rustc_mir_transform/
large_enums.rs

1use rustc_abi::{HasDataLayout, Size, TagEncoding, Variants};
2use rustc_const_eval::interpret::{Scalar, alloc_range};
3use rustc_data_structures::fx::FxHashMap;
4use rustc_middle::mir::interpret::AllocId;
5use rustc_middle::mir::*;
6use rustc_middle::ty::util::IntTypeExt;
7use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt};
8use rustc_session::Session;
9
10use crate::patch::MirPatch;
11
12/// A pass that seeks to optimize unnecessary moves of large enum types, if there is a large
13/// enough discrepancy between them.
14///
15/// i.e. If there are two variants:
16/// ```
17/// enum Example {
18///   Small,
19///   Large([u32; 1024]),
20/// }
21/// ```
22/// Instead of emitting moves of the large variant, perform a memcpy instead.
23/// Based off of [this HackMD](https://hackmd.io/@ft4bxUsFT5CEUBmRKYHr7w/rJM8BBPzD).
24///
25/// In summary, what this does is at runtime determine which enum variant is active,
26/// and instead of copying all the bytes of the largest possible variant,
27/// copy only the bytes for the currently active variant. The number of bytes to copy is determined
28/// by a lookup table: a discriminant-indexed array indicating the size of each variant.
29pub(super) struct EnumSizeOpt {
30    pub(crate) discrepancy: u64,
31}
32
33impl<'tcx> crate::MirPass<'tcx> for EnumSizeOpt {
34    fn is_enabled(&self, sess: &Session) -> bool {
35        // There are some differences in behavior on wasm and ARM that are not properly
36        // understood, so we conservatively treat this optimization as unsound:
37        // https://github.com/rust-lang/rust/issues/154413
38        sess.opts.unstable_opts.unsound_mir_opts && sess.mir_opt_level() >= 3
39    }
40
41    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
42        // NOTE: This pass may produce different MIR based on the alignment of the target
43        // platform, but it will still be valid.
44
45        let mut alloc_cache = FxHashMap::default();
46        let typing_env = body.typing_env(tcx);
47
48        let mut patch = MirPatch::new(body);
49
50        for (block, data) in body.basic_blocks.as_mut().iter_enumerated_mut() {
51            for (statement_index, st) in data.statements.iter_mut().enumerate() {
52                let StatementKind::Assign(box (
53                    lhs,
54                    Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)),
55                )) = &st.kind
56                else {
57                    continue;
58                };
59
60                let location = Location { block, statement_index };
61
62                let ty = lhs.ty(&body.local_decls, tcx).ty;
63
64                let Some((adt_def, num_variants, alloc_id)) =
65                    self.candidate(tcx, typing_env, ty, &mut alloc_cache)
66                else {
67                    continue;
68                };
69
70                let span = st.source_info.span;
71
72                let tmp_ty = Ty::new_array(tcx, tcx.types.usize, num_variants as u64);
73                let size_array_local = patch.new_temp(tmp_ty, span);
74
75                let store_live = StatementKind::StorageLive(size_array_local);
76
77                let place = Place::from(size_array_local);
78                let constant_vals = ConstOperand {
79                    span,
80                    user_ty: None,
81                    const_: Const::Val(
82                        ConstValue::Indirect { alloc_id, offset: Size::ZERO },
83                        tmp_ty,
84                    ),
85                };
86                let rval = Rvalue::Use(Operand::Constant(Box::new(constant_vals)));
87                let const_assign = StatementKind::Assign(Box::new((place, rval)));
88
89                let discr_place =
90                    Place::from(patch.new_temp(adt_def.repr().discr_type().to_ty(tcx), span));
91                let store_discr =
92                    StatementKind::Assign(Box::new((discr_place, Rvalue::Discriminant(*rhs))));
93
94                let discr_cast_place = Place::from(patch.new_temp(tcx.types.usize, span));
95                let cast_discr = StatementKind::Assign(Box::new((
96                    discr_cast_place,
97                    Rvalue::Cast(CastKind::IntToInt, Operand::Copy(discr_place), tcx.types.usize),
98                )));
99
100                let size_place = Place::from(patch.new_temp(tcx.types.usize, span));
101                let store_size = StatementKind::Assign(Box::new((
102                    size_place,
103                    Rvalue::Use(Operand::Copy(Place {
104                        local: size_array_local,
105                        projection: tcx.mk_place_elems(&[PlaceElem::Index(discr_cast_place.local)]),
106                    })),
107                )));
108
109                let dst = Place::from(patch.new_temp(Ty::new_mut_ptr(tcx, ty), span));
110                let dst_ptr =
111                    StatementKind::Assign(Box::new((dst, Rvalue::RawPtr(RawPtrKind::Mut, *lhs))));
