Skip Drop terminators for enum variants without drop glue
#68943
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When doing drop elaboration for an `enum` that may or may not be moved out of (an open drop), we check the discriminant of the `enum` to see whether the live variant has any drop flags and then check the drop flags to see whether we need to drop each field. Sometimes, however, the live variant has no move paths. In this case, we still emit a drop terminator for the entire enum after checking the enum discriminant. This commit skips emitting the drop terminator when the "otherwise" variants, those without move paths, have no drop glue. This was frequently the case with `Option`, as the `None` variant has no drop glue and no move path.
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r? @eddyb (rust_highfive has picked a reviewer for you, use r? to override) |
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⌛ Trying commit b23d910 with merge 57cd8438f4c735173fc5383cd91a064c41cc4082... |
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Queued 57cd8438f4c735173fc5383cd91a064c41cc4082 with parent 8498c5f, future comparison URL. |
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Mark other variants as uninitialized after switch on discriminant During drop elaboration, which builds the drop ladder that handles destruction during stack unwinding, we attempt to remove MIR `Drop` terminators that will never be reached in practice. This reduces the number of basic blocks that are passed to LLVM, which should improve performance. In #66753, a user pointed out that unreachable `Drop` terminators are common in functions like `Option::unwrap`, which move out of an `enum`. While discussing possible remedies for that issue, @eddyb suggested moving const-checking after drop elaboration. This would allow the former, which looks for `Drop` terminators and replicates a small amount of drop elaboration to determine whether a dropped local has been moved out, leverage the work done by the latter. However, it turns out that drop elaboration is not as precise as it could be when it comes to eliminating useless drop terminators. For example, let's look at the code for `unwrap_or`. ```rust fn unwrap_or<T>(opt: Option<T>, default: T) -> T { match opt { Some(inner) => inner, None => default, } } ``` `opt` never needs to be dropped, since it is either moved out of (if it is `Some`) or has no drop glue (if it is `None`), and `default` only needs to be dropped if `opt` is `Some`. This is not reflected in the MIR we currently pass to codegen.  @eddyb also suggested the solution to this problem. When we switch on an enum discriminant, we should be marking all fields in other variants as definitely uninitialized. I implemented this on top of alongside a small optimization (split out into #68943) that suppresses drop terminators for enum variants with no fields (e.g. `Option::None`). This is the resulting MIR for `unwrap_or`.  In concert with #68943, this change speeds up many [optimized and debug builds](https://perf.rust-lang.org/compare.html?start=d55f3e9f1da631c636b54a7c22c1caccbe4bf0db&end=0077a7aa11ebc2462851676f9f464d5221b17d6a). We need to carefully investigate whether I have introduced any miscompilations before merging this. Code that never drops anything would be very fast indeed until memory is exhausted.
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Oh this shouldn't have been assigned to me. r? @matthewjasper or @pnkfelix |
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@bors r+ rollup=never |
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📌 Commit b23d910 has been approved by |
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This was referenced Mar 2, 2020
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May 16, 2020
Pkgsrc changes:
* Bump rust bootstrap version to 1.42.0, except for Darwin/i686 where the
bootstrap is not (yet?) available.
Upstream changes:
Version 1.43.0 (2020-04-23)
==========================
Language
--------
- [Fixed using binary operations with `&{number}` (e.g. `&1.0`) not having
the type inferred correctly.][68129]
- [Attributes such as `#[cfg()]` can now be used on `if` expressions.][69201]
**Syntax only changes**
- [Allow `type Foo: Ord` syntactically.][69361]
- [Fuse associated and extern items up to defaultness.][69194]
- [Syntactically allow `self` in all `fn` contexts.][68764]
- [Merge `fn` syntax + cleanup item parsing.][68728]
- [`item` macro fragments can be interpolated into `trait`s, `impl`s,
and `extern` blocks.][69366]
For example, you may now write:
```rust
macro_rules! mac_trait {
($i:item) => {
trait T { $i }
}
}
mac_trait! {
fn foo() {}
}
```
These are still rejected *semantically*, so you will likely receive an error but
these changes can be seen and parsed by macros and
conditional compilation.
Compiler
--------
- [You can now pass multiple lint flags to rustc to override the previous
flags.][67885] For example; `rustc -D unused -A unused-variables` denies
everything in the `unused` lint group except `unused-variables` which
is explicitly allowed. However, passing `rustc -A unused-variables -D unused` denies
everything in the `unused` lint group **including** `unused-variables` since
the allow flag is specified before the deny flag (and therefore overridden).
- [rustc will now prefer your system MinGW libraries over its bundled libraries
if they are available on `windows-gnu`.][67429]
- [rustc now buffers errors/warnings printed in JSON.][69227]
Libraries
---------
- [`Arc<[T; N]>`, `Box<[T; N]>`, and `Rc<[T; N]>`, now implement
`TryFrom<Arc<[T]>>`,`TryFrom<Box<[T]>>`, and `TryFrom<Rc<[T]>>`
respectively.][69538] **Note** These conversions are only available when `N`
is `0..=32`.
- [You can now use associated constants on floats and integers directly, rather
than having to import the module.][68952] e.g. You can now write `u32::MAX` or
`f32::NAN` with no imports.
