Can I hint the optimizer by giving the range of an integer?

by

Yes, it is possible. For example, for gcc you can use __builtin_unreachable to tell the compiler about impossible conditions, like so:

if (value < 0 || value > 36) __builtin_unreachable();

We can wrap the condition above in a macro:

#define assume(cond) do { if (!(cond)) __builtin_unreachable(); } while (0)

And use it like so:

assume(x >= 0 && x <= 10);

As you can see, gcc performs optimizations based on this information:

#define assume(cond) do { if (!(cond)) __builtin_unreachable(); } while (0)

int func(int x){
    assume(x >=0 && x <= 10);

    if (x > 11){
        return 2;
    }
    else{
        return 17;
    }
}

Produces:

func(int):
    mov     eax, 17
    ret

One downside, however, that if your code ever breaks such assumptions, you get undefined behavior.

It doesn’t notify you when this happens, even in debug builds. To debug/test/catch bugs with assumptions more easily, you can use a hybrid assume/assert macro (credits to @David Z), like this one:

#if defined(NDEBUG)
#define assume(cond) do { if (!(cond)) __builtin_unreachable(); } while (0)
#else
#include <cassert>
#define assume(cond) assert(cond)
#endif

In debug builds (with NDEBUG not defined), it works like an ordinary assert, printing error message and abort‘ing program, and in release builds it makes use of an assumption, producing optimized code.

Note, however, that it is not a substitute for regular assert – cond remains in release builds, so you should not do something like assume(VeryExpensiveComputation()).

More Related Contents:

  1. gcc optimization flag -O3 makes code slower than -O2
  2. Why can lambdas be better optimized by the compiler than plain functions?
  3. When do programmers use Empty Base Optimization (EBO)
  4. How can I convert a std::string to int?
  5. How can I use cout
  6. Why are elementwise additions much faster in separate loops than in a combined loop?
  7. Which, if any, C++ compilers do tail-recursion optimization?
  8. Generate random numbers using C++11 random library
  9. Why is transposing a matrix of 512×512 much slower than transposing a matrix of 513×513?
  10. How to write a large buffer into a binary file in C++, fast?
  11. Is it better to use std::memcpy() or std::copy() in terms to performance?
  12. Why don’t compilers merge redundant std::atomic writes?
  13. Why does optimisation kill this function?
  14. Why do we use volatile keyword? [duplicate]
  15. How to get IOStream to perform better?
  16. What C/C++ compiler can use push pop instructions for creating local variables, instead of just increasing esp once?
  17. Efficient way to determine number of digits in an integer
  18. performance of unsigned vs signed integers
  19. Why is std::fill(0) slower than std::fill(1)?
  20. Why should I ever use inline code?
  21. Does a c/c++ compiler optimize constant divisions by power-of-two value into shifts?
  22. Optimization of raw new[]/delete[] vs std::vector
  23. C++ error: ‘_mm_sin_ps’ was not declared in this scope
  24. Fast multiplication/division by 2 for floats and doubles (C/C++)
  25. Loop with a zero execution time
  26. std::vector reserve() and push_back() is faster than resize() and array index, why?
  27. Which is faster/preferred: memset or for loop to zero out an array of doubles?
  28. How can adding code to a loop make it faster?
  29. Fastest way to produce a mask with n ones starting at position i
  30. VS: unexpected optimization behavior with _BitScanReverse64 intrinsic

Leave a Comment