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Auto merge of #72135 - oli-obk:const_prop_deaggregates, r=wesleywiser

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Propagate locals, even if they have unpropagatable assignments somewhere

Second try for https://github.com/rust-lang/rust/pull/71946#discussion_r422967292

r? @wesleywiser

cc @rust-lang/wg-mir-opt @RalfJung
This commit is contained in:
bors 2020-05-17 09:18:12 +00:00
commit 7faeae0d38
5 changed files with 189 additions and 1 deletions
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25
src/librustc_mir/transform/const_prop.rs
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@ -781,6 +781,10 @@ impl<'tcx> Visitor<'tcx> for CanConstProp {
// Projections are fine, because `&mut foo.x` will be caught by
// `MutatingUseContext::Borrow` elsewhere.
MutatingUse(MutatingUseContext::Projection)
// These are just stores, where the storing is not propagatable, but there may be later
// mutations of the same local via `Store`
| MutatingUse(MutatingUseContext::Call)
// Actual store that can possibly even propagate a value
| MutatingUse(MutatingUseContext::Store) => {
if !self.found_assignment.insert(local) {
match &mut self.can_const_prop[local] {
@ -805,7 +809,21 @@ impl<'tcx> Visitor<'tcx> for CanConstProp {
| NonMutatingUse(NonMutatingUseContext::Inspect)
| NonMutatingUse(NonMutatingUseContext::Projection)
| NonUse(_) => {}
_ => {
// These could be propagated with a smarter analysis or just some careful thinking about
// whether they'd be fine right now.
MutatingUse(MutatingUseContext::AsmOutput)
| MutatingUse(MutatingUseContext::Yield)
| MutatingUse(MutatingUseContext::Drop)
| MutatingUse(MutatingUseContext::Retag)
// These can't ever be propagated under any scheme, as we can't reason about indirect
// mutation.
| NonMutatingUse(NonMutatingUseContext::SharedBorrow)
| NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
| NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
| NonMutatingUse(NonMutatingUseContext::AddressOf)
| MutatingUse(MutatingUseContext::Borrow)
| MutatingUse(MutatingUseContext::AddressOf) => {
trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
self.can_const_prop[local] = ConstPropMode::NoPropagation;
}
@ -881,6 +899,11 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
// ```
// FIXME: we overzealously erase the entire local, because that's easier to
// implement.
trace!(
"propagation into {:?} failed.
Nuking the entire site from orbit, it's the only way to be sure",
place,
);
Self::remove_const(&mut self.ecx, place.local);
}
}

14
src/test/mir-opt/const_prop/mutable_variable_aggregate_partial_read.rs Normal file
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@ -0,0 +1,14 @@
// compile-flags: -O
// EMIT_MIR rustc.main.ConstProp.diff
fn main() {
let mut x: (i32, i32) = foo();
x.1 = 99;
x.0 = 42;
let y = x.1;
}
#[inline(never)]
fn foo() -> (i32, i32) {
unimplemented!()
}

