Learning Question about jsonkit HOT 3 CLOSED

The JK_EXPECT_T and JK_EXPECT_F are preprocessor macros that are used to abstract a feature often provided by optimizing compilers. Specifically, it allows you to give the optimizer a hint as to which condition is most likely to occur: the true condition, or the false condition. The __builtin_expect() "function" has the following behavior:

• __builtin_expect(x, y), where x is 0, returns a result of 0.
• __builtin_expect(x, y), where x is != 0, returns a result of 1.

In either case of x, y does not change the result that is "returned". In other words, __builtin_expect() effectively "does nothing".

However, internally the optimizer makes a note of the value of y. When a modern CPU encounters a conditional branch, it may not have all the information needed at that point in time to know exactly which branch it should follow- the true or false side (this is due to pipelining, cache and memory latencies, etc). Modern CPU's will typically being to speculatively execute one of the paths, and if it later determines that it picked the wrong one, it requires an expensive flush of the CPU state to undo all the speculatively execute instructions, and then restart instruction execution on the correct path.

By using __builtin_expect(), you can help the CPU pick the correct path to follow "most of the time", which translates in to better performance.

from jsonkit.

Thanks John

from jsonkit.

If i get this right, clang/llvm doesn't use these hints?

from jsonkit.