Thread

1. wahern · 1789747013 · focus · HN ↗

   > When both conditions are met, the loop body is replaced with a call to std::this_thread::yield(). This gives execution of the loop the forward-progress semantics it previously lacked.

   That's the epitome of the hidden code downside that Linus and many others dislike about C++. For constructors and destructors it's somewhat unavoidable and not so random, though Rust does better at limiting the blast radius of non-local code, at least in the drop case.

   If they didn't want to adopt the C11 rule, the C++ committee should've explored a rule that required the compiler to emit a diagnostic or error for trivial loops (whether as defined by C11 or otherwise), requiring the programmer to explicitly insert ::yield or similar. No hidden code, and less opportunity for the compiler to do surprising things.

   The C committee has been rigorously enumerating UB cases in the standard and addressing each case in turn, often by requiring a diagnostic, error, or by turning it into implemention defined behavior. But inserting code like that would be unthinkable.

   1. gpderetta · 1789769288 · focus · HN ↗

      As the only observable behaviour of this_thread::yield is forward progress, because of the as-if rule, the compiler doesn't actually need to replace the loop, when running on a runtime that guarantees preemption. That's the case when std::threads are backed by kernel threads. On a M:N implementation, then yes, a yield would need to be added, but that would be desirable.

      Interestingly, posix realtime FIFO scheduling doesn't preempt even on kernel thread based implementations, so one reading of the standard would require yield on this case. But that can actually be potentially catastrophic as FIFO scheduling is expected to be deterministic. But realtime scheduling is already beyond the standard: I doubt gcc and clang will do the transformation by default.

      In practice the equivalence is necessary to make some obscure corner of the memory model work and prevent some undesirable optimizations; I expect that in practice the compilers, if they implement this at all, will provide an opt-in flag, but they will optimize as-if the call was there.

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