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Auto merge of #139327 - cjgillot:gvn-place, r=oli-obk

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Allow GVN to produce places and not just locals.

That may be too big of a hammer, as we may introduce new deref projections (possible UB footgun + probably not good for perf).

The second commit opts out of introducing projections that don't have a stable offset, which is probably what we want. Hence no new Deref and no new Index projections.

Fixes https://github.com/rust-lang/rust/issues/138936
cc `@scottmcm` `@dianqk`
This commit is contained in:
bors 2025-04-09 08:50:33 +00:00
commit f06e5c1e35
14 changed files with 206 additions and 55 deletions
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74
compiler/rustc_mir_transform/src/gvn.rs
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@ -980,27 +980,17 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
}
}
let tcx = self.tcx;
let mut projection = SmallVec::<[PlaceElem<'tcx>; 1]>::new();
loop {
if let Some(local) = self.try_as_local(copy_from_local_value, location) {
projection.reverse();
let place = Place { local, projection: tcx.mk_place_elems(projection.as_slice()) };
if rvalue.ty(self.local_decls, tcx) == place.ty(self.local_decls, tcx).ty {
self.reused_locals.insert(local);
*rvalue = Rvalue::Use(Operand::Copy(place));
return Some(copy_from_value);
}
return None;
} else if let Value::Projection(pointer, proj) = *self.get(copy_from_local_value)
&& let Some(proj) = self.try_as_place_elem(proj, location)
{
projection.push(proj);
copy_from_local_value = pointer;
} else {
return None;
// Allow introducing places with non-constant offsets, as those are still better than
// reconstructing an aggregate.
if let Some(place) = self.try_as_place(copy_from_local_value, location, true) {
if rvalue.ty(self.local_decls, self.tcx) == place.ty(self.local_decls, self.tcx).ty {
self.reused_locals.insert(place.local);
*rvalue = Rvalue::Use(Operand::Copy(place));
return Some(copy_from_local_value);
}
}
None
}
fn simplify_aggregate(
@ -1672,14 +1662,14 @@ fn op_to_prop_const<'tcx>(
}
impl<'tcx> VnState<'_, 'tcx> {
/// If either [`Self::try_as_constant`] as [`Self::try_as_local`] succeeds,
/// If either [`Self::try_as_constant`] as [`Self::try_as_place`] succeeds,
/// returns that result as an [`Operand`].
fn try_as_operand(&mut self, index: VnIndex, location: Location) -> Option<Operand<'tcx>> {
if let Some(const_) = self.try_as_constant(index) {
Some(Operand::Constant(Box::new(const_)))
} else if let Some(local) = self.try_as_local(index, location) {
self.reused_locals.insert(local);
Some(Operand::Copy(local.into()))
} else if let Some(place) = self.try_as_place(index, location, false) {
self.reused_locals.insert(place.local);
Some(Operand::Copy(place))
} else {
None
}
@ -1712,6 +1702,35 @@ impl<'tcx> VnState<'_, 'tcx> {
Some(ConstOperand { span: DUMMY_SP, user_ty: None, const_ })
}
/// Construct a place which holds the same value as `index` and for which all locals strictly
/// dominate `loc`. If you used this place, add its base local to `reused_locals` to remove
/// storage statements.
#[instrument(level = "trace", skip(self), ret)]
fn try_as_place(
&mut self,
mut index: VnIndex,
loc: Location,
allow_complex_projection: bool,
) -> Option<Place<'tcx>> {
let mut projection = SmallVec::<[PlaceElem<'tcx>; 1]>::new();
loop {
if let Some(local) = self.try_as_local(index, loc) {
projection.reverse();
let place =
Place { local, projection: self.tcx.mk_place_elems(projection.as_slice()) };
return Some(place);
} else if let Value::Projection(pointer, proj) = *self.get(index)
&& (allow_complex_projection || proj.is_stable_offset())
&& let Some(proj) = self.try_as_place_elem(proj, loc)
{
projection.push(proj);
index = pointer;
} else {
return None;
}
}
}
/// If there is a local which is assigned `index`, and its assignment strictly dominates `loc`,
/// return it. If you used this local, add it to `reused_locals` to remove storage statements.
fn try_as_local(&mut self, index: VnIndex, loc: Location) -> Option<Local> {
@ -1762,11 +1781,12 @@ impl<'tcx> MutVisitor<'tcx> for VnState<'_, 'tcx> {
if let Some(value) = value {
if let Some(const_) = self.try_as_constant(value) {
*rvalue = Rvalue::Use(Operand::Constant(Box::new(const_)));
} else if let Some(local) = self.try_as_local(value, location)
&& *rvalue != Rvalue::Use(Operand::Move(local.into()))
} else if let Some(place) = self.try_as_place(value, location, false)
&& *rvalue != Rvalue::Use(Operand::Move(place))
&& *rvalue != Rvalue::Use(Operand::Copy(place))
{
*rvalue = Rvalue::Use(Operand::Copy(local.into()));
self.reused_locals.insert(local);
*rvalue = Rvalue::Use(Operand::Copy(place));
self.reused_locals.insert(place.local);
}
}
}