4 ms·

Note that the generated instructions - even if they are never run as in [1] - still consume cache and might hinder further optimizations. Which is why `noexcept

Note that the generated instructions - even if they are never run as in [1] - still consume cache and might hinder further optimizations. Which is why `noexcept` is becoming more popular. And because there is no GC, code must be written to be exception-safe in all conditions which is often forgotten. 1: https://godbolt.org/z/bKfG14P64 https://godbolt.org/z/bKfG14P64 - the difference between `e()` and `n()` is that one is marked `noexcept`. Both `f()` and `g()` run directly to `ret` (return) in the normal case.

vlovich123 5y ago
Isn’t exception handling code placed in the cold section of the binary these days so that the impact on cache is nonexistent? Yup, the generated assembly does this. There’s some minimal extra instructions still to setup the frame but the bulk of the exception handling lives elsewhere.
bregma 5y ago
Yeah, noexcept(true) is identical to wrapping the function with a try-catch construct in which the catch clause simply calls std::terminate(). Your godbolt example doesn't include the definition of n() so it doesn't show that. Adding noexcept(true) has a cost (because it's a kind of exception handling) but also allows the compiler to optimize out some of that cost under some circumstances. Nevertheless, an example that demonstrates the exception path doesn't say much about the "zero-cost" non-exception path. In C++ exception handling is expensive. Outside of actually directly handling exceptions they have zero cost. There are no generated instructions. They do not consume cache. They do not hinder further optimizations.
- zeotroph 5y ago
  Indeed, in the following case of a 4-deep call stack, each with its own exception, the bulk of the handling code can be moved elsewhere (already marked as cold by the compiler), but nonetheless there are a few instructions which won't matter in most cases but are still required to jump there and thus end up in the instruction cache. https://godbolt.org/z/eh1d4K1M7 https://godbolt.org/z/eh1d4K1M7
pjmlp 5y ago
There is GC in C++/CLI, Unreal C++, and the optional C++11 API, which is getting dropped in C++23, though.

vlovich123 5y ago

Isn’t exception handling code placed in the cold section of the binary these days so that the impact on cache is nonexistent? Yup, the generated assembly does this. There’s some minimal extra instructions still to setup the frame but the bulk of the exception handling lives elsewhere.

bregma 5y ago

Yeah, noexcept(true) is identical to wrapping the function with a try-catch construct in which the catch clause simply calls std::terminate(). Your godbolt example doesn't include the definition of n() so it doesn't show that. Adding noexcept(true) has a cost (because it's a kind of exception handling) but also allows the compiler to optimize out some of that cost under some circumstances. Nevertheless, an example that demonstrates the exception path doesn't say much about the "zero-cost" non-exception path. In C++ exception handling is expensive. Outside of actually directly handling exceptions they have zero cost. There are no generated instructions. They do not consume cache. They do not hinder further optimizations.

zeotroph 5y ago
Indeed, in the following case of a 4-deep call stack, each with its own exception, the bulk of the handling code can be moved elsewhere (already marked as cold by the compiler), but nonetheless there are a few instructions which won't matter in most cases but are still required to jump there and thus end up in the instruction cache. https://godbolt.org/z/eh1d4K1M7 https://godbolt.org/z/eh1d4K1M7

pjmlp 5y ago

There is GC in C++/CLI, Unreal C++, and the optional C++11 API, which is getting dropped in C++23, though.