[tirrex]

> And how much impact does it have on runtime performance? Well even computing a sum for a relative small number 1000, according to cppbench, the signed version is 430X faster.

Although you can find these kind of examples for a few lines of code snippet, the question is what is the impact on overall program? Nowadays, I guess it has no impact for almost all programs. Because memory access patterns, system call overhead, operating system interaction etc. have much more impact on overall performance compared to optimizations enabled by undefined behaviors.

[fooker]

You are wrong.

Optimizations directly enabled by Undefined behavior are only a very negligible part of the performance benefits of the existence of UB.

For example, consider the fact that array access out of bounds is UB. Because of this a compiler can assume (without proof) that all accesses are actually going to be in range. This enables a boatload of loop optimizations.

All non trivial optimizations done by a compiler usually assume such facts.

[AlotOfReading]

The problem has always been that in practice any nontrivial codebase had UB somewhere (invalidating the entire program!) and diagnosing any particular instance was generally painful until recently. Compilers didn't point most things out, sanitizers didn't exist, and prior to 2011, I don't think there was even a list of UB in C besides the entire standard. C++ is still largely in that position AFAIK.

It's a complete disaster on all sides.

[jcelerier]

> Because memory access patterns, system call overhead, operating system interaction etc. have much more impact on overall performance

In most media apps, the actually processing intensive part definitely does not do syscalls or OS interaction. It's pure computations for as long as possible (and often non-parallelizable, e.g. x[i] *= x[i-1] sort of things). Disabling those optimisations is a killer.

[foota]

I'm not so sure, while these certainly have a large impact on performance, I know that compiler optimizations have a huge impact as well, now what part of that is enabled by assuming the lack of undefined behavior I don't know.