Signed or Unsigned Integer

Related C++ Core guidelines

• ES.100: Don’t mix signed and unsigned arithmetic
• ES.101: Use unsigned types for bit manipulation
• ES.102: Use signed types for arithmetic

Operations note

• ADD - 1 cycle
• IMUL - 3 cycles
• DIV - at least 20 times slower then IMUL

Code samples

uint64_t arc_unsigned(uint64_t n) {
   uint64_t sum = 0;
   for(uint64_t i = 1; i <=n; i++) {
      sum += i;
   }
   return sum;
}

int64_t arc_signed(int64_t n) {
   int64_t sum = 0;
   for(int64_t i = 1; i <=n; i++) {
      sum += i;
   }
   return sum;
}

• Undefined behavior in signed utilized in performance improvement in clang-12 …
• … and no effect in clang-17

Recommendations

• Use warning flags -Wall, -Wextra, -pedantic, -Werror
• Use newer compilers - they optimize better!
• Use sanitizers - -fsanitize=signed-integer-oveflow,-fsanitize=unsigned-integer-oveflow
• Use special types for better performance - int_fastN_t, uint_fastN_t
• Know your CPU
• Use special helpers from the standard - MAKE_SIGNED, MAKE_UNSIGNED
• Use C++20 safe comparators
• Avoid using auto when not sure about the type
• Use concrete types when possible!
• Use modern loops as much as you can (instead of indexed ones)
• Use strong types

Reference

• To Int or to Uint, This is the Question - Alex Dathskovsky - CppCon 2024
• UndefinedBehaviorSanitizer
• Fixed width integer types