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[MIR] Cache drops for early scope exits - #34307
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rust-highfive
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Jun 16, 2016
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r? @arielb1 (rust_highfive has picked a reviewer for you, use r? to override) |
nagisa
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Jun 16, 2016
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cc @eddyb (the patch you wanted to see) |
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Are the fancy quotes intentional here?
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Yes. These are preferred characters for quotation characters in english prose.
Previously we would rebuild all drops on every early exit from a scope, which for code like:
```rust
match x {
a => return 1,
b => return 2,
...
z => return 27
}
```
would produce 27 exactly same chains of drops for each return, a O(n*m) explosion in drops.arielb1
commented
Jun 16, 2016
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@bors r+ |
bors
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Jun 16, 2016
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📌 Commit 04d63cc has been approved by |
Manishearth added a commit
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that referenced
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Jun 16, 2016
[MIR] Cache drops for early scope exits
Previously we would rebuild all drops on every early exit from a scope, which for code like:
```rust
match x {
A => return 1,
B => return 2,
...
C => return 27
}
```
would produce 27 exactly same chains of drops for each return, basically a `O(n*m)` explosion. [This](https://cloud.githubusercontent.com/assets/679122/16125192/3355e32c-33fb-11e6-8564-c37cab2477a0.png) is such a case for a match on 80-variant enum with 3 droppable variables in scope.
For [`::core::iter::Iterator::partial_cmp`](https://github.com/rust-lang/rust/blob/6edea2cfda2818f0a76f4bac2d18a30feb54c137/src/libcore/iter/iterator.rs#L1909) the CFG looked like [this](https://cloud.githubusercontent.com/assets/679122/16122708/ce0024d8-33f0-11e6-93c2-e1c44b910db2.png) (after initial SimplifyCfg). With this patch the CFG looks like [this](https://cloud.githubusercontent.com/assets/679122/16122806/294fb16e-33f1-11e6-95f6-16c5438231af.png) instead.
Some numbers (overall very small wins, however neither of the crates have many cases which abuse this corner case):
| | old time | old rss | new time | new rss |
|-------------------------|----------|---------|----------|----------|
| core dump | 0.879 | 224MB | 0.871 | 223MB |
| core MIR passes | 0.759 | 224MB | 0.718 | 223MB |
| core MIR codegen passes | 1.762 | 230MB | 1.442 | 228MB |
| core trans | 3.263 | 279MB | 3.116 | 278MB |
| core llvm passes | 5.611 | 263MB | 5.565 | 263MB |
| std dump | 0.487 | 190MB | 0.475 | 192MB |
| std MIR passes | 0.311 | 190MB | 0.288 | 192MB |
| std MIR codegen passes | 0.753 | 195MB | 0.720 | 197MB |
| std trans | 2.589 | 287MB | 2.523 | 287MB |
| std llvm passes | 7.268 | 245MB | 7.447 | 246MB |
eddyb added a commit
to eddyb/rust
that referenced
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Aug 18, 2016
Fix the invalidation of the MIR early exit cache ~~The rust-lang#34307 introduced a cache for early exits in order to help with O(n*m) explosion of cleanup blocks but the cache is invalidated incorrectly and I can’t seem to figure out why (caching is hard!)~~ ~~Remove the cache for now to fix the immediate correctness issue and worry about the performance later.~~ Cache invalidation got fixed. Fixesrust-lang#35737 r? @nikomatsakis
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Previously we would rebuild all drops on every early exit from a scope, which for code like:
would produce 27 exactly same chains of drops for each return, basically a
O(n*m)explosion. This is such a case for a match on 80-variant enum with 3 droppable variables in scope.For
::core::iter::Iterator::partial_cmpthe CFG looked like this (after initial SimplifyCfg). With this patch the CFG looks like this instead.Some numbers (overall very small wins, however neither of the crates have many cases which abuse this corner case):