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Auto merge of #80475 - simonvandel:fix-77355, r=oli-obk

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New mir-opt pass to simplify gotos with const values (reopening #77486)

Reopening PR #77486

Fixes #77355

This pass optimizes the following sequence
```rust
bb2: {
    _2 = const true;
    goto -> bb3;
}

bb3: {
    switchInt(_2) -> [false: bb4, otherwise: bb5];
}
```
into
```rust
bb2: {
    _2 = const true;
    goto -> bb5;
}
```
This commit is contained in:
bors 2021-02-24 07:23:54 +00:00
commit 6b56603e35
22 changed files with 652 additions and 374 deletions
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37
compiler/rustc_mir/src/transform/simplify.rs
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@ -320,28 +320,31 @@ pub struct SimplifyLocals;
impl<'tcx> MirPass<'tcx> for SimplifyLocals {
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
trace!("running SimplifyLocals on {:?}", body.source);
simplify_locals(body, tcx);
}
}
// First, we're going to get a count of *actual* uses for every `Local`.
let mut used_locals = UsedLocals::new(body);
pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>) {
// First, we're going to get a count of *actual* uses for every `Local`.
let mut used_locals = UsedLocals::new(body);
// Next, we're going to remove any `Local` with zero actual uses. When we remove those
// `Locals`, we're also going to subtract any uses of other `Locals` from the `used_locals`
// count. For example, if we removed `_2 = discriminant(_1)`, then we'll subtract one from
// `use_counts[_1]`. That in turn might make `_1` unused, so we loop until we hit a
// fixedpoint where there are no more unused locals.
remove_unused_definitions(&mut used_locals, body);
// Next, we're going to remove any `Local` with zero actual uses. When we remove those
// `Locals`, we're also going to subtract any uses of other `Locals` from the `used_locals`
// count. For example, if we removed `_2 = discriminant(_1)`, then we'll subtract one from
// `use_counts[_1]`. That in turn might make `_1` unused, so we loop until we hit a
// fixedpoint where there are no more unused locals.
remove_unused_definitions(&mut used_locals, body);
// Finally, we'll actually do the work of shrinking `body.local_decls` and remapping the `Local`s.
let map = make_local_map(&mut body.local_decls, &used_locals);
// Finally, we'll actually do the work of shrinking `body.local_decls` and remapping the `Local`s.
let map = make_local_map(&mut body.local_decls, &used_locals);
// Only bother running the `LocalUpdater` if we actually found locals to remove.
if map.iter().any(Option::is_none) {
// Update references to all vars and tmps now
let mut updater = LocalUpdater { map, tcx };
updater.visit_body(body);
// Only bother running the `LocalUpdater` if we actually found locals to remove.
if map.iter().any(Option::is_none) {
// Update references to all vars and tmps now
let mut updater = LocalUpdater { map, tcx };
updater.visit_body(body);
body.local_decls.shrink_to_fit();
}
body.local_decls.shrink_to_fit();
}
}