diff options
Diffstat (limited to 'compiler/rustc_mir_transform/src')
| -rw-r--r-- | compiler/rustc_mir_transform/src/gvn.rs | 179 | ||||
| -rw-r--r-- | compiler/rustc_mir_transform/src/instsimplify.rs | 23 |
2 files changed, 139 insertions, 63 deletions
diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs index 283ed94b615..71bec38c405 100644 --- a/compiler/rustc_mir_transform/src/gvn.rs +++ b/compiler/rustc_mir_transform/src/gvn.rs @@ -1366,57 +1366,108 @@ impl<'body, 'tcx> VnState<'body, 'tcx> { return self.new_opaque(); } - let mut was_updated = false; - - // If that cast just casts away the metadata again, - if let PtrToPtr = kind - && let Value::Aggregate(AggregateTy::RawPtr { data_pointer_ty, .. }, _, fields) = - self.get(value) - && let ty::RawPtr(to_pointee, _) = to.kind() - && to_pointee.is_sized(self.tcx, self.typing_env()) - { - from = *data_pointer_ty; - value = fields[0]; - was_updated = true; - if *data_pointer_ty == to { - return Some(fields[0]); + let mut was_ever_updated = false; + loop { + let mut was_updated_this_iteration = false; + + // Transmuting between raw pointers is just a pointer cast so long as + // they have the same metadata type (like `*const i32` <=> `*mut u64` + // or `*mut [i32]` <=> `*const [u64]`), including the common special + // case of `*const T` <=> `*mut T`. + if let Transmute = kind + && from.is_unsafe_ptr() + && to.is_unsafe_ptr() + && self.pointers_have_same_metadata(from, to) + { + *kind = PtrToPtr; + was_updated_this_iteration = true; } - } - // PtrToPtr-then-PtrToPtr can skip the intermediate step - if let PtrToPtr = kind - && let Value::Cast { kind: inner_kind, value: inner_value, from: inner_from, to: _ } = - *self.get(value) - && let PtrToPtr = inner_kind - { - from = inner_from; - value = inner_value; - was_updated = true; - if inner_from == to { - return Some(inner_value); + // If a cast just casts away the metadata again, then we can get it by + // casting the original thin pointer passed to `from_raw_parts` + if let PtrToPtr = kind + && let Value::Aggregate(AggregateTy::RawPtr { data_pointer_ty, .. }, _, fields) = + self.get(value) + && let ty::RawPtr(to_pointee, _) = to.kind() + && to_pointee.is_sized(self.tcx, self.typing_env()) + { + from = *data_pointer_ty; + value = fields[0]; + was_updated_this_iteration = true; + if *data_pointer_ty == to { + return Some(fields[0]); + } } - } - // PtrToPtr-then-Transmute can just transmute the original, so long as the - // PtrToPtr didn't change metadata (and thus the size of the pointer) - if let Transmute = kind - && let Value::Cast { - kind: PtrToPtr, + // Aggregate-then-Transmute can just transmute the original field value, + // so long as the bytes of a value from only from a single field. + if let Transmute = kind + && let Value::Aggregate(_aggregate_ty, variant_idx, field_values) = self.get(value) + && let Some((field_idx, field_ty)) = + self.value_is_all_in_one_field(from, *variant_idx) + { + from = field_ty; + value = field_values[field_idx.as_usize()]; + was_updated_this_iteration = true; + if field_ty == to { + return Some(value); + } + } + + // Various cast-then-cast cases can be simplified. + if let Value::Cast { + kind: inner_kind, value: inner_value, from: inner_from, to: inner_to, } = *self.get(value) - && self.pointers_have_same_metadata(inner_from, inner_to) - { - from = inner_from; - value = inner_value; - was_updated = true; - if inner_from == to { - return Some(inner_value); + { + let new_kind = match (inner_kind, *kind) { + // Even if there's a narrowing cast in here that's fine, because + // things like `*mut [i32] -> *mut i32 -> *const i32` and + // `*mut [i32] -> *const [i32] -> *const i32` can skip the middle in MIR. + (PtrToPtr, PtrToPtr) => Some(PtrToPtr), + // PtrToPtr-then-Transmute is fine so long as the pointer cast is identity: + // `*const T -> *mut T -> NonNull<T>` is fine, but we need to check for narrowing + // to skip things like `*const [i32] -> *const i32 -> NonNull<T>`. + (PtrToPtr, Transmute) + if