4 ms·

> And it is in incorrect...in C++ Hardware converged on two's complement arithmetic for signed arithmetic decades ago. C/C++ has really polluted people's think

by titzer 5y ago

> And it is in incorrect...in C++ Hardware converged on two's complement arithmetic for signed arithmetic decades ago. C/C++ has really polluted people's thinking in a profound and devious way. The real reason that C++ continues to insist on undefined behavior is because compilers optimize code in ways that are incorrect w.r.t. overflow and compiler writers have too much power in C++ standardization. What is the value of such optimizations? Mum.

agent327 5y ago
This particular UB is used for optimisation in the following manner: when you write a loop that iterates over an array using a signed index variable, it doesn't check that the index doesn't overflow. Apparently doing it 'properly' would cost something like 6% performance loss in such code. Even so, I feel the proper solution would be to use the correct type for array indexing (either one that has the same word size as a pointer, in which case it cannot overflow, or just a special index_t type defined for this purpose). Forcing everything else to have to put up with UB in the least expected places is not, in my mind, a good way to go. Unfortunately I don't think this battle can be won. They won't even remove the UB from functions like tolower(), even though it would cost precisely nothing...
- titzer 5y ago
  I've heard many variations of the above, and the solution to almost all of them is loop versioning as appropriate to the particular case. In this case, a single check before the loop would suffice, and then that 6% disappears. The battle can't be won because of a perspective difference. It's really difficult to communicate the importance of a sane programming model when a sane programming model has not been a priority for C or C++ since their respective inceptions. They've prioritized low-level manipulations and performance over everything else and ironically painted themselves into silly corners like this one; even when you want the hardware instruction that does signed two's complement arithmetic, you can't get it, because the other firmly-held belief that compilers are never wrong.

by titzer 5y ago

agent327 5y ago

This particular UB is used for optimisation in the following manner: when you write a loop that iterates over an array using a signed index variable, it doesn't check that the index doesn't overflow. Apparently doing it 'properly' would cost something like 6% performance loss in such code. Even so, I feel the proper solution would be to use the correct type for array indexing (either one that has the same word size as a pointer, in which case it cannot overflow, or just a special index_t type defined for this purpose). Forcing everything else to have to put up with UB in the least expected places is not, in my mind, a good way to go. Unfortunately I don't think this battle can be won. They won't even remove the UB from functions like tolower(), even though it would cost precisely nothing...

titzer 5y ago
I've heard many variations of the above, and the solution to almost all of them is loop versioning as appropriate to the particular case. In this case, a single check before the loop would suffice, and then that 6% disappears. The battle can't be won because of a perspective difference. It's really difficult to communicate the importance of a sane programming model when a sane programming model has not been a priority for C or C++ since their respective inceptions. They've prioritized low-level manipulations and performance over everything else and ironically painted themselves into silly corners like this one; even when you want the hardware instruction that does signed two's complement arithmetic, you can't get it, because the other firmly-held belief that compilers are never wrong.