CODEWITHSUNDEEP
cif-statement
Spacebrain
Spacebrain

Reputation: 97

Definitive if check

Consider this kind of if-statment in a function:

int Foo( void )
{
    if ( HostCPUInfo::IsSSE3Supported() )
        // SSE3 intrinsic
    else
        // C code
}

I check during the runtime if SSE3 instructions are supported by the host CPU to permit the use of faster intrinsic. But, as I doubt the the return of HostCPUInfo::IsSSE3Supported() is going to change during the life of my program, is there a way to only make this kind of checks once (maybe during the initialization) ?

Upvotes: 1

Views: 72

Answers (3)

Nisse Engström
Nisse Engström

Reputation: 4752

You could use a function pointer that is initialized to a function that performs the test. This way, you don't have to call a special initialization function before using foo():

int foo_init (void);
int foo1 (void);
int foo2 (void);

int (*Foo) (void) = foo_init;

int foo_init (void)
{
  Foo = (HostCPUInfo::IsSSE3Supported() ? foo1 : foo2);
  return Foo();
}

...

int main (void)
{
  Foo();

  ...
}

This might have implications for performance if the compiler is unable to inline the foo functions.

Upvotes: 1

Emil Laine
Emil Laine

Reputation: 42838

Check HostCPUInfo::IsSSE3Supported() once before entering Foo and set some boolean depending on the return value.

Then in Foo check only that boolean, which will always remain the same regardless of whether the return value of HostCPUInfo::IsSSE3Supported() changes.

Upvotes: 0

cnicutar
cnicutar

Reputation: 182639

Here's one way with function pointers:

int (*foo)() = synthetic_foo;

void init() {
    if (HostCPUInfo::IsSSE3Supported()) {
        foo = sse3_foo;
    }
}

Upvotes: 4

Related Questions