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::PassPolicy;
11use crate::patch::MirPatch;
12
13/// A pass that seeks to optimize unnecessary moves of large enum types, if there is a large
14/// enough discrepancy between them.
15///
16/// i.e. If there are two variants:
17/// ```
18/// enum Example {
19///   Small,
20///   Large([u32; 1024]),
21/// }
22/// ```
23/// Instead of emitting moves of the large variant, perform a memcpy instead.
24/// Based off of [this HackMD](https://hackmd.io/@ft4bxUsFT5CEUBmRKYHr7w/rJM8BBPzD).
25///
26/// In summary, what this does is at runtime determine which enum variant is active,
27/// and instead of copying all the bytes of the largest possible variant,
28/// copy only the bytes for the currently active variant. The number of bytes to copy is determined
29/// by a lookup table: a discriminant-indexed array indicating the size of each variant.
30pub(super) struct EnumSizeOpt {
31    pub(crate) discrepancy: u64,
32}
33
34impl<'tcx> crate::MirPass<'tcx> for EnumSizeOpt {
35    fn policy(&self, sess: &Session) -> PassPolicy {
36        // There are some differences in behavior on wasm and ARM that are not properly
37        // understood, so we conservatively treat this optimization as unsound:
38        // https://github.com/rust-lang/rust/issues/154413
39        PassPolicy::optimization(
40            sess.opts.unstable_opts.unsound_mir_opts && sess.mir_opt_level() >= 3,
41        )
42    }
43
44    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
45        // NOTE: This pass may produce different MIR based on the alignment of the target
46        // platform, but it will still be valid.
47
48        let mut alloc_cache = FxHashMap::default();
49        let typing_env = body.typing_env(tcx);
50
51        let mut patch = MirPatch::new(body);
52
53        for (block, data) in body.basic_blocks.as_mut().iter_enumerated_mut() {
54            for (statement_index, st) in data.statements.iter_mut().enumerate() {
55                let StatementKind::Assign((
56                    lhs,
57                    Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs), _),
58                )) = &st.kind
59                else {
60                    continue;
61                };
62
63                let location = Location { block, statement_index };
64
65                let ty = lhs.ty(&body.local_decls, tcx).ty;
66
67                let Some((adt_def, num_variants, alloc_id)) =
68                    self.candidate(tcx, typing_env, ty, &mut alloc_cache)
69                else {
70                    continue;
71                };
72
73                let span = st.source_info.span;
74
75                let tmp_ty = Ty::new_array(tcx, tcx.types.usize, num_variants as u64);
76                let size_array_local = patch.new_temp(tmp_ty, span);
77
78                let store_live = StatementKind::StorageLive(size_array_local);
79
80                let place = Place::from(size_array_local);
81                let constant_vals = ConstOperand {
82                    span,
83                    user_ty: None,
84                    const_: Const::Val(
85                        ConstValue::Indirect { alloc_id, offset: Size::ZERO },
86                        tmp_ty,
87                    ),
88                };
89                let rval = Rvalue::Use(Operand::Constant(Box::new(constant_vals)), WithRetag::No);
90                let const_assign = StatementKind::Assign(Box::new((place, rval)));
91
92                let discr_place =
93                    Place::from(patch.new_temp(adt_def.repr().discr_type().to_ty(tcx), span));
94                let store_discr =
95                    StatementKind::Assign(Box::new((discr_place, Rvalue::Discriminant(*rhs))));
96
97                let discr_cast_place = Place::from(patch.new_temp(tcx.types.usize, span));
98                let cast_discr = StatementKind::Assign(Box::new((
99                    discr_cast_place,
100                    Rvalue::Cast(CastKind::IntToInt, Operand::Copy(discr_place), tcx.types.usize),
101                )));
102
103                let size_place = Place::from(patch.new_temp(tcx.types.usize, span));
104                let store_size = StatementKind::Assign(Box::new((
105                    size_place,
106                    Rvalue::Use(
107                        Operand::Copy(Place {
108                            local: size_array_local,
109                            projection: tcx
110                                .mk_place_elems(&[PlaceElem::Index(discr_cast_place.local)]),
111                        }),
112                        WithRetag::No,
113                    ),
114                )));
115
116                let dst = Place::from(patch.new_temp(Ty::new_mut_ptr(tcx, ty), span));
117                let dst_ptr =
118                    StatementKind::Assign(Box::new((dst, Rvalue::RawPtr(RawPtrKind::Mut, *lhs))));
119
