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Some cleanups to DataflowConstProp #110685

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merged 4 commits into from
Apr 24, 2023
Merged

Some cleanups to DataflowConstProp #110685

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0e68cbd
Remove useless special case.
cjgillot Apr 15, 2023
629cdb4
Move eval_discriminant.
cjgillot Apr 15, 2023
dd452ae
Simplify logic.
cjgillot Apr 15, 2023
dd78b99
Reduce rightward drift.
cjgillot Apr 15, 2023
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125 changes: 52 additions & 73 deletions compiler/rustc_mir_transform/src/dataflow_const_prop.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -70,22 +70,6 @@ struct ConstAnalysis<'a, 'tcx> {
param_env: ty::ParamEnv<'tcx>,
}

impl<'tcx> ConstAnalysis<'_, 'tcx> {
fn eval_discriminant(
&self,
enum_ty: Ty<'tcx>,
variant_index: VariantIdx,
) -> Option<ScalarTy<'tcx>> {
if !enum_ty.is_enum() {
return None;
}
let discr = enum_ty.discriminant_for_variant(self.tcx, variant_index)?;
let discr_layout = self.tcx.layout_of(self.param_env.and(discr.ty)).ok()?;
let discr_value = Scalar::try_from_uint(discr.val, discr_layout.size)?;
Some(ScalarTy(discr_value, discr.ty))
}
}

impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
type Value = FlatSet<ScalarTy<'tcx>>;

Expand Down Expand Up @@ -126,59 +110,55 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
// we must make sure that all `target as Variant#i` are `Top`.
state.flood(target.as_ref(), self.map());

if let Some(target_idx) = self.map().find(target.as_ref()) {
let (variant_target, variant_index) = match **kind {
AggregateKind::Tuple | AggregateKind::Closure(..) => {
(Some(target_idx), None)
}
AggregateKind::Adt(def_id, variant_index, ..) => {
match self.tcx.def_kind(def_id) {
DefKind::Struct => (Some(target_idx), None),
DefKind::Enum => (
self.map.apply(target_idx, TrackElem::Variant(variant_index)),
Some(variant_index),
),
_ => (None, None),
}
}
_ => (None, None),
};
if let Some(variant_target_idx) = variant_target {
for (field_index, operand) in operands.iter().enumerate() {
if let Some(field) = self.map().apply(
variant_target_idx,
TrackElem::Field(FieldIdx::from_usize(field_index)),
) {
let result = self.handle_operand(operand, state);
state.insert_idx(field, result, self.map());
}
let Some(target_idx) = self.map().find(target.as_ref()) else { return };

let (variant_target, variant_index) = match **kind {
AggregateKind::Tuple | AggregateKind::Closure(..) => (Some(target_idx), None),
AggregateKind::Adt(def_id, variant_index, ..) => {
match self.tcx.def_kind(def_id) {
DefKind::Struct => (Some(target_idx), None),
DefKind::Enum => (
self.map.apply(target_idx, TrackElem::Variant(variant_index)),
Some(variant_index),
),
_ => return,
}
}
if let Some(variant_index) = variant_index
&& let Some(discr_idx) = self.map().apply(target_idx, TrackElem::Discriminant)
{
// We are assigning the discriminant as part of an aggregate.
// This discriminant can only alias a variant field's value if the operand
// had an invalid value for that type.
// Using invalid values is UB, so we are allowed to perform the assignment
// without extra flooding.
let enum_ty = target.ty(self.local_decls, self.tcx).ty;
if let Some(discr_val) = self.eval_discriminant(enum_ty, variant_index) {
state.insert_value_idx(discr_idx, FlatSet::Elem(discr_val), &self.map);
_ => return,
};
if let Some(variant_target_idx) = variant_target {
for (field_index, operand) in operands.iter().enumerate() {
if let Some(field) = self.map().apply(
variant_target_idx,
TrackElem::Field(FieldIdx::from_usize(field_index)),
) {
let result = self.handle_operand(operand, state);
state.insert_idx(field, result, self.map());
}
}
}
if let Some(variant_index) = variant_index
&& let Some(discr_idx) = self.map().apply(target_idx, TrackElem::Discriminant)
{
// We are assigning the discriminant as part of an aggregate.
// This discriminant can only alias a variant field's value if the operand
// had an invalid value for that type.
// Using invalid values is UB, so we are allowed to perform the assignment
// without extra flooding.
let enum_ty = target.ty(self.local_decls, self.tcx).ty;
if let Some(discr_val) = self.eval_discriminant(enum_ty, variant_index) {
state.insert_value_idx(discr_idx, FlatSet::Elem(discr_val), &self.map);
}
}
}
Rvalue::CheckedBinaryOp(op, box (left, right)) => {
// Flood everything now, so we can use `insert_value_idx` directly later.
state.flood(target.as_ref(), self.map());

let target = self.map().find(target.as_ref());
let Some(target) = self.map().find(target.as_ref()) else { return };

let value_target = target
.and_then(|target| self.map().apply(target, TrackElem::Field(0_u32.into())));
let overflow_target = target
.and_then(|target| self.map().apply(target, TrackElem::Field(1_u32.into())));
let value_target = self.map().apply(target, TrackElem::Field(0_u32.into()));
let overflow_target = self.map().apply(target, TrackElem::Field(1_u32.into()));

if value_target.is_some() || overflow_target.is_some() {
let (val, overflow) = self.binary_op(state, *op, left, right);
Expand Down Expand Up @@ -377,6 +357,20 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
}
}

fn eval_discriminant(
&self,
enum_ty: Ty<'tcx>,
variant_index: VariantIdx,
) -> Option<ScalarTy<'tcx>> {
if !enum_ty.is_enum() {
return None;
}
let discr = enum_ty.discriminant_for_variant(self.tcx, variant_index)?;
let discr_layout = self.tcx.layout_of(self.param_env.and(discr.ty)).ok()?;
let discr_value = Scalar::try_from_uint(discr.val, discr_layout.size)?;
Some(ScalarTy(discr_value, discr.ty))
}

fn wrap_scalar(&self, scalar: Scalar, ty: Ty<'tcx>) -> FlatSet<ScalarTy<'tcx>> {
FlatSet::Elem(ScalarTy(scalar, ty))
}
Expand Down Expand Up @@ -520,21 +514,6 @@ impl<'tcx, 'map, 'a> Visitor<'tcx> for OperandCollector<'tcx, 'map, 'a> {
_ => (),
}
}

fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
match rvalue {
Rvalue::Discriminant(place) => {
match self.state.get_discr(place.as_ref(), self.visitor.map) {
FlatSet::Top => (),
FlatSet::Elem(value) => {
self.visitor.before_effect.insert((location, *place), value);
}
FlatSet::Bottom => (),
}
}
_ => self.super_rvalue(rvalue, location),
}
}
}

struct DummyMachine;
Expand Down