1 //! Evaluating const expressions. 2 3 use crate::runtime::vm::{Instance, VMGcRef, ValRaw, I31}; 4 use crate::store::AutoAssertNoGc; 5 use crate::{ 6 prelude::*, ArrayRef, ArrayRefPre, ArrayType, StructRef, StructRefPre, StructType, Val, 7 }; 8 use smallvec::SmallVec; 9 use wasmtime_environ::{ 10 ConstExpr, ConstOp, FuncIndex, GlobalIndex, ModuleInternedTypeIndex, WasmCompositeType, 11 WasmSubType, 12 }; 13 14 /// An interpreter for const expressions. 15 /// 16 /// This can be reused across many const expression evaluations to reuse 17 /// allocated resources, if any. 18 #[derive(Default)] 19 pub struct ConstExprEvaluator { 20 stack: SmallVec<[ValRaw; 2]>, 21 } 22 23 /// The context within which a particular const expression is evaluated. 24 pub struct ConstEvalContext<'a> { 25 pub(crate) instance: &'a mut Instance, 26 } 27 28 impl<'a> ConstEvalContext<'a> { 29 /// Create a new context. 30 pub fn new(instance: &'a mut Instance) -> Self { 31 Self { instance } 32 } 33 34 fn global_get(&mut self, store: &mut AutoAssertNoGc<'_>, index: GlobalIndex) -> Result<ValRaw> { 35 unsafe { 36 let global = self 37 .instance 38 .defined_or_imported_global_ptr(index) 39 .as_ref() 40 .unwrap(); 41 let mut gc_store = store.unwrap_gc_store_mut(); 42 Ok(global.to_val_raw( 43 &mut gc_store, 44 self.instance.env_module().globals[index].wasm_ty, 45 )) 46 } 47 } 48 49 fn ref_func(&mut self, index: FuncIndex) -> Result<ValRaw> { 50 Ok(ValRaw::funcref( 51 self.instance.get_func_ref(index).unwrap().cast(), 52 )) 53 } 54 55 #[cfg(feature = "gc")] 56 fn struct_fields_len(&self, struct_type_index: ModuleInternedTypeIndex) -> usize { 57 let module = self 58 .instance 59 .runtime_module() 60 .expect("should never be allocating a struct type defined in a dummy module"); 61 62 let struct_ty = match &module.types()[struct_type_index].composite_type { 63 WasmCompositeType::Struct(s) => s, 64 _ => unreachable!(), 65 }; 66 67 struct_ty.fields.len() 68 } 69 70 /// Safety: field values must be of the correct types. 71 #[cfg(feature = "gc")] 72 unsafe fn struct_new( 73 &mut self, 74 store: &mut AutoAssertNoGc<'_>, 75 struct_type_index: ModuleInternedTypeIndex, 76 fields: &[ValRaw], 77 ) -> Result<ValRaw> { 78 let module = self 79 .instance 80 .runtime_module() 81 .expect("should never be allocating a struct type defined in a dummy module"); 82 let shared_ty = module 83 .signatures() 84 .shared_type(struct_type_index) 85 .expect("should have an engine type for module type"); 86 87 let struct_ty = StructType::from_shared_type_index(store.engine(), shared_ty); 88 let fields = fields 89 .iter() 90 .zip(struct_ty.fields()) 91 .map(|(raw, ty)| { 92 let ty = ty.element_type().unpack(); 93 Val::_from_raw(store, *raw, ty) 94 }) 95 .collect::<Vec<_>>(); 96 97 let allocator = StructRefPre::_new(store, struct_ty); 98 let struct_ref = StructRef::_new(store, &allocator, &fields)?; 99 let raw = struct_ref.to_anyref()._to_raw(store)?; 100 Ok(ValRaw::anyref(raw)) 101 } 102 103 #[cfg(feature = "gc")] 104 fn struct_new_default( 105 &mut self, 106 store: &mut AutoAssertNoGc<'_>, 107 struct_type_index: ModuleInternedTypeIndex, 108 ) -> Result<ValRaw> { 109 let module = self 110 .instance 111 .runtime_module() 112 .expect("should never be allocating a struct type defined in a dummy module"); 113 114 let shared_ty = module 115 .signatures() 116 .shared_type(struct_type_index) 117 .expect("should have an engine type for module type"); 118 119 let borrowed = module 120 .engine() 121 .signatures() 122 .borrow(shared_ty) 123 .expect("should have a registered type for struct"); 124 let WasmSubType { 125 composite_type: WasmCompositeType::Struct(struct_ty), 126 .. 127 } = &*borrowed 128 else { 129 unreachable!("registered type should be a struct"); 130 }; 131 132 let fields = struct_ty 133 .fields 134 .iter() 135 .map(|ty| match &ty.element_type { 136 wasmtime_environ::WasmStorageType::I8 | wasmtime_environ::WasmStorageType::I16 => { 137 ValRaw::i32(0) 138 } 139 wasmtime_environ::WasmStorageType::Val(v) => match v { 140 wasmtime_environ::WasmValType::I32 => ValRaw::i32(0), 141 wasmtime_environ::WasmValType::I64 => ValRaw::i64(0), 142 wasmtime_environ::WasmValType::F32 => ValRaw::f32(0.0f32.to_bits()), 143 wasmtime_environ::WasmValType::F64 => ValRaw::f64(0.0f64.to_bits()), 144 wasmtime_environ::WasmValType::V128 => ValRaw::v128(0), 145 wasmtime_environ::WasmValType::Ref(r) => { 146 assert!