62
src/test/mir-opt/const_prop/mutable_variable_aggregate_partial_read/rustc.main.ConstProp.diff Normal file
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@ -0,0 +1,62 @@
- // MIR for `main` before ConstProp
+ // MIR for `main` after ConstProp
fn main() -> () {
let mut _0: (); // return place in scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:4:11: 4:11
let mut _1: (i32, i32) as UserTypeProjection { base: UserType(0), projs: [] }; // in scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:5:9: 5:14
scope 1 {
debug x => _1; // in scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:5:9: 5:14
let _2: i32; // in scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:8:9: 8:10
scope 2 {
debug y => _2; // in scope 2 at $DIR/mutable_variable_aggregate_partial_read.rs:8:9: 8:10
}
}
bb0: {
StorageLive(_1); // scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:5:9: 5:14
_1 = const foo() -> bb1; // scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:5:29: 5:34
// ty::Const
// + ty: fn() -> (i32, i32) {foo}
// + val: Value(Scalar(<ZST>))
// mir::Constant
// + span: $DIR/mutable_variable_aggregate_partial_read.rs:5:29: 5:32
// + literal: Const { ty: fn() -> (i32, i32) {foo}, val: Value(Scalar(<ZST>)) }
}
bb1: {
(_1.1: i32) = const 99i32; // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:6:5: 6:13
// ty::Const
// + ty: i32
// + val: Value(Scalar(0x00000063))
// mir::Constant
// + span: $DIR/mutable_variable_aggregate_partial_read.rs:6:11: 6:13
// + literal: Const { ty: i32, val: Value(Scalar(0x00000063)) }
(_1.0: i32) = const 42i32; // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:7:5: 7:13
// ty::Const
// + ty: i32
// + val: Value(Scalar(0x0000002a))
// mir::Constant
// + span: $DIR/mutable_variable_aggregate_partial_read.rs:7:11: 7:13
// + literal: Const { ty: i32, val: Value(Scalar(0x0000002a)) }
StorageLive(_2); // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:8:9: 8:10
- _2 = (_1.1: i32); // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:8:13: 8:16
+ _2 = const 99i32; // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:8:13: 8:16
+ // ty::Const
+ // + ty: i32
+ // + val: Value(Scalar(0x00000063))
+ // mir::Constant
+ // + span: $DIR/mutable_variable_aggregate_partial_read.rs:8:13: 8:16
+ // + literal: Const { ty: i32, val: Value(Scalar(0x00000063)) }
_0 = const (); // scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:4:11: 9:2
// ty::Const
// + ty: ()
// + val: Value(Scalar(<ZST>))
// mir::Constant
// + span: $DIR/mutable_variable_aggregate_partial_read.rs:4:11: 9:2
// + literal: Const { ty: (), val: Value(Scalar(<ZST>)) }
StorageDead(_2); // scope 1 at $DIR/mutable_variable_aggregate_partial_read.rs:9:1: 9:2
StorageDead(_1); // scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:9:1: 9:2
return; // scope 0 at $DIR/mutable_variable_aggregate_partial_read.rs:9:2: 9:2
}
}

15
src/test/mir-opt/const_prop/mutable_variable_unprop_assign.rs Normal file
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@ -0,0 +1,15 @@
// compile-flags: -O
// EMIT_MIR rustc.main.ConstProp.diff
fn main() {
let a = foo();
let mut x: (i32, i32) = (1, 2);
x.1 = a;
let y = x.1;
let z = x.0; // this could theoretically be allowed, but we can't handle it right now
}
#[inline(never)]
fn foo() -> i32 {
unimplemented!()
}

