Branch Prediction Macros In GCC

Branch Prediction Macros In GCC

One commonly used optimization technique in the Linux kernel is “__builtin_expect.” When dealing with conditional code, such as if-else statements, there is often prior knowledge about which branch is more likely to be true and which is less likely. When the compiler is aware of this information, it can generate highly optimized code to improve performance. This technique is particularly valuable for enhancing the efficiency of code execution.

Refer the macro definition of “likely()” and “unlikely()” macros from Linux kernel code “http://lxr.linux.no/linux+v3.6.5/include/linux/compiler.h” .

Let us see macro definition of “likely()” and “unlikely()” macros from linux kernel code “http://lxr.linux.no/linux+v3.6.5/include/linux/compiler.h” [line no 146 and 147].

We are marking branches in the example given below:

const char *home_dir ; 
  
home_dir = getenv("HOME"); 
if (likely(home_dir))  
    printf("home directory: %s\n", home_dir); 
else
    perror("getenv");

In the example above, we’ve used the “likely()” macro to indicate that the “if” condition is more likely to be true. This allows the compiler to optimize the code by placing the true branch immediately after the branch instruction and the false branch within the branch instruction. This can result in significant performance improvements. However, it’s essential not to use “likely()” and “unlikely()” macros blindly. If the prediction is correct, there is no penalty in terms of jump instructions, but if the prediction is wrong, it can lead to a performance hit as the processor needs to flush its pipeline, which is worse than having no prediction.

Memory access is one of the slowest CPU operations compared to other operations. To address this limitation, CPUs use CPU caches, such as L1-cache and L2-cache, to store a portion of memory directly within the CPU. Accessing cache memory is much faster than accessing other types of memory. However, cache memory has a limited size, so the CPU has to make educated guesses about which memory will be needed in the near future and load that memory into the CPU cache. The “likely()” and “unlikely()” macros serve as hints to the CPU to help it load the appropriate memory into the cache, optimizing memory access and improving overall performance.

FAQ- Branch Prediction Macros In GCC