Can GCC Force Branch Prediction Outcomes on Intel Architectures?

Customizing Branch Prediction with GCC

Question: Can GCC be directed to force branch prediction outcomes in specified directions for Intel architectures?

Answer:

Yes, GCC offers the __builtin_expect function to guide its code generation for branch prediction. This function takes two arguments: exp (the condition expression) and c (the expected outcome).

To instruct GCC to consistently predict a certain branch path as true, use the following syntax:

<code class="c">if (__builtin_expect(exp, 1))</code>

For instance, in the provided code snippet, you can add:

<code class="c">if (__builtin_expect(normal, 1))</code>

Additionally, to simplify the syntax, you can define custom macros:

<code class="c">#define likely(x)    __builtin_expect (!!(x), 1)
#define unlikely(x)  __builtin_expect (!!(x), 0)</code>

Considerations:

• __builtin_expect is not a standard feature.
• Compiler and CPU branch predictors are generally adept at decision-making. This technique should be considered only in highly performance-sensitive scenarios.

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