How to Utilize GCC\'s Compiler Hint for Branch Prediction?

Compiler Hint for Branch Prediction in GCC

Modern processors employ branch predictors to optimize program execution by anticipating the direction of upcoming branches. However, certain situations may arise where a specific branch prediction is desired to enhance performance. In this article, we will explore if GCC provides a mechanism to force branch prediction a particular way and discuss hardware support for this feature.

GCC supports a compiler hint known as __builtin_expect(), which allows programmers to convey their expectations about the likelihood of a branch being taken. This hint takes two arguments: the condition being evaluated and the expected outcome. To force the compiler to always predict a specific branch, simply specify that outcome as the expected value.

For example, consider the following code:

<code class="cpp">for (;;) {
  if (normal) {
    doSomethingNormal();
  } else {
    exceptionalCase();
  }
}</code>

In this example, it is known that the normal case is likely to be executed more frequently than the exceptionalCase. To optimize performance, the compiler can be instructed to always predict that the normal branch will be taken:

<code class="cpp">for (;;) {
  if (__builtin_expect(normal, 1)) {
    doSomethingNormal();
  } else {
    exceptionalCase();
  }
}</code>

Intel architectures do support forced branch prediction using the PREFETCHW instruction. However, this instruction is typically used for caching data and not for altering branch prediction behavior. Other compilers or hardware platforms may have their own mechanisms for influencing branch prediction.

It is important to note that compiler hints are non-standard and may not always produce the desired results. Additionally, modern compilers and processors have sophisticated branch prediction algorithms that may already be optimizing branch behavior more effectively than any manual intervention. Premature micro-optimizations should be approached with caution.

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