- [`u8::is_ascii` is now `const`.][68984]
- [`String` now implements `AsMut<str>`.][68742]
- [Added the `primitive` module to `std` and `core`.][67637] This module
reexports Rust's primitive types. This is mainly useful in macros
where you want avoid these types being shadowed.
- [Relaxed some of the trait bounds on `HashMap` and `HashSet`.][67642]
- [`string::FromUtf8Error` now implements `Clone + Eq`.][68738]
Stabilized APIs
---------------
- [`Once::is_completed`]
- [`f32::LOG10_2`]
- [`f32::LOG2_10`]
- [`f64::LOG10_2`]
- [`f64::LOG2_10`]
- [`iter::once_with`]
Cargo
-----
- [You can now set config `[profile]`s in your `.cargo/config`, or through
your environment.][cargo/7823]
- [Cargo will now set `CARGO_BIN_EXE_<name>` pointing to a binary's
executable path when running integration tests or benchmarks.][cargo/7697]
`<name>` is the name of your binary as-is e.g. If you wanted the executable
path for a binary named `my-program`you would use
`env!("CARGO_BIN_EXE_my-program")`.
Misc
----
- [Certain checks in the `const_err` lint were deemed unrelated to const
evaluation][69185], and have been moved to the `unconditional_panic` and
`arithmetic_overflow` lints.
Compatibility Notes
-------------------
- [Having trailing syntax in the `assert!` macro is now a hard error.][69548]
This has been a warning since 1.36.0.
- [Fixed `Self` not having the correctly inferred type.][69340] This incorrectly
led to some instances being accepted, and now correctly emits a hard error.
[69340]: rust-lang/rust#69340
Internal Only
-------------
These changes provide no direct user facing benefits, but represent significant
improvements to the internals and overall performance of `rustc` and
related tools.
- [All components are now built with `opt-level=3` instead of `2`.][67878]
- [Improved how rustc generates drop code.][67332]
- [Improved performance from `#[inline]`-ing certain hot functions.][69256]
- [traits: preallocate 2 Vecs of known initial size][69022]
- [Avoid exponential behaviour when relating types][68772]
- [Skip `Drop` terminators for enum variants without drop glue][68943]
- [Improve performance of coherence checks][68966]
- [Deduplicate types in the generator witness][68672]
- [Invert control in struct_lint_level.][68725]
[67332]: rust-lang/rust#67332
[67429]: rust-lang/rust#67429
[67637]: rust-lang/rust#67637
[67642]: rust-lang/rust#67642
[67878]: rust-lang/rust#67878
[67885]: rust-lang/rust#67885
[68129]: rust-lang/rust#68129
[68672]: rust-lang/rust#68672
[68725]: rust-lang/rust#68725
[68728]: rust-lang/rust#68728
[68738]: rust-lang/rust#68738
[68742]: rust-lang/rust#68742
[68764]: rust-lang/rust#68764
[68772]: rust-lang/rust#68772
[68943]: rust-lang/rust#68943
[68952]: rust-lang/rust#68952
[68966]: rust-lang/rust#68966
[68984]: rust-lang/rust#68984
[69022]: rust-lang/rust#69022
[69185]: rust-lang/rust#69185
[69194]: rust-lang/rust#69194
[69201]: rust-lang/rust#69201
[69227]: rust-lang/rust#69227
[69548]: rust-lang/rust#69548
[69256]: rust-lang/rust#69256
[69361]: rust-lang/rust#69361
[69366]: rust-lang/rust#69366
[69538]: rust-lang/rust#69538
[cargo/7823]: rust-lang/cargo#7823
[cargo/7697]: rust-lang/cargo#7697
[`Once::is_completed`]: https://doc.rust-lang.org/std/sync/struct.Once.html#method.is_completed
[`f32::LOG10_2`]: https://doc.rust-lang.org/std/f32/consts/constant.LOG10_2.html
[`f32::LOG2_10`]: https://doc.rust-lang.org/std/f32/consts/constant.LOG2_10.html
[`f64::LOG10_2`]: https://doc.rust-lang.org/std/f64/consts/constant.LOG10_2.html
[`f64::LOG2_10`]: https://doc.rust-lang.org/std/f64/consts/constant.LOG2_10.html
[`iter::once_with`]: https://doc.rust-lang.org/std/iter/fn.once_with.html
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Split out from #68528.
When doing drop elaboration for an
enumthat may or may not be moved out of (an open drop), we check the discriminant of theenumto see whether the live variant has any drop flags and then check the drop flags to see whether we need to drop each field. Sometimes, however, the live variant has no move paths and thus no drop flags. In this case, we still emit a drop terminator for the entire enum after checking the enum discriminant. This drop shim will check the discriminant of the enum again and then drop the fields of the active variant. If the active variant has no drop glue, nothing will be done.This commit skips emitting the drop terminator during drop elaboration when the "otherwise" variants, those without move paths, have no drop glue. A common example of this scenario is when an
Optionis moved from, sinceOption::Nonenever needs drop glue. Below is a fragment the pre-codegen CFG forOption::unwrap_orin which we check the drop flag (_5) forself(_1), before and after the change.Before:
After:
This change doesn't do much on its own, but it is a prerequisite to get the perf gains from #68528.
cc @arielb1