Writing a function in 64 bit assembly - c

Question

I am trying to write a function (max) in 64 bit assembly and I don't know what I am doing wrong, maybe some of you guys can determine what I am doing wrong :/
Here's the function:
int max(int a, int b) {
/* Return the larger of the two integers `a' and `b'. */
if (a > b)
return a;
else
return b;
}
And here is my assembly code (with comments):
push %rbp
mov %rsp, %rbp
mov %rdi, %rax
mov %rsi, %rcx
test %rax, %rax // Checking if first parameter is signed
js .signedRAX
test %rcx, %rcx // Checking if second parameter is signed
js .signedRCX
jmp .compare // If either one signed then jump to .compare
.signedRAX:
test %rcx, %rcx // Checking if both are signed
js .signedRAXandRCX
mov %rcx, %rax // If not then return the positive number
jmp .end // finish the function
.signedRCX:
jmp .end // If only the second parameter is signed then jump
.signedRAXandRCX: // straight to end of function and return %rax
cmp %rax, %rcx // If both are signed compare which one is the max
jl .end
mov %rcx, %rax
jmp .end
.compare:
cmp %rax, %rcx // If both are positive then compare which one is
jg .end // the max
mov %rcx, %rax
.end:
mov %rbp, %rsp
pop %rbp
ret
I am getting the wrong output when comparing two parameters that are both signed and then both positive.

Answer 1

You work nuch too complicated.
If I input your program to gcc -S, I get
max:
.LFB0:
.cfi_startproc
pushl %ebp
.cfi_def_cfa_offset 8
.cfi_offset 5, -8
movl %esp, %ebp
.cfi_def_cfa_register 5
movl 8(%ebp), %eax
cmpl 12(%ebp), %eax
jle .L2
movl 8(%ebp), %eax
jmp .L3
.L2:
movl 12(%ebp), %eax
.L3:
popl %ebp
.cfi_restore 5
.cfi_def_cfa 4, 4
ret
.cfi_endproc
.LFE0:
If I take over your "ABI" and way of passing arguments,
I get
max:
push %rbp
mov %rsp, %rbp
mov %rdi, %rax
mov %rsi, %rcx
cmpl %rcx, %rax
jle .L2
movl %rcx, %rax
.L2:
mov %rbp, %rsp
pop %rbp
ret

Answer 2

Here is equivalent C-pseudo code for ASM from question. You can see, for a >= 0 and b < 0 it return b. For a < 0 and b >= 0 it return a. That's incorrect. There may be another errors in the code, because so simple operation encoded really messy. It's really hard to see anything in the code. Don't make simple things so complex. Follow KISS principle.
// test %rax, %rax
// js .signedRAX
if (a >= 0) {
// test %rcx, %rcx
// js .signedRCX
if (b >= 0) {
// .compare
// cmp %rax, %rcx // If both are positive then compare which one is
// jg .end // the max
if (a > b) {
b = a;
}
return b;
} else {
// .signedRCX
return b;
}
} else {
// .signedRAX
// test %rcx, %rcx // Checking if both are signed
// js .signedRAXandRCX
if (b >= 0) {
b = a;
return b;
} else {
// .signedRAXandRCX
// cmp %rax, %rcx // If both are signed compare which one is the max
// jl .end
if (a < b) {
b = a;
}
return b;
}
}