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axboe / liburing

Library providing helpers for the Linux kernel io_uring support
MIT License
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Feature request: mark functions `noexcept` in C++ #1117

Open rohanlean opened 3 months ago

rohanlean commented 3 months ago

Hi,

first of all, many thanks for creating io_uring!

Because you seem to be using raw syscalls everywhere instead of POSIX wrappers that could be cancellation points, the functions in liburing thankfully do not throw and probably never will. Yet they are not marked noexcept, which inhibits optimisations.

I propose that all functions be marked noexcept.

isilence commented 3 months ago

Apparently, extern "C" doesn't imply noexcept and it might be worth to add. I wonder though, have you tried it? Any what's the difference in binary size / assembly?

rohanlean commented 3 months ago

Yes, it absolutely makes a difference. A missing noexcept can prevent code motion, dead-code elimination, and a cascade of other optimisation. For a simple example consider T x; for (;;) io_uring_enter(...). Unless the compiler knows that io_uring_enter does not throw, it needs to insert a call to (and possibly generate) the destructor of T, possibly insert a call to (and link) std::terminate, generate unwind tables, etc.

It is worth also marking the the functions with __attribute__(nothrow) in C (if supported), by the way. Some C code is compiled with exceptions enabled, and it can make a difference there. For example Fedora compiles everything with exceptions enabled. You can look at how glibc defines its __THROW macro in sys/cdefs.h.