, %rax
movq %rax, %rdi
call fib
movq %rax, %rbx
movq -24(%rbp), %rax
subq $2, %rax
movq %rax, %rdi
call fib
addq %rbx, %rax
.L3:
addq $24, %rsp
popq %rbx
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE5:
.size fib, .-fib
.globl main
.type main, @function
main:
.LFB6:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl %edi, -4(%rbp)
movq %rsi, -16(%rbp)
movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE6:
.size main, .-main
.ident "GCC: (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0"
.section .note.GNU-stack,"",@progbits
我的问题是：
Why do we decrement the stack pointer by 24, subq $24, %rsp? As I see it, we store one element only, first argument n in %rdi, on the stack after the initial two pushes. So why don't we just decrement the stack pointer by 8 and then move n to -8(%rbp)? So
subq $8, %rsp
movq %rdi, -8(%rbp)
