Reduce compile time of bevy_ptr::OwnedPtr::make function #15644
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bushrat011899
added
D-Trivial
| // SAFETY: The value behind the pointer will not get dropped or observed later, | ||
| // so it's safe to promote it to an owning pointer. |
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This can probably be removed now, right? Since the unsafe code was moved into make_internal?
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Yes. I tried multiple ways to rearrange the code and forgot to remove the comment on the latest one.
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make_internal should be an unsafe method. The safety is still unheld in this method.
crates/bevy_ptr/src/lib.rs
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| @@ -397,13 +397,20 @@ impl<'a, T: ?Sized> From<&'a mut T> for PtrMut<'a> { | |||
| } | |||
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| impl<'a> OwningPtr<'a> { | |||
| /// This exists mostly to reduce compile times; | |||
| /// code is only duplicated per type, rather than per function called. | |||
| fn make_internal<T>(temp: &mut ManuallyDrop<T>) -> OwningPtr<'_> { | |||
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This should be an unsafe method and have the same safety requirements as the promote method.
crates/bevy_ptr/src/lib.rs
Outdated
| /// This exists mostly to reduce compile times; | ||
| /// code is only duplicated per type, rather than per function called. | ||
| fn make_internal<T>(temp: &mut ManuallyDrop<T>) -> OwningPtr<'_> { | ||
| // SAFETY: The value behind the pointer will not get dropped or observed later, |
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| // SAFETY: The value behind the pointer will not get dropped or observed later, | |
| // SAFETY: Safety is unheld by the caller. |
The following line should be deleted.
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I get making the function itself unsafe, but what value does this safety comment add? If safety is unheld, shouldn't we not be using unsafe code?
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Oops typo. It should be upheld
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The caller lets go of all safety constraints and becomes free.
| // SAFETY: The value behind the pointer will not get dropped or observed later, | ||
| // so it's safe to promote it to an owning pointer. |
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make_internal should be an unsafe method. The safety is still unheld in this method.
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I fixed |
alice-i-cecile
added
D-Straightforward
Methodology
A good metric that correlates with compile time is the amount of code generated by the compiler itself; even if the end binary is exactly the same size, having more copies of the same code can really slow down compile time, since it has to figure out whether it needs to include them or not.
The measurement for this used was the
cargo-llvm-linescrate which can measure which functions are generating the most lines of LLVM IR, which generally means more code compiled. The example compiled was thebreakoutgame, to choose something that touches a decent portion of the engine.Solution
Based upon the measurements,
bevy_ptr::OwnedPtr::makewas taking up 4061 lines of LLVM IR in the example code. So, I separated part of this function into a less-monomorphised version to reduce the amount of generated code. This was by far the most lines emitted by any single function.Results
After this change, only 2560 lines are emitted, accounting for a 36% decrease. I tried timing the results and it seemed like it did decrease compile times a bit, but honestly, the data is really noisy and I can't be bothered to compile bevy for hours on end to get enough data points.
The tweak feels like an improvement, so, I'll offer it, however small.