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Const prop should finish propagation into user defined variables #67130

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Dec 21, 2019
Merged

Const prop should finish propagation into user defined variables #67130

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52 changes: 34 additions & 18 deletions src/librustc_mir/transform/const_prop.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -262,7 +262,7 @@ struct ConstPropagator<'mir, 'tcx> {
ecx: InterpCx<'mir, 'tcx, ConstPropMachine>,
tcx: TyCtxt<'tcx>,
source: MirSource<'tcx>,
can_const_prop: IndexVec<Local, bool>,
can_const_prop: IndexVec<Local, ConstPropMode>,
param_env: ParamEnv<'tcx>,
// FIXME(eddyb) avoid cloning these two fields more than once,
// by accessing them through `ecx` instead.
Expand Down Expand Up @@ -708,17 +708,28 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
}
}

/// The mode that `ConstProp` is allowed to run in for a given `Local`.
#[derive(Clone, Copy, Debug, PartialEq)]
enum ConstPropMode {
/// The `Local` can be propagated into and reads of this `Local` can also be propagated.
FullConstProp,
/// The `Local` can be propagated into but reads cannot be propagated.
OnlyPropagateInto,
/// No propagation is allowed at all.
NoPropagation,
}

struct CanConstProp {
can_const_prop: IndexVec<Local, bool>,
can_const_prop: IndexVec<Local, ConstPropMode>,
// false at the beginning, once set, there are not allowed to be any more assignments
found_assignment: IndexVec<Local, bool>,
}

impl CanConstProp {
/// returns true if `local` can be propagated
fn check(body: ReadOnlyBodyAndCache<'_, '_>) -> IndexVec<Local, bool> {
fn check(body: ReadOnlyBodyAndCache<'_, '_>) -> IndexVec<Local, ConstPropMode> {
let mut cpv = CanConstProp {
can_const_prop: IndexVec::from_elem(true, &body.local_decls),
can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
found_assignment: IndexVec::from_elem(false, &body.local_decls),
};
for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
Expand All @@ -728,10 +739,10 @@ impl CanConstProp {
// FIXME(oli-obk): lint variables until they are used in a condition
// FIXME(oli-obk): lint if return value is constant
let local_kind = body.local_kind(local);
*val = local_kind == LocalKind::Temp || local_kind == LocalKind::ReturnPointer;

if !*val {
trace!("local {:?} can't be propagated because it's not a temporary", local);
if local_kind == LocalKind::Arg || local_kind == LocalKind::Var {
*val = ConstPropMode::OnlyPropagateInto;
trace!("local {:?} can't be const propagated because it's not a temporary", local);
}
}
cpv.visit_body(body);
Expand All @@ -753,7 +764,7 @@ impl<'tcx> Visitor<'tcx> for CanConstProp {
// only occur in independent execution paths
MutatingUse(MutatingUseContext::Store) => if self.found_assignment[local] {
trace!("local {:?} can't be propagated because of multiple assignments", local);
self.can_const_prop[local] = false;
self.can_const_prop[local] = ConstPropMode::NoPropagation;
} else {
self.found_assignment[local] = true
},
Expand All @@ -766,7 +777,7 @@ impl<'tcx> Visitor<'tcx> for CanConstProp {
NonUse(_) => {},
_ => {
trace!("local {:?} can't be propagaged because it's used: {:?}", local, context);
self.can_const_prop[local] = false;
self.can_const_prop[local] = ConstPropMode::NoPropagation;
},
}
}
Expand Down Expand Up @@ -800,10 +811,10 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
if let Some(local) = place.as_local() {
let source = statement.source_info;
let can_const_prop = self.can_const_prop[local];
if let Some(()) = self.const_prop(rval, place_layout, source, place) {
if self.can_const_prop[local] {
trace!("propagated into {:?}", local);

if can_const_prop == ConstPropMode::FullConstProp ||
can_const_prop == ConstPropMode::OnlyPropagateInto {
if let Some(value) = self.get_const(local) {
if self.should_const_prop(value) {
trace!("replacing {:?} with {:?}", rval, value);
Expand All @@ -812,21 +823,26 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
value,
statement.source_info,
);

if can_const_prop == ConstPropMode::FullConstProp {
trace!("propagated into {:?}", local);
}
}
}
} else {
trace!("can't propagate into {:?}", local);
if local != RETURN_PLACE {
self.remove_const(local);
}
}
}
if self.can_const_prop[local] != ConstPropMode::FullConstProp {
trace!("can't propagate into {:?}", local);
if local != RETURN_PLACE {
self.remove_const(local);
}
}
}
}
} else {
match statement.kind {
StatementKind::StorageLive(local) |
StatementKind::StorageDead(local) if self.can_const_prop[local] => {
StatementKind::StorageDead(local) => {
let frame = self.ecx.frame_mut();
frame.locals[local].value =
if let StatementKind::StorageLive(_) = statement.kind {
Expand Down
2 changes: 1 addition & 1 deletion src/test/mir-opt/const_prop/aggregate.rs
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Expand Up @@ -19,7 +19,7 @@ fn main() {
// ...
// _3 = (const 0i32, const 1i32, const 2i32);
// _2 = const 1i32;
// _1 = Add(move _2, const 0i32);
// _1 = const 1i32;
// ...
// }
// END rustc.main.ConstProp.after.mir
2 changes: 1 addition & 1 deletion src/test/mir-opt/const_prop/array_index.rs
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Expand Up @@ -26,7 +26,7 @@ fn main() {
// assert(const true, "index out of bounds: the len is move _4 but the index is _3") -> bb1;
// }
// bb1: {
// _1 = _2[_3];
// _1 = const 2u32;
// ...
// return;
// }
Expand Down
149 changes: 149 additions & 0 deletions src/test/mir-opt/const_prop/optimizes_into_variable.rs
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@@ -0,0 +1,149 @@
// compile-flags: -C overflow-checks=on

struct Point {
x: u32,
y: u32,
}

fn main() {
let x = 2 + 2;
let y = [0, 1, 2, 3, 4, 5][3];
let z = (Point { x: 12, y: 42}).y;
}