self.pointers_have_same_metadata(inner_from, inner_to) => + { + Some(Transmute) + } + // Similarly, for Transmute-then-PtrToPtr. Note that we need to check different + // variables for their metadata, and thus this can't merge with the previous arm. + (Transmute, PtrToPtr) if self.pointers_have_same_metadata(from, to) => { + Some(Transmute) + } + // If would be legal to always do this, but we don't want to hide information + // from the backend that it'd otherwise be able to use for optimizations. + (Transmute, Transmute) + if !self.type_may_have_niche_of_interest_to_backend(inner_to) => + { + Some(Transmute) + } + _ => None, + }; + if let Some(new_kind) = new_kind { + *kind = new_kind; + from = inner_from; + value = inner_value; + was_updated_this_iteration = true; + if inner_from == to { + return Some(inner_value); + } + } + } + + if was_updated_this_iteration { + was_ever_updated = true; + } else { + break; } } - if was_updated && let Some(op) = self.try_as_operand(value, location) { + if was_ever_updated && let Some(op) = self.try_as_operand(value, location) { *operand = op; } @@ -1438,6 +1489,54 @@ impl<'body, 'tcx> VnState<'body, 'tcx> { false } } + + /// Returns `false` if we know for sure that this type has no interesting niche, + /// and thus we can skip transmuting through it without worrying. + /// + /// The backend will emit `assume`s when transmuting between types with niches, + /// so we want to preserve `i32 -> char -> u32` so that that data is around, + /// but it's fine to skip whole-range-is-value steps like `A -> u32 -> B`. + fn type_may_have_niche_of_interest_to_backend(&self, ty: Ty<'tcx>) -> bool { + let Ok(layout) = self.ecx.layout_of(ty) else { + // If it's too generic or something, then assume it might be interesting later. + return true; + }; + + match layout.backend_repr { + BackendRepr::Uninhabited => true, + BackendRepr::Scalar(a) => !a.is_always_valid(&self.ecx), + BackendRepr::ScalarPair(a, b) => { + !a.is_always_valid(&self.ecx) || !b.is_always_valid(&self.ecx) + } + BackendRepr::Vector { .. } | BackendRepr::Memory { .. } => false, + } + } + + fn value_is_all_in_one_field( + &self, + ty: Ty<'tcx>, + variant: VariantIdx, + ) -> Option<(FieldIdx, Ty<'tcx>)> { + if let Ok(layout) = self.ecx.layout_of(ty) + && let abi::Variants::Single { index } = layout.variants + && index == variant + && let Some((field_idx, field_layout)) = layout.non_1zst_field(&self.ecx) + && layout.size == field_layout.size + { + // We needed to check the variant to avoid trying to read the tag + // field from an enum where no fields have variants, since that tag + // field isn't in the `Aggregate` from which we're getting values. + Some((FieldIdx::from_usize(field_idx), field_layout.ty)) + } else if let ty::Adt(adt, args) = ty.kind() + && adt.is_struct() + && adt.repr().transparent() + && let [single_field] = adt.non_enum_variant().fields.raw.as_slice() + { + Some((FieldIdx::ZERO, single_field.ty(self.tcx, args))) + } else { + None + } + } } fn op_to_prop_const<'tcx>( diff --git a/compiler/rustc_mir_transform/src/instsimplify.rs b/compiler/rustc_mir_transform/src/instsimplify.rs index 1a65affe812..9e024508c58 100644 --- a/compiler/rustc_mir_transform/src/instsimplify.rs +++ b/compiler/rustc_mir_transform/src/instsimplify.rs @@ -173,29 +173,6 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> { *kind = CastKind::IntToInt; return; } - - // Transmuting a transparent struct/union to a field's type is a projection - if let ty::Adt(adt_def, args) = operand_ty.kind() - && adt_def.repr().transparent() - && (adt_def.is_struct() || adt_def.is_union()) - && let Some(place) = operand.place() - { - let variant = adt_def.non_enum_variant(); - for (i, field) in variant.fields.iter_enumerated() { - let field_ty = field.ty(self.tcx, args); - if field_ty == *cast_ty { - let place = place - .project_deeper(&[ProjectionElem::Field(i, *cast_ty)], self.tcx); - let operand = if operand.is_move() { - Operand::Move(place) - } else { - Operand::Copy(place) - }; - *rvalue = Rvalue::Use(operand); - return; - } - } - } } } } |