Visual Studio performance optimization in branching

Question

Consider the following

while(true)
{
    if(x>5)
     // Run function A
    else
     // Run function B
}

if x is always less than 5, does visual studio compiler do any optimization? ie like never checks if x is larger than 5 and always run function B

Answer 1

It depends on whether or not the compiler "knows" that x will always be less than 5 .

Yes, nearly all modern compilers are capable of removing the branch. But the compiler needs to be able to prove that the branch will always go one direction.

---

Here's an example that can be optimized:

int x = 1;

if (x > 5)
    printf("Hello\n");
else
    printf("World\n");

The disassembly is:

    sub rsp, 40                 ; 00000028H
    lea rcx, OFFSET FLAT:??_C@_06DKJADKFF@World?6?$AA@
    call    QWORD PTR __imp_printf

x = 1 is provably less than 5 . So the compiler is able to remove the branch.

---

But in this example, even if you always input less than 5, the compiler doesn't know that. It must assume any input.

int x;
cin >> x;

if (x > 5)
    printf("Hello\n");
else
    printf("World\n");

The disassembly is:

    cmp DWORD PTR x$[rsp], 5
    lea rcx, OFFSET FLAT:??_C@_06NJBIDDBG@Hello?6?$AA@
    jg  SHORT $LN5@main
    lea rcx, OFFSET FLAT:??_C@_06DKJADKFF@World?6?$AA@
$LN5@main:
    call    QWORD PTR __imp_printf

The branch stays. But note that it actually hoisted the function call out of the branch. So it really optimized the code down to something like this:

const char *str = "Hello\n";

if (!(x > 5))
    str = "World\n";

printf(str);