// END RUST SOURCE
// START rustc.main.ConstProp.before.mir
// let mut _0: ();
// let _1: i32;
// let mut _2: (i32, bool);
// let mut _4: [i32; 6];
// let _5: usize;
// let mut _6: usize;
// let mut _7: bool;
// let mut _9: Point;
// scope 1 {
// debug x => _1;
// let _3: i32;
// scope 2 {
// debug y => _3;
// let _8: u32;
// scope 3 {
// debug z => _8;
// }
// }
// }
// bb0: {
// StorageLive(_1);
// _2 = CheckedAdd(const 2i32, const 2i32);
// assert(!move (_2.1: bool), "attempt to add with overflow") -> bb1;
// }
// bb1: {
// _1 = move (_2.0: i32);
// StorageLive(_3);
// StorageLive(_4);
// _4 = [const 0i32, const 1i32, const 2i32, const 3i32, const 4i32, const 5i32];
// StorageLive(_5);
// _5 = const 3usize;
// _6 = const 6usize;
// _7 = Lt(_5, _6);
// assert(move _7, "index out of bounds: the len is move _6 but the index is _5") -> bb2;
// }
// bb2: {
// _3 = _4[_5];
// StorageDead(_5);
// StorageDead(_4);
// StorageLive(_8);
// StorageLive(_9);
// _9 = Point { x: const 12u32, y: const 42u32 };
// _8 = (_9.1: u32);
// StorageDead(_9);
// _0 = ();
// StorageDead(_8);
// StorageDead(_3);
// StorageDead(_1);
// return;
// }
// END rustc.main.ConstProp.before.mir
// START rustc.main.ConstProp.after.mir
// let mut _0: ();
// let _1: i32;
// let mut _2: (i32, bool);
// let mut _4: [i32; 6];
// let _5: usize;
// let mut _6: usize;
// let mut _7: bool;
// let mut _9: Point;
// scope 1 {
// debug x => _1;
// let _3: i32;
// scope 2 {
// debug y => _3;
// let _8: u32;
// scope 3 {
// debug z => _8;
// }
// }
// }
// bb0: {
// StorageLive(_1);
// _2 = (const 4i32, const false);
// assert(!const false, "attempt to add with overflow") -> bb1;
// }
// bb1: {
// _1 = const 4i32;
// StorageLive(_3);
// StorageLive(_4);
// _4 = [const 0i32, const 1i32, const 2i32, const 3i32, const 4i32, const 5i32];
// StorageLive(_5);
// _5 = const 3usize;
// _6 = const 6usize;
// _7 = const true;
// assert(const true, "index out of bounds: the len is move _6 but the index is _5") -> bb2;
// }
// bb2: {
// _3 = const 3i32;
// StorageDead(_5);
// StorageDead(_4);
// StorageLive(_8);
// StorageLive(_9);
// _9 = Point { x: const 12u32, y: const 42u32 };
// _8 = const 42u32;
// StorageDead(_9);
// _0 = ();
// StorageDead(_8);
// StorageDead(_3);
// StorageDead(_1);
// return;
// }
// END rustc.main.ConstProp.after.mir
// START rustc.main.SimplifyLocals.after.mir
// let mut _0: ();
// let _1: i32;
// let mut _3: [i32; 6];
// scope 1 {
// debug x => _1;
// let _2: i32;
// scope 2 {
// debug y => _2;
// let _4: u32;
// scope 3 {
// debug z => _4;
// }
// }
// }
// bb0: {
// StorageLive(_1);
// _1 = const 4i32;
// StorageLive(_2);
// StorageLive(_3);
// _3 = [const 0i32, const 1i32, const 2i32, const 3i32, const 4i32, const 5i32];
// _2 = const 3i32;
// StorageDead(_3);
// StorageLive(_4);
// _4 = const 42u32;
// StorageDead(_4);
// StorageDead(_2);
// StorageDead(_1);
// return;
// }
// END rustc.main.SimplifyLocals.after.mir
2 changes: 1 addition & 1 deletion src/test/mir-opt/const_prop/read_immutable_static.rs
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Expand Up @@ -25,7 +25,7 @@ fn main() {
// _2 = const 2u8;
// ...
// _4 = const 2u8;
// _1 = Add(move _2, move _4);
// _1 = const 4u8;
// ...
// }
// END rustc.main.ConstProp.after.mir
2 changes: 1 addition & 1 deletion src/test/mir-opt/const_prop/repeat.rs
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Expand Up @@ -30,7 +30,7 @@ fn main() {
// }
// bb1: {
// _2 = const 42u32;
// _1 = Add(move _2, const 0u32);
// _1 = const 42u32;
// ...
// return;
// }
Expand Down