Add shared memories

This change adds the ability to use shared memories in Wasmtime when the
[threads proposal] is enabled. Shared memories are annotated as `shared`
in the WebAssembly syntax, e.g., `(memory 1 1 shared)`, and are
protected from concurrent access during `memory.size` and `memory.grow`.

[threads proposal]: https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md

In order to implement this in Wasmtime, there are two main cases to
cover:
    - a program may simply create a shared memory and possibly export it;
    this means that Wasmtime itself must be able to create shared
    memories
    - a user may create a shared memory externally and pass it in as an
    import during instantiation; this is the case when the program
    contains code like `(import "env" "memory" (memory 1 1
    shared))`--this case is handled by a new Wasmtime API
    type--`SharedMemory`

Because of the first case, this change allows any of the current
memory-creation mechanisms to work as-is. Wasmtime can still create
either static or dynamic memories in either on-demand or pooling modes,
and any of these memories can be considered shared. When shared, the
`Memory` runtime container will lock appropriately during `memory.size`
and `memory.grow` operations; since all memories use this container, it
is an ideal place for implementing the locking once and once only.

The second case is covered by the new `SharedMemory` structure. It uses
the same `Mmap` allocation under the hood as non-shared memories, but
allows the user to perform the allocation externally to Wasmtime and
share the memory across threads (via an `Arc`). The pointer address to
the actual memory is carefully wired through and owned by the
`SharedMemory` structure itself. This means that there are differing
views of where to access the pointer (i.e., `VMMemoryDefinition`): for
owned memories (the default), the `VMMemoryDefinition` is stored
directly by the `VMContext`; in the `SharedMemory` case, however, this
`VMContext` must point to this separate structure.

To ensure that the `VMContext` can always point to the correct
`VMMemoryDefinition`, this change alters the `VMContext` structure.
Since a `SharedMemory` owns its own `VMMemoryDefinition`, the
`defined_memories` table in the `VMContext` becomes a sequence of
pointers--in the shared memory case, they point to the
`VMMemoryDefinition` owned by the `SharedMemory` and in the owned memory
case (i.e., not shared) they point to `VMMemoryDefinition`s stored in a
new table, `owned_memories`.

This change adds an additional indirection (through the `*mut
VMMemoryDefinition` pointer) that could add overhead. Using an imported
memory as a proxy, we measured a 1-3% overhead of this approach on the
`pulldown-cmark` benchmark. To avoid this, Cranelift-generated code will
special-case the owned memory access (i.e., load a pointer directly to
the `owned_memories` entry) for `memory.size` so that only
shared memories (and imported memories, as before) incur the indirection
cost.

Author: abrown

Cargo.lock

@@ -200,7 +200,7 @@ fn libcall_function() {
 
 #[test]
 #[should_panic(
-    expected = "Result::unwrap()` on an `Err` value: Backend(Symbol \"function\\0with\\0nul\\0bytes\" has a null byte, which is disallowed"
+    expected = "Result::unwrap()` on an `Err` value: Backend(Symbol \"function\\u{0}with\\u{0}nul\\u{0}bytes\" has a null byte, which is disallowed"
 )]
 fn reject_nul_byte_symbol_for_func() {
     let flag_builder = settings::builder();
@@ -224,7 +224,7 @@ fn reject_nul_byte_symbol_for_func() {
 
 #[test]
 #[should_panic(
-    expected = "Result::unwrap()` on an `Err` value: Backend(Symbol \"data\\0with\\0nul\\0bytes\" has a null byte, which is disallowed"
+    expected = "Result::unwrap()` on an `Err` value: Backend(Symbol \"data\\u{0}with\\u{0}nul\\u{0}bytes\" has a null byte, which is disallowed"
 )]
 fn reject_nul_byte_symbol_for_data() {
     let flag_builder = settings::builder();