74
src/test/mir-opt/const_prop/mutable_variable_unprop_assign/rustc.main.ConstProp.diff Normal file
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@ -0,0 +1,74 @@
- // MIR for `main` before ConstProp
+ // MIR for `main` after ConstProp
fn main() -> () {
let mut _0: (); // return place in scope 0 at $DIR/mutable_variable_unprop_assign.rs:4:11: 4:11
let _1: i32; // in scope 0 at $DIR/mutable_variable_unprop_assign.rs:5:9: 5:10
let mut _3: i32; // in scope 0 at $DIR/mutable_variable_unprop_assign.rs:7:11: 7:12
scope 1 {
debug a => _1; // in scope 1 at $DIR/mutable_variable_unprop_assign.rs:5:9: 5:10
let mut _2: (i32, i32) as UserTypeProjection { base: UserType(0), projs: [] }; // in scope 1 at $DIR/mutable_variable_unprop_assign.rs:6:9: 6:14
scope 2 {
debug x => _2; // in scope 2 at $DIR/mutable_variable_unprop_assign.rs:6:9: 6:14
let _4: i32; // in scope 2 at $DIR/mutable_variable_unprop_assign.rs:8:9: 8:10
scope 3 {
debug y => _4; // in scope 3 at $DIR/mutable_variable_unprop_assign.rs:8:9: 8:10
let _5: i32; // in scope 3 at $DIR/mutable_variable_unprop_assign.rs:9:9: 9:10
scope 4 {
debug z => _5; // in scope 4 at $DIR/mutable_variable_unprop_assign.rs:9:9: 9:10
}
}
}
}
bb0: {
StorageLive(_1); // scope 0 at $DIR/mutable_variable_unprop_assign.rs:5:9: 5:10
_1 = const foo() -> bb1; // scope 0 at $DIR/mutable_variable_unprop_assign.rs:5:13: 5:18
// ty::Const
// + ty: fn() -> i32 {foo}
// + val: Value(Scalar(<ZST>))
// mir::Constant
// + span: $DIR/mutable_variable_unprop_assign.rs:5:13: 5:16
// + literal: Const { ty: fn() -> i32 {foo}, val: Value(Scalar(<ZST>)) }
}
bb1: {
StorageLive(_2); // scope 1 at $DIR/mutable_variable_unprop_assign.rs:6:9: 6:14
_2 = (const 1i32, const 2i32); // scope 1 at $DIR/mutable_variable_unprop_assign.rs:6:29: 6:35
// ty::Const
// + ty: i32
// + val: Value(Scalar(0x00000001))
// mir::Constant
- // + span: $DIR/mutable_variable_unprop_assign.rs:6:30: 6:31
+ // + span: $DIR/mutable_variable_unprop_assign.rs:6:29: 6:35
// + literal: Const { ty: i32, val: Value(Scalar(0x00000001)) }
// ty::Const
// + ty: i32
// + val: Value(Scalar(0x00000002))
// mir::Constant
- // + span: $DIR/mutable_variable_unprop_assign.rs:6:33: 6:34
+ // + span: $DIR/mutable_variable_unprop_assign.rs:6:29: 6:35
// + literal: Const { ty: i32, val: Value(Scalar(0x00000002)) }
StorageLive(_3); // scope 2 at $DIR/mutable_variable_unprop_assign.rs:7:11: 7:12
_3 = _1; // scope 2 at $DIR/mutable_variable_unprop_assign.rs:7:11: 7:12
(_2.1: i32) = move _3; // scope 2 at $DIR/mutable_variable_unprop_assign.rs:7:5: 7:12
StorageDead(_3); // scope 2 at $DIR/mutable_variable_unprop_assign.rs:7:11: 7:12
StorageLive(_4); // scope 2 at $DIR/mutable_variable_unprop_assign.rs:8:9: 8:10
_4 = (_2.1: i32); // scope 2 at $DIR/mutable_variable_unprop_assign.rs:8:13: 8:16
StorageLive(_5); // scope 3 at $DIR/mutable_variable_unprop_assign.rs:9:9: 9:10
_5 = (_2.0: i32); // scope 3 at $DIR/mutable_variable_unprop_assign.rs:9:13: 9:16
_0 = const (); // scope 0 at $DIR/mutable_variable_unprop_assign.rs:4:11: 10:2
// ty::Const
// + ty: ()
// + val: Value(Scalar(<ZST>))
// mir::Constant
// + span: $DIR/mutable_variable_unprop_assign.rs:4:11: 10:2
// + literal: Const { ty: (), val: Value(Scalar(<ZST>)) }
StorageDead(_5); // scope 3 at $DIR/mutable_variable_unprop_assign.rs:10:1: 10:2
StorageDead(_4); // scope 2 at $DIR/mutable_variable_unprop_assign.rs:10:1: 10:2
StorageDead(_2); // scope 1 at $DIR/mutable_variable_unprop_assign.rs:10:1: 10:2
StorageDead(_1); // scope 0 at $DIR/mutable_variable_unprop_assign.rs:10:1: 10:2
return; // scope 0 at $DIR/mutable_variable_unprop_assign.rs:10:2: 10:2
}
}