Experiment in removing ptr->int->ptr casts #7664
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steven-johnson
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buildbot_test_everything
abadams
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Jun 27, 2023
| #if defined(__has_builtin) && __has_builtin(__builtin_align_up) | ||
| return __builtin_align_up(value, alignment); | ||
| #else | ||
| if constexpr (std::is_pointer<SomeType>::value) { |
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How about this implementation for align_up:
ptr += ((-(uintptr_t)(ptr)) & (alignment - 1));
Curious if that passes the clang-tidy checker. It just adds a computed offset to a pointer, where the offset depends on casting the pointer to an int. It never casts an int to a ptr.
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While looking over
clang-tidychecks that we had disabled, I came acrossperformance-no-int-to-ptrand ended up in a fascinating rabbit hole. TL;DR: doing ptr->int->ptr casting reduces opportunities for compiler optimizations (and that's what this check attempts to warn you about). I had no idea this was even a thing, but https://www.ralfj.de/blog/2020/12/14/provenance.html and https://www.ralfj.de/blog/2022/04/11/provenance-exposed.html provided interesting backstory.Running clang-tidy with this enabled revealed a lot of places in our runtime code that might run afoul of this... but it's not clear whether any of them are on a critical path that makes this inefficiency critical (except, perhaps, for the bit-tagging we do in
synchronization_common).Anyway, I experimentally converted a few places to follow what I think are the right guidelines (and they no longer trigger this check). Not sure if this is worth pursuing any further, but I thought it worthwhile to post here as a Draft for people to take a look at.