Why GCC didn't optimize this tail call? - c

Question

I have the code working with lined lists. I use tail calls. Unfortunately, GCC does not optimise the calls.
Here is C code of the function that recursively calculates length of the linked list:
size_t ll_length(const ll_t* list) {
return ll_length_rec(list, 0);
}
size_t ll_length_rec(const ll_t* list, size_t size_so_far)
{
if (list) {
return ll_length_rec(list->next, size_so_far + 1);
} else {
return size_so_far;
}
}
and here is the assembler code:
.globl _ll_length_rec
_ll_length_rec:
LFB8:
.loc 1 47 0
pushq %rbp
LCFI6:
movq %rsp, %rbp
LCFI7:
subq $32, %rsp
LCFI8:
movq %rdi, -8(%rbp)
movq %rsi, -16(%rbp)
.loc 1 48 0
cmpq $0, -8(%rbp)
je L8
.loc 1 49 0
movq -16(%rbp), %rsi
incq %rsi
movq -8(%rbp), %rax
movq 8(%rax), %rdi
call _ll_length_rec # < THIS SHOUD BE OPTIMIZED
movq %rax, -24(%rbp)
jmp L10
If GCC would optimize it, there would be no call in the asm. I compile it with:
gcc -S -fnested-functions -foptimize-sibling-calls \
-03 -g -Wall -o llist llist.c
and GCC version is:
i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5666) (dot 3)

Answer 1

If I add -O3 to your compilation line, it does not seem to generate the offending call, while without it, I get the unoptimised call. I don't know all gcc options in my head, but is -03 a typo for -O3 or intentional?
Ltmp2:
pushq %rbp
Ltmp0:
movq %rsp, %rbp
Ltmp1:
jmp LBB1_1
.align 4, 0x90
LBB1_3:
addq $2, %rsi
Ltmp3:
movq (%rax), %rdi
Ltmp4:
LBB1_1:
Ltmp5:
testq %rdi, %rdi
je LBB1_5
Ltmp6:
movq (%rdi), %rax
testq %rax, %rax
jne LBB1_3
incq %rsi
LBB1_5:
movq %rsi, %rax
Ltmp7:
Ltmp8:
popq %rbp
ret

Answer 2

Most likely because neither of your functions are declared as static, which means that the symbols must be visible to the linker in case any other compilation units need them at link time. Try to compile with the -fwhole-program flag and see what happens.

Answer 3

Probably depends on the version of GCC and specific build. This is what I get from GCC 3.4.4 on Windows starting from -O2 and up
.globl _ll_length_rec
.def _ll_length_rec; .scl 2; .type 32; .endef
_ll_length_rec:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %edx
movl 12(%ebp), %eax
jmp L3
.p2align 4,,7
L6:
movl (%edx), %edx
incl %eax
L3:
testl %edx, %edx
jne L6
popl %ebp
ret