(r.nullable); 147 ValRaw::null() 148 } 149 }, 150 }) 151 .collect::<SmallVec<[_; 8]>>(); 152 153 unsafe { self.struct_new(store, struct_type_index, &fields) } 154 } 155 } 156 157 impl ConstExprEvaluator { 158 /// Evaluate the given const expression in the given context. 159 /// 160 /// # Unsafety 161 /// 162 /// The given const expression must be valid within the given context, 163 /// e.g. the const expression must be well-typed and the context must return 164 /// global values of the expected types. This evaluator operates directly on 165 /// untyped `ValRaw`s and does not and cannot check that its operands are of 166 /// the correct type. 167 pub unsafe fn eval( 168 &mut self, 169 context: &mut ConstEvalContext<'_>, 170 expr: &ConstExpr, 171 ) -> Result<ValRaw> { 172 self.stack.clear(); 173 174 let mut store = (*context.instance.store()).store_opaque_mut(); 175 176 // Ensure that we don't permanently root any GC references we allocate 177 // during const evaluation, keeping them alive for the duration of the 178 // store's lifetime. 179 #[cfg(feature = "gc")] 180 let mut store = crate::OpaqueRootScope::new(&mut store); 181 182 // We cannot allow GC during const evaluation because the stack of 183 // `ValRaw`s are not rooted. If we had a GC reference on our stack, and 184 // then performed a collection, that on-stack reference's object could 185 // be reclaimed or relocated by the collector, and then when we use the 186 // reference again we would basically get a use-after-free bug. 187 let mut store = AutoAssertNoGc::new(&mut store); 188 189 for op in expr.ops() { 190 match op { 191 ConstOp::I32Const(i) => self.stack.push(ValRaw::i32(*i)), 192 ConstOp::I64Const(i) => self.stack.push(ValRaw::i64(*i)), 193 ConstOp::F32Const(f) => self.stack.push(ValRaw::f32(*f)), 194 ConstOp::F64Const(f) => self.stack.push(ValRaw::f64(*f)), 195 ConstOp::V128Const(v) => self.stack.push(ValRaw::v128(*v)), 196 ConstOp::GlobalGet(g) => self.stack.push(context.global_get(&mut store, *g)?), 197 ConstOp::RefNull => self.stack.push(ValRaw::null()), 198 ConstOp::RefFunc(f) => self.stack.push(context.ref_func(*f)?), 199 ConstOp::RefI31 => { 200 let i = self.pop()?.get_i32(); 201 let i31 = I31::wrapping_i32(i); 202 let raw = VMGcRef::from_i31(i31).as_raw_u32(); 203 self.stack.push(ValRaw::anyref(raw)); 204 } 205 ConstOp::I32Add => { 206 let b = self.pop()?.get_i32(); 207 let a = self.pop()?.get_i32(); 208 self.stack.push(ValRaw::i32(a.wrapping_add(b))); 209 } 210 ConstOp::I32Sub => { 211 let b = self.pop()?.get_i32(); 212 let a = self.pop()?.get_i32(); 213 self.stack.push(ValRaw::i32(a.wrapping_sub(b))); 214 } 215 ConstOp::I32Mul => { 216 let b = self.pop()?.get_i32(); 217 let a = self.pop()?.get_i32(); 218 self.stack.push(ValRaw::i32(a.wrapping_mul(b))); 219 } 220 ConstOp::I64Add => { 221 let b = self.pop()?.get_i64(); 222 let a = self.pop()?.get_i64(); 223 self.stack.push(ValRaw::i64(a.wrapping_add(b))); 224 } 225 ConstOp::I64Sub => { 226 let b = self.pop()?.get_i64(); 227 let a = self.pop()?.get_i64(); 228 self.stack.push(ValRaw::i64(a.wrapping_sub(b))); 229 } 230 ConstOp::I64Mul => { 231 let b = self.pop()?.get_i64(); 232 let a = self.pop()?.get_i64(); 233 self.stack.push(ValRaw::i64(a.wrapping_mul(b))); 234 } 235 236 #[cfg(not(feature = "gc"))] 237 ConstOp::StructNew { .. } 238 | ConstOp::StructNewDefault { .. } 239 | ConstOp::ArrayNew { .. } 240 | ConstOp::ArrayNewDefault { .. } 241 | ConstOp::ArrayNewFixed { .. } => { 242 bail!( 243 "const expr evaluation error: struct operations are not \ 244 supported without the `gc` feature" 245 ) 246 } 247 248 #[cfg(feature = "gc")] 249 ConstOp::StructNew { struct_type_index } => { 250 let interned_type_index = 251 context.instance.env_module().types[*struct_type_index]; 252 let len = context.struct_fields_len(interned_type_index); 253 254 if self.stack.len() < len { 255 bail!