I have some code which needs to check if a bit in a bit field is set. I've been impressed with how optimized std::bitset.count compiles down, so thought this would be a good use of it as well. I'm surprised that when I use bitset.test, the compiler is still checking for exceptions, even when I add noexcept.
I have other checks to ensure that I won't test a bit which is out of range. Is there anything else I can do to optimize this code?
bool bitset_test(int n, size_t m) noexcept {
// The caller already verifies m is within range
return std::bitset<64>( n ).test( m );
}
Compiled output: https://godbolt.org/g/uRggXD
Is there anything else I can do to optimize this code?
Yes! Just like other containers, std::bitset has a function to get a specific bit with and without bounds checking. test is the bounds checking function, which throws.
But if you have other tests to make sure that m is not too high, then you can use std::bitset's operator[], which doesn't have any exception handling in the resulting assembly, because it assumes that you are passing a correct value:
bool bitset_test(int n, size_t m) noexcept {
return std::bitset<64>(n)[m];
}
As others have mentioned, the reason why the compiler still generates exceptions in the resulting assembly is because the compiler still has to call std::terminate if an exception is thrown, so it can't just ignore the exception.