<functional>: make move_only_function do less work
#5328
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AlexGuteniev
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<functiona>: make move_only_function doing less work<functional>: make move_only_function doing less work
StephanTLavavej
changed the title
<functional>: make move_only_function doing less work<functional>: make move_only_function do less work
StephanTLavavej
approved these changes
Mar 8, 2025
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Thanks! 😻 On my 5950X I actually observe
However, this case seems unrealistic (how often are users default-constructing and immediately moving), and if the code had originally been written this way, I would never have asked for an additional pointer to be nulled. |
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I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed. |
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Thanks for improving this code and benchmarking it! ⏱️ 🚀 😻 |
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Primarily I treat this more as a clarity improvement.
Yet, the benchmark results prove this PR deserving
performancelabel.Resolves #5316
⏱️ Benchmark results
🥇 Results interpretation
mof_constructis the only that changed more than results variation, it shows some improvement due to doing less work.mof_construct_and_moveaddresses the concern of potential slowdown due to failed store-to-load forwarding.The concern was due to storing zero as one pointer but then loading two pointers, which can be loaded in the same vector register. If the load happens shortly after the store, it has to wait for the store to completely reach the cache, as the wider value cannot be obtained from the narrower store buffer entry.
The situation apparently does not happen, as before the actual two-pointers move, there's vtable obtaining and inspection, which takes enough time for the store to complete.
mof_noneandmof_movegive some idea on the corresponding baseline timings. They are not expected to change.