( 256 "const expr evaluation error: expected at least {len} values on the stack, found {}", 257 self.stack.len() 258 ) 259 } 260 261 let start = self.stack.len() - len; 262 let s = context.struct_new( 263 &mut store, 264 interned_type_index, 265 &self.stack[start..], 266 )?; 267 self.stack.truncate(start); 268 self.stack.push(s); 269 } 270 271 #[cfg(feature = "gc")] 272 ConstOp::StructNewDefault { struct_type_index } => { 273 let interned_type_index = 274 context.instance.env_module().types[*struct_type_index]; 275 self.stack 276 .push(context.struct_new_default(&mut store, interned_type_index)?); 277 } 278 279 #[cfg(feature = "gc")] 280 ConstOp::ArrayNew { array_type_index } => { 281 let interned_type_index = 282 context.instance.env_module().types[*array_type_index]; 283 let module = context.instance.runtime_module().expect( 284 "should never be allocating a struct type defined in a dummy module", 285 ); 286 let shared_ty = module 287 .signatures() 288 .shared_type(interned_type_index) 289 .expect("should have an engine type for module type"); 290 let ty = ArrayType::from_shared_type_index(store.engine(), shared_ty); 291 292 #[allow(clippy::cast_sign_loss)] 293 let len = self.pop()?.get_i32() as u32; 294 295 let elem = Val::_from_raw(&mut store, self.pop()?, ty.element_type().unpack()); 296 297 let pre = ArrayRefPre::_new(&mut store, ty); 298 let array = ArrayRef::_new(&mut store, &pre, &elem, len)?; 299 300 self.stack 301 .push(ValRaw::anyref(array.to_anyref()._to_raw(&mut store)?)); 302 } 303 304 #[cfg(feature = "gc")] 305 ConstOp::ArrayNewDefault { array_type_index } => { 306 let interned_type_index = 307 context.instance.env_module().types[*array_type_index]; 308 let module = context.instance.runtime_module().expect( 309 "should never be allocating a struct type defined in a dummy module", 310 ); 311 let shared_ty = module 312 .signatures() 313 .shared_type(interned_type_index) 314 .expect("should have an engine type for module type"); 315 let ty = ArrayType::from_shared_type_index(store.engine(), shared_ty); 316 317 #[allow(clippy::cast_sign_loss)] 318 let len = self.pop()?.get_i32() as u32; 319 320 let elem = Val::default_for_ty(ty.element_type().unpack()) 321 .expect("type should have a default value"); 322 323 let pre = ArrayRefPre::_new(&mut store, ty); 324 let array = ArrayRef::_new(&mut store, &pre, &elem, len)?; 325 326 self.stack 327 .push(ValRaw::anyref(array.to_anyref()._to_raw(&mut store)?)); 328 } 329 330 #[cfg(feature = "gc")] 331 ConstOp::ArrayNewFixed { 332 array_type_index, 333 array_size, 334 } => { 335 let interned_type_index = 336 context.instance.env_module().types[*array_type_index]; 337 let module = context.instance.runtime_module().expect( 338 "should never be allocating a struct type defined in a dummy module", 339 ); 340 let shared_ty = module 341 .signatures() 342 .shared_type(interned_type_index) 343 .expect("should have an engine type for module type"); 344 let ty = ArrayType::from_shared_type_index(store.engine(), shared_ty); 345 346 let array_size = usize::try_from(*array_size).unwrap(); 347 if self.stack.len() < array_size { 348 bail!( 349 "const expr evaluation error: expected at least {array_size} values on the stack, found {}", 350 self.stack.len() 351 ) 352 } 353 354 let start = self.stack.len() - array_size; 355 356 let elem_ty = ty.element_type(); 357 let elem_ty = elem_ty.unpack(); 358 359 let elems = self 360 .stack 361 .drain(start..) 362 .map(|raw| Val::_from_raw(&mut store, raw, elem_ty)) 363 .collect::<SmallVec<[_; 8]>>(); 364 365 let pre = ArrayRefPre::_new(&mut store, ty); 366 let array = ArrayRef::_new_fixed(&mut store, &pre, &elems)?; 367 368 self.stack 369 .push(ValRaw::anyref(array.to_anyref()._to_raw(&mut store)?)); 370 } 371 } 372 } 373 374 if self.stack.len() == 1 { 375 Ok(self.stack[0]) 376 } else { 377 bail!( 378 "const expr evaluation error: expected 1 resulting value, found {}", 379 self.stack.len() 380 ) 381 } 382 } 383 384 fn pop(&mut self) -> Result<ValRaw> { 385 self.stack.pop().ok_or_else(|| { 386 anyhow!( 387 "const expr evaluation error: attempted to pop from an empty \ 388 evaluation stack" 389 ) 390 }) 391 } 392 } 393