112
113                let dst_cast_ty = Ty::new_mut_ptr(tcx, tcx.types.u8);
114                let dst_cast_place = Place::from(patch.new_temp(dst_cast_ty, span));
115                let dst_cast = StatementKind::Assign(Box::new((
116                    dst_cast_place,
117                    Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(dst), dst_cast_ty),
118                )));
119
120                let src = Place::from(patch.new_temp(Ty::new_imm_ptr(tcx, ty), span));
121                let src_ptr =
122                    StatementKind::Assign(Box::new((src, Rvalue::RawPtr(RawPtrKind::Const, *rhs))));
123
124                let src_cast_ty = Ty::new_imm_ptr(tcx, tcx.types.u8);
125                let src_cast_place = Place::from(patch.new_temp(src_cast_ty, span));
126                let src_cast = StatementKind::Assign(Box::new((
127                    src_cast_place,
128                    Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(src), src_cast_ty),
129                )));
130
131                let copy_bytes = StatementKind::Intrinsic(Box::new(
132                    NonDivergingIntrinsic::CopyNonOverlapping(CopyNonOverlapping {
133                        src: Operand::Copy(src_cast_place),
134                        dst: Operand::Copy(dst_cast_place),
135                        count: Operand::Copy(size_place),
136                    }),
137                ));
138
139                let store_dead = StatementKind::StorageDead(size_array_local);
140
141                let stmts = [
142                    store_live,
143                    const_assign,
144                    store_discr,
145                    cast_discr,
146                    store_size,
147                    dst_ptr,
148                    dst_cast,
149                    src_ptr,
150                    src_cast,
151                    copy_bytes,
152                    store_dead,
153                ];
154                for stmt in stmts {
155                    patch.add_statement(location, stmt);
156                }
157
158                st.make_nop(true);
159            }
160        }
161
162        patch.apply(body);
163    }
164
165    fn is_required(&self) -> bool {
166        false
167    }
168}
169
170impl EnumSizeOpt {
171    fn candidate<'tcx>(
172        &self,
173        tcx: TyCtxt<'tcx>,
174        typing_env: ty::TypingEnv<'tcx>,
175        ty: Ty<'tcx>,
176        alloc_cache: &mut FxHashMap<Ty<'tcx>, AllocId>,
177    ) -> Option<(AdtDef<'tcx>, usize, AllocId)> {
178        let adt_def = match ty.kind() {
179            ty::Adt(adt_def, _args) if adt_def.is_enum() => adt_def,
180            _ => return None,
181        };
182        let layout = tcx.layout_of(typing_env.as_query_input(ty)).ok()?;
183        let variants = match &layout.variants {
184            Variants::Single { .. } | Variants::Empty => return None,
185            Variants::Multiple { tag_encoding: TagEncoding::Niche { .. }, .. } => return None,
186
187            Variants::Multiple { variants, .. } if variants.len() <= 1 => return None,
188            Variants::Multiple { variants, .. } => variants,
189        };
190        let min = variants.iter().map(|v| v.size).min().unwrap();
191        let max = variants.iter().map(|v| v.size).max().unwrap();
192        if max.bytes() - min.bytes() < self.discrepancy {
193            return None;
194        }
195
196        let num_discrs = adt_def.discriminants(tcx).count();
197        if variants.iter_enumerated().any(|(var_idx, _)| {
198            let discr_for_var = adt_def.discriminant_for_variant(tcx, var_idx).val;
199            (discr_for_var > usize::MAX as u128) || (discr_for_var as usize >= num_discrs)
200        }) {
201            return None;
202        }
203        if let Some(alloc_id) = alloc_cache.get(&ty) {
204            return Some((*adt_def, num_discrs, *alloc_id));
205        }
206
207        // Construct an in-memory array mapping discriminant idx to variant size.
208        let data_layout = tcx.data_layout();
209        let ptr_size = data_layout.pointer_size();
210        let mut alloc = interpret::Allocation::from_bytes(
211            vec![0; ptr_size.bytes_usize() * num_discrs],
212            tcx.data_layout.ptr_sized_integer().align(&tcx.data_layout).abi,
213            Mutability::Mut,
214            (),
215        );
216        for (var_idx, layout) in variants.iter_enumerated() {
217            let curr_idx = ptr_size * adt_def.discriminant_for_variant(tcx, var_idx).val as u64;
218            let val = Scalar::from_target_usize(layout.size.bytes(), &tcx);
219            alloc.write_scalar(&tcx, alloc_range(curr_idx, val.size()), val).unwrap();
220        }
221        alloc.mutability = Mutability::Not;
222        let alloc = tcx.reserve_and_set_memory_alloc(tcx.mk_const_alloc(alloc));
223
224        Some((*adt_def, num_discrs, *alloc_cache.entry(ty).or_insert(alloc)))
225    }
226}
rustc_mir_transform/large_enums.rs

rustc_mir_transform/
large_enums.rs
