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InstCombine: Don't optimize &mut *x into x
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@jonas-schievink
jonas-schievink committed May 31, 2020
1 parent b6fa392 commit 309661e
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Showing 4 changed files with 25 additions and 14 deletions.
18 changes: 8 additions & 10 deletions src/librustc_mir/transform/instcombine.rs
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Original file line number Diff line number Diff line change
@@ -1,11 +1,12 @@
//! Performs various peephole optimizations.
use crate::transform::{MirPass, MirSource};
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_hir::Mutability;
use rustc_index::vec::Idx;
use rustc_middle::mir::visit::{MutVisitor, Visitor};
use rustc_middle::mir::{
Body, Constant, Local, Location, Mutability, Operand, Place, PlaceRef, ProjectionElem, Rvalue,
Body, Constant, Local, Location, Operand, Place, PlaceRef, ProjectionElem, Rvalue,
};
use rustc_middle::ty::{self, TyCtxt};
use std::mem;
Expand Down Expand Up @@ -39,7 +40,7 @@ impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
}

fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, location: Location) {
if let Some(mtbl) = self.optimizations.and_stars.remove(&location) {
if self.optimizations.and_stars.remove(&location) {
debug!("replacing `&*`: {:?}", rvalue);
let new_place = match rvalue {
Rvalue::Ref(_, _, place) => {
Expand All @@ -57,10 +58,7 @@ impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
}
_ => bug!("Detected `&*` but didn't find `&*`!"),
};
*rvalue = Rvalue::Use(match mtbl {
Mutability::Mut => Operand::Move(new_place),
Mutability::Not => Operand::Copy(new_place),
});
*rvalue = Rvalue::Use(Operand::Copy(new_place))
}

if let Some(constant) = self.optimizations.arrays_lengths.remove(&location) {
Expand Down Expand Up @@ -93,8 +91,8 @@ impl Visitor<'tcx> for OptimizationFinder<'b, 'tcx> {
{
// The dereferenced place must have type `&_`.
let ty = Place::ty_from(local, proj_base, self.body, self.tcx).ty;
if let ty::Ref(_, _, mtbl) = ty.kind {
self.optimizations.and_stars.insert(location, mtbl);
if let ty::Ref(_, _, Mutability::Not) = ty.kind {
self.optimizations.and_stars.insert(location);
}
}
}
Expand All @@ -114,6 +112,6 @@ impl Visitor<'tcx> for OptimizationFinder<'b, 'tcx> {

#[derive(Default)]
struct OptimizationList<'tcx> {
and_stars: FxHashMap<Location, Mutability>,
and_stars: FxHashSet<Location>,
arrays_lengths: FxHashMap<Location, Constant<'tcx>>,
}
6 changes: 5 additions & 1 deletion src/test/mir-opt/inline/issue-58867-inline-as-ref-as-mut/rustc.a.Inline.after.mir
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Original file line number Diff line number Diff line change
Expand Up @@ -8,14 +8,18 @@ fn a(_1: &mut [T]) -> &mut [T] {
let mut _4: &mut [T]; // in scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:6
scope 1 {
debug self => _4; // in scope 1 at $SRC_DIR/libcore/convert/mod.rs:LL:COL
let mut _5: &mut [T]; // in scope 1 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:15
}

bb0: {
StorageLive(_2); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:15
StorageLive(_3); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:15
StorageLive(_4); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:6
_4 = &mut (*_1); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:6
_3 = move _4; // scope 1 at $SRC_DIR/libcore/convert/mod.rs:LL:COL
StorageLive(_5); // scope 1 at $SRC_DIR/libcore/convert/mod.rs:LL:COL
_5 = &mut (*_4); // scope 1 at $SRC_DIR/libcore/convert/mod.rs:LL:COL
_3 = &mut (*_5); // scope 1 at $SRC_DIR/libcore/convert/mod.rs:LL:COL
StorageDead(_5); // scope 1 at $SRC_DIR/libcore/convert/mod.rs:LL:COL
_2 = &mut (*_3); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:15
StorageDead(_4); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:14: 3:15
_0 = &mut (*_2); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:3:5: 3:15
Expand Down
8 changes: 6 additions & 2 deletions src/test/mir-opt/inline/issue-58867-inline-as-ref-as-mut/rustc.b.Inline.after.mir
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@ fn b(_1: &mut std::boxed::Box<T>) -> &mut T {
scope 1 {
debug self => _4; // in scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
let mut _5: &mut T; // in scope 1 at $DIR/issue-58867-inline-as-ref-as-mut.rs:8:5: 8:15
let mut _6: &mut T; // in scope 1 at $DIR/issue-58867-inline-as-ref-as-mut.rs:8:5: 8:15
}

bb0: {
Expand All @@ -17,8 +18,11 @@ fn b(_1: &mut std::boxed::Box<T>) -> &mut T {
StorageLive(_4); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:8:5: 8:6
_4 = &mut (*_1); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:8:5: 8:6
StorageLive(_5); // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
_5 = &mut (*(*_4)); // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
_3 = move _5; // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
StorageLive(_6); // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
_6 = &mut (*(*_4)); // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
_5 = &mut (*_6); // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
_3 = &mut (*_5); // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
StorageDead(_6); // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
StorageDead(_5); // scope 1 at $SRC_DIR/liballoc/boxed.rs:LL:COL
_2 = &mut (*_3); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:8:5: 8:15
StorageDead(_4); // scope 0 at $DIR/issue-58867-inline-as-ref-as-mut.rs:8:14: 8:15
Expand Down
7 changes: 6 additions & 1 deletion src/test/mir-opt/nrvo-simple/rustc.nrvo.RenameReturnPlace.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -26,12 +26,17 @@
// + span: $DIR/nrvo-simple.rs:3:20: 3:21
// + literal: Const { ty: u8, val: Value(Scalar(0x00)) }
StorageLive(_3); // scope 1 at $DIR/nrvo-simple.rs:4:5: 4:19
StorageLive(_5); // scope 1 at $DIR/nrvo-simple.rs:4:10: 4:18
StorageLive(_6); // scope 1 at $DIR/nrvo-simple.rs:4:10: 4:18
- _6 = &mut _2; // scope 1 at $DIR/nrvo-simple.rs:4:10: 4:18
+ _6 = &mut _0; // scope 1 at $DIR/nrvo-simple.rs:4:10: 4:18
_3 = move _1(move _6) -> bb1; // scope 1 at $DIR/nrvo-simple.rs:4:5: 4:19
_5 = &mut (*_6); // scope 1 at $DIR/nrvo-simple.rs:4:10: 4:18
_3 = move _1(move _5) -> bb1; // scope 1 at $DIR/nrvo-simple.rs:4:5: 4:19
}

bb1: {
StorageDead(_5); // scope 1 at $DIR/nrvo-simple.rs:4:18: 4:19
StorageDead(_6); // scope 1 at $DIR/nrvo-simple.rs:4:19: 4:20
StorageDead(_3); // scope 1 at $DIR/nrvo-simple.rs:4:19: 4:20
- _0 = _2; // scope 1 at $DIR/nrvo-simple.rs:5:5: 5:8
- StorageDead(_2); // scope 0 at $DIR/nrvo-simple.rs:6:1: 6:2
Expand Down

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