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Rollup merge of rust-lang#80458 - RalfJung:promotion-refactor, r=oli-obk
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Some Promotion Refactoring

Clean up promotion a bit:
* factor out some common code
* more exhaustive matches

This *should* not break anything... the only potentially-breaking change is that `BorrowKind::Shallow | BorrowKind::Unique` are now rejected for internal references.

r? `@oli-obk`
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@Dylan-DPC
Dylan-DPC committed Dec 30, 2020
2 parents fd364d6 + 51cec58 commit ea43757
Showing 1 changed file with 115 additions and 99 deletions.
214 changes: 115 additions & 99 deletions compiler/rustc_mir/src/transform/promote_consts.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -90,7 +90,7 @@ pub enum TempState {
impl TempState {
pub fn is_promotable(&self) -> bool {
debug!("is_promotable: self={:?}", self);
matches!(self, TempState::Defined { .. } )
matches!(self, TempState::Defined { .. })
}
}

Expand Down Expand Up @@ -309,50 +309,26 @@ impl<'tcx> Validator<'_, 'tcx> {
let statement = &self.body[loc.block].statements[loc.statement_index];
match &statement.kind {
StatementKind::Assign(box (_, Rvalue::Ref(_, kind, place))) => {
match kind {
BorrowKind::Shared | BorrowKind::Mut { .. } => {}

// FIXME(eddyb) these aren't promoted here but *could*
// be promoted as part of a larger value because
// `validate_rvalue` doesn't check them, need to
// figure out what is the intended behavior.
BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),
}

// We can only promote interior borrows of promotable temps (non-temps
// don't get promoted anyway).
self.validate_local(place.local)?;

// The reference operation itself must be promotable.
// (Needs to come after `validate_local` to avoid ICEs.)
self.validate_ref(*kind, place)?;

// We do not check all the projections (they do not get promoted anyway),
// but we do stay away from promoting anything involving a dereference.
if place.projection.contains(&ProjectionElem::Deref) {
return Err(Unpromotable);
}
if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
return Err(Unpromotable);
}

// FIXME(eddyb) this duplicates part of `validate_rvalue`.
let has_mut_interior =
self.qualif_local::<qualifs::HasMutInterior>(place.local);
if has_mut_interior {
// We cannot promote things that need dropping, since the promoted value
// would not get dropped.
if self.qualif_local::<qualifs::NeedsDrop>(place.local) {
return Err(Unpromotable);
}

if let BorrowKind::Mut { .. } = kind {
let ty = place.ty(self.body, self.tcx).ty;

// In theory, any zero-sized value could be borrowed
// mutably without consequences. However, only &mut []
// is allowed right now.
if let ty::Array(_, len) = ty.kind() {
match len.try_eval_usize(self.tcx, self.param_env) {
Some(0) => {}
_ => return Err(Unpromotable),
}
} else {
return Err(Unpromotable);
}
}

Ok(())
}
_ => bug!(),
Expand Down Expand Up @@ -572,58 +548,115 @@ impl<'tcx> Validator<'_, 'tcx> {
}
}

fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
match *rvalue {
Rvalue::Cast(CastKind::Misc, ref operand, cast_ty) => {
let operand_ty = operand.ty(self.body, self.tcx);
let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
// ptr-to-int casts are not possible in consts and thus not promotable
fn validate_ref(&self, kind: BorrowKind, place: &Place<'tcx>) -> Result<(), Unpromotable> {
match kind {
// Reject these borrow types just to be safe.
// FIXME(RalfJung): could we allow them? Should we? No point in it until we have a usecase.
BorrowKind::Shallow | BorrowKind::Unique => return Err(Unpromotable),

BorrowKind::Shared => {
let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
if has_mut_interior {
return Err(Unpromotable);
}
}

Rvalue::BinaryOp(op, ref lhs, _) => {
if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind() {
assert!(
op == BinOp::Eq
|| op == BinOp::Ne
|| op == BinOp::Le
|| op == BinOp::Lt
|| op == BinOp::Ge
|| op == BinOp::Gt
|| op == BinOp::Offset
);
BorrowKind::Mut { .. } => {
let ty = place.ty(self.body, self.tcx).ty;

// raw pointer operations are not allowed inside consts and thus not promotable
// In theory, any zero-sized value could be borrowed
// mutably without consequences. However, only &mut []
// is allowed right now.
if let ty::Array(_, len) = ty.kind() {
match len.try_eval_usize(self.tcx, self.param_env) {
Some(0) => {}
_ => return Err(Unpromotable),
}
} else {
return Err(Unpromotable);
}
}

Rvalue::NullaryOp(NullOp::Box, _) => return Err(Unpromotable),

// FIXME(RalfJung): the rest is *implicitly considered promotable*... that seems dangerous.
_ => {}
}

