4 ms·

The “if (debug) log” example in the article is likely compiled to an always taken conditional jump past the logging call. And therefore not quite free. Nitpicki

• xenadu02 5y ago
  You can easily verify this in your specific code base but I believe the compiler prefers not-taken jumps for paths it predicts to be less frequently taken. You can provide hints that the path is a cold one to ensure the compiler uses the most efficient conditional branch layout. On some architectures the compiler will also provide this hint to the CPU (with a prefix, the instruction it selects, or a flag bit in the instruction) which avoids the perf hit on the first iteration. Both Clang and GCC recognize __builtin_expect which can be wrapped in a macro easily: #define unlikely(x) __builtin_expect(!!(x), 0)
  • the_duke 5y ago
    C++ 20 introduces standard likely/unlikely attributes: https://en.cppreference.com/w/cpp/language/attributes/likely https://en.cppreference.com/w/cpp/language/attributes/likely
    • astrange 5y ago
      clang also has `__builtin_unpredictable` which is at least as useful as likely/unlikely.