120                let dst_cast_ty = Ty::new_mut_ptr(tcx, tcx.types.u8);
121                let dst_cast_place = Place::from(patch.new_temp(dst_cast_ty, span));
122                let dst_cast = StatementKind::Assign(Box::new((
123                    dst_cast_place,
124                    Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(dst), dst_cast_ty),
125                )));
126
127                let src = Place::from(patch.new_temp(Ty::new_imm_ptr(tcx, ty), span));
128                let src_ptr =
129                    StatementKind::Assign(Box::new((src, Rvalue::RawPtr(RawPtrKind::Const, *rhs))));
130
131                let src_cast_ty = Ty::new_imm_ptr(tcx, tcx.types.u8);
132                let src_cast_place = Place::from(patch.new_temp(src_cast_ty, span));
133                let src_cast = StatementKind::Assign(Box::new((
134                    src_cast_place,
135                    Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(src), src_cast_ty),
136                )));
137
138                let copy_bytes = StatementKind::Intrinsic(Box::new(
139                    NonDivergingIntrinsic::CopyNonOverlapping(CopyNonOverlapping {
140                        src: Operand::Copy(src_cast_place),
141                        dst: Operand::Copy(dst_cast_place),
142                        count: Operand::Copy(size_place),
143                    }),
144                ));
145
146                let store_dead = StatementKind::StorageDead(size_array_local);
147
148                let stmts = [
149                    store_live,
150                    const_assign,
151                    store_discr,
152                    cast_discr,
153                    store_size,
154                    dst_ptr,
155                    dst_cast,
156                    src_ptr,
157                    src_cast,
158                    copy_bytes,
159                    store_dead,
160                ];
161                for stmt in stmts {
162                    patch.add_statement(location, stmt);
163                }
164
165                st.make_nop(true);
166            }
167        }
168
169        patch.apply(body);
170    }
171}
172
173impl EnumSizeOpt {
174    fn candidate<'tcx>(
175        &self,
176        tcx: TyCtxt<'tcx>,
177        typing_env: ty::TypingEnv<'tcx>,
178        ty: Ty<'tcx>,
179        alloc_cache: &mut FxHashMap<Ty<'tcx>, AllocId>,
180    ) -> Option<(AdtDef<'tcx>, usize, AllocId)> {
181        let adt_def = match ty.kind() {
182            ty::Adt(adt_def, _args) if adt_def.is_enum() => adt_def,
183            _ => return None,
184        };
185        let layout = tcx.layout_of(typing_env.as_query_input(ty)).ok()?;
186        let variants = match &layout.variants {
187            Variants::Single { .. } | Variants::Empty => return None,
188            Variants::Multiple { tag_encoding: TagEncoding::Niche { .. }, .. } => return None,
189
190            Variants::Multiple { variants, .. } if variants.len() <= 1 => return None,
191            Variants::Multiple { variants, .. } => variants,
192        };
193        let min = variants.iter().map(|v| v.size).min().unwrap();
194        let max = variants.iter().map(|v| v.size).max().unwrap();
195        if max.bytes() - min.bytes() < self.discrepancy {
196            return None;
197        }
198
199        let num_discrs = adt_def.discriminants(tcx).count();
200        if variants.iter_enumerated().any(|(var_idx, _)| {
201            let discr_for_var = adt_def.discriminant_for_variant(tcx, var_idx).val;
202            (discr_for_var > usize::MAX as u128) || (discr_for_var as usize >= num_discrs)
203        }) {
204            return None;
205        }
206        if let Some(alloc_id) = alloc_cache.get(&ty) {
207            return Some((*adt_def, num_discrs, *alloc_id));
208        }
209
210        // Construct an in-memory array mapping discriminant idx to variant size.
211        let data_layout = tcx.data_layout();
212        let ptr_size = data_layout.pointer_size();
213        let mut alloc = interpret::Allocation::from_bytes(
214            vec![0; ptr_size.bytes_usize() * num_discrs],
215            tcx.data_layout.ptr_sized_integer().align(&tcx.data_layout).abi,
216            Mutability::Mut,
217            (),
218        );
219        for (var_idx, layout) in variants.iter_enumerated() {
220            let curr_idx = ptr_size * adt_def.discriminant_for_variant(tcx, var_idx).val as u64;
221            let val = Scalar::from_target_usize(layout.size.bytes(), &tcx);
222            alloc.write_scalar(&tcx, alloc_range(curr_idx, val.size()), val).unwrap();
223        }
224        alloc.mutability = Mutability::Not;
225        let alloc = tcx.reserve_and_set_memory_alloc(tcx.mk_const_alloc(alloc));
226
227        Some((*adt_def, num_discrs, *alloc_cache.entry(ty).or_insert(alloc)))
228    }
229}
rustc_mir_transform/large_enums.rs

rustc_mir_transform/
large_enums.rs