Ok(())
}

fn validate_rvalue(&self, rvalue: &Rvalue<'tcx>) -> Result<(), Unpromotable> {
match rvalue {
Rvalue::ThreadLocalRef(_) => Err(Unpromotable),
Rvalue::Use(operand)
| Rvalue::Repeat(operand, _)
| Rvalue::UnaryOp(UnOp::Not | UnOp::Neg, operand) => {
self.validate_operand(operand)?;
}

Rvalue::NullaryOp(..) => Ok(()),
Rvalue::Discriminant(place) | Rvalue::Len(place) => {
self.validate_place(place.as_ref())?
}

Rvalue::Discriminant(place) | Rvalue::Len(place) => self.validate_place(place.as_ref()),
Rvalue::ThreadLocalRef(_) => return Err(Unpromotable),

Rvalue::Use(operand)
| Rvalue::Repeat(operand, _)
| Rvalue::UnaryOp(_, operand)
| Rvalue::Cast(_, operand, _) => self.validate_operand(operand),
Rvalue::Cast(kind, operand, cast_ty) => {
if matches!(kind, CastKind::Misc) {
let operand_ty = operand.ty(self.body, self.tcx);
let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
if let (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) = (cast_in, cast_out) {
// ptr-to-int casts are not possible in consts and thus not promotable
return Err(Unpromotable);
}
// int-to-ptr casts are fine, they just use the integer value at pointer type.
}

self.validate_operand(operand)?;
}

Rvalue::BinaryOp(op, lhs, rhs) | Rvalue::CheckedBinaryOp(op, lhs, rhs) => {
let op = *op;
if let ty::RawPtr(_) | ty::FnPtr(..) = lhs.ty(self.body, self.tcx).kind() {
// raw pointer operations are not allowed inside consts and thus not promotable
assert!(matches!(
op,
BinOp::Eq
| BinOp::Ne
| BinOp::Le
| BinOp::Lt
| BinOp::Ge
| BinOp::Gt
| BinOp::Offset
));
return Err(Unpromotable);
}

match op {
// FIXME: reject operations that can fail -- namely, division and modulo.
BinOp::Eq
| BinOp::Ne
| BinOp::Le
| BinOp::Lt
| BinOp::Ge
| BinOp::Gt
| BinOp::Offset
| BinOp::Add
| BinOp::Sub
| BinOp::Mul
| BinOp::Div
| BinOp::Rem
| BinOp::BitXor
| BinOp::BitAnd
| BinOp::BitOr
| BinOp::Shl
| BinOp::Shr => {}
}

Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
self.validate_operand(lhs)?;
self.validate_operand(rhs)
self.validate_operand(rhs)?;
}

Rvalue::NullaryOp(op, _) => match op {
NullOp::Box => return Err(Unpromotable),
NullOp::SizeOf => {}
},

Rvalue::AddressOf(_, place) => {
// We accept `&raw *`, i.e., raw reborrows -- creating a raw pointer is
// no problem, only using it is.
Expand All @@ -636,53 +669,36 @@ impl<'tcx> Validator<'_, 'tcx> {
});
}
}
Err(Unpromotable)
return Err(Unpromotable);
}

Rvalue::Ref(_, kind, place) => {
if let BorrowKind::Mut { .. } = kind {
let ty = place.ty(self.body, self.tcx).ty;

// In theory, any zero-sized value could be borrowed
// mutably without consequences. However, only &mut []
// is allowed right now.
if let ty::Array(_, len) = ty.kind() {
match len.try_eval_usize(self.tcx, self.param_env) {
Some(0) => {}
_ => return Err(Unpromotable),
}
} else {
return Err(Unpromotable);
}
}

// Special-case reborrows to be more like a copy of the reference.
let mut place = place.as_ref();
if let [proj_base @ .., ProjectionElem::Deref] = &place.projection {
let base_ty = Place::ty_from(place.local, proj_base, self.body, self.tcx).ty;
let mut place_simplified = place.as_ref();
if let [proj_base @ .., ProjectionElem::Deref] = &place_simplified.projection {
let base_ty =
Place::ty_from(place_simplified.local, proj_base, self.body, self.tcx).ty;
if let ty::Ref(..) = base_ty.kind() {
place = PlaceRef { local: place.local, projection: proj_base };
place_simplified =
PlaceRef { local: place_simplified.local, projection: proj_base };
}
}

self.validate_place(place)?;

let has_mut_interior = self.qualif_local::<qualifs::HasMutInterior>(place.local);
if has_mut_interior {
return Err(Unpromotable);
}
self.validate_place(place_simplified)?;

Ok(())
// Check that the reference is fine (using the original place!).
// (Needs to come after `validate_place` to avoid ICEs.)
self.validate_ref(*kind, place)?;
}

Rvalue::Aggregate(_, ref operands) => {
Rvalue::Aggregate(_, operands) => {
for o in operands {
self.validate_operand(o)?;
}

Ok(())
}
}

Ok(())
}

fn validate_call(
Expand Down

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