Is there a compiler hint for GCC to force branch prediction to always go a certain way?

Question

For the Intel architectures, is there a way to instruct the GCC compiler to generate code that always forces branch prediction a particular way in my code? Does the Intel h

Answer 1

gcc has long __builtin_expect (long exp, long c) (emphasis mine):

You may use __builtin_expect to provide the compiler with branch prediction information. In general, you should prefer to use actual profile feedback for this (-fprofile-arcs), as programmers are notoriously bad at predicting how their programs actually perform. However, there are applications in which this data is hard to collect.

The return value is the value of exp, which should be an integral expression. The semantics of the built-in are that it is expected that exp == c. For example:

if (__builtin_expect (x, 0))
   foo ();

indicates that we do not expect to call foo, since we expect x to be zero. Since you are limited to integral expressions for exp, you should use constructions such as

if (__builtin_expect (ptr != NULL, 1))
   foo (*ptr);

when testing pointer or floating-point values.

As the documentation notes you should prefer to use actual profile feedback and this article shows a practical example of this and how it in their case at least ends up being an improvement over using __builtin_expect. Also see How to use profile guided optimizations in g++?.

We can also find a Linux kernel newbies article on the kernal macros likely() and unlikely() which use this feature:

#define likely(x)       __builtin_expect(!!(x), 1)
#define unlikely(x)     __builtin_expect(!!(x), 0)

Note the !! used in the macro we can find the explanation for this in Why use !!(condition) instead of (condition)?.

Just because this technique is used in the Linux kernel does not mean it always makes sense to use it. We can see from this question I recently answered difference between the function performance when passing parameter as compile time constant or variable that many hand rolled optimizations techniques don't work in the general case. We need to profile code carefully to understand whether a technique is effective. Many old techniques may not even be relevant with modern compiler optimizations.

Note, although builtins are not portable clang also supports __builtin_expect.

Also on some architectures it may not make a difference.