cranelift/object/tests/basic.rs

@@ -16,7 +16,7 @@ use cranelift_codegen::{MachSrcLoc, MachStackMap};
 use cranelift_entity::{EntityRef, PrimaryMap};
 use cranelift_frontend::FunctionBuilder;
 use cranelift_wasm::{
-    DefinedFuncIndex, DefinedMemoryIndex, FuncIndex, FuncTranslator, MemoryIndex, SignatureIndex,
+    DefinedFuncIndex, FuncIndex, FuncTranslator, MemoryIndex, OwnedMemoryIndex, SignatureIndex,
     WasmFuncType,
 };
 use object::write::Object;
@@ -332,8 +332,9 @@ impl wasmtime_environ::Compiler for Compiler {
             let memory_offset = if ofs.num_imported_memories > 0 {
                 ModuleMemoryOffset::Imported(ofs.vmctx_vmmemory_import(MemoryIndex::new(0)))
             } else if ofs.num_defined_memories > 0 {
+                // TODO shared?
                 ModuleMemoryOffset::Defined(
-                    ofs.vmctx_vmmemory_definition_base(DefinedMemoryIndex::new(0)),
+                    ofs.vmctx_vmmemory_definition_base(OwnedMemoryIndex::new(0)),
                 )
             } else {
                 ModuleMemoryOffset::None

crates/cranelift/src/compiler.rs

@@ -1368,18 +1368,37 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
 
     fn make_heap(&mut self, func: &mut ir::Function, index: MemoryIndex) -> WasmResult<ir::Heap> {
         let pointer_type = self.pointer_type();
-
+        let is_shared = self.module.memory_plans[index].memory.shared;
         let (ptr, base_offset, current_length_offset) = {
             let vmctx = self.vmctx(func);
             if let Some(def_index) = self.module.defined_memory_index(index) {
-                let base_offset =
-                    i32::try_from(self.offsets.vmctx_vmmemory_definition_base(def_index)).unwrap();
-                let current_length_offset = i32::try_from(
-                    self.offsets
-                        .vmctx_vmmemory_definition_current_length(def_index),
-                )
-                .unwrap();
-                (vmctx, base_offset, current_length_offset)
+                if is_shared {
+                    // As with imported memory, the `VMMemoryDefinition` for a
+                    // shared memory is stored elsewhere. We store a `*mut
+                    // VMMemoryDefinition` to it and dereference that when
+                    // atomically growing it.
+                    let from_offset = self.offsets.vmctx_vmmemory_pointer(def_index);
+                    let memory = func.create_global_value(ir::GlobalValueData::Load {
+                        base: vmctx,
+                        offset: Offset32::new(i32::try_from(from_offset).unwrap()),
+                        global_type: pointer_type,
+                        readonly: true,
+                    });
+                    let base_offset = i32::from(self.offsets.vmmemory_definition_base());
+                    let current_length_offset =
+                        i32::from(self.offsets.vmmemory_definition_current_length());
+                    (memory, base_offset, current_length_offset)
+                } else {
+                    let owned_index = self.module.owned_memory_index(def_index).expect("TODO");
+                    let owned_base_offset =
+                        self.offsets.vmctx_vmmemory_definition_base(owned_index);
+                    let owned_length_offset = self
+                        .offsets
+                        .vmctx_vmmemory_definition_current_length(owned_index);
+                    let current_base_offset = i32::try_from(owned_base_offset).unwrap();
+                    let current_length_offset = i32::try_from(owned_length_offset).unwrap();
+                    (vmctx, current_base_offset, current_length_offset)
+                }
             } else {
                 let from_offset = self.offsets.vmctx_vmmemory_import_from(index);
                 let memory = func.create_global_value(ir::GlobalValueData::Load {
@@ -1693,16 +1712,33 @@ impl<'module_environment> cranelift_wasm::FuncEnvironment for FuncEnvironment<'m
     ) -> WasmResult<ir::Value> {
         let pointer_type = self.pointer_type();
         let vmctx = self.vmctx(&mut pos.func);
+        let is_shared = self.module.memory_plans[index].memory.shared;
         let base = pos.ins().global_value(pointer_type, vmctx);
         let current_length_in_bytes = match self.module.defined_memory_index(index) {
             Some(def_index) => {
-                let offset = i32::try_from(
-                    self.offsets
-                        .vmctx_vmmemory_definition_current_length(def_index),
-                )
-                .unwrap();
-                pos.ins()
-                    .load(pointer_type, ir::MemFlags::trusted(), base, offset)
+                if is_shared {
+                    let offset =
+                        i32::try_from(self.offsets.vmctx_vmmemory_pointer(def_index)).unwrap();
+                    let vmmemory_ptr =
+                        pos.ins()
+                            .load(pointer_type, ir::MemFlags::trusted(), base, offset);
+                    // TODO should be an atomic_load (need a way to to do atomic_load + offset).
+                    pos.ins().load(
+                        pointer_type,
+                        ir::MemFlags::trusted(),
+                        vmmemory_ptr,