Optimize Iterator for IIterator<T>
#3476
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kennykerr
commented
Feb 6, 2025
Comment on lines
+464
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+468
| let result = if self.HasCurrent().unwrap_or(false) { | ||
| self.Current().ok() | ||
| } else { | ||
| None | ||
| }; |
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This is the only change - the rest are the output of code generation used for validation.
talagrand
approved these changes
Feb 6, 2025
|
Nice! |
JamesMcNellis
approved these changes
Feb 7, 2025
|
lgtm |
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C++/WinRT uses exceptions for error propagation and unfortunately exceptions continue to be very slow. I hadn't realized how much slower until a customer shared some logs. I decided to take a fresh look and compare the Rust implementation of
IIterator<T>::CurrentreturningE_BOUNDSto C++/WinRT's implementation that throws before returningE_BOUNDS. For a million calls:RoOriginate): 3.95sSo that's quite a difference. Ideally we can get C++/WinRT to avoid exceptions but this probably merits a tweak in Rust to avoid this pathological case as changing C++ is hard.
This change tweaks the
Iteratortrait implementation on the Rust side to useHasCurrentto avoid calls toCurrentand thus avoid those exceptions. With this change, the cost of iteration for C++ implementations approaches that of Rust.Thanks to @JamesMcNellis for prodding me to look into this a bit more closely. He also gave a great talk on how exceptions work that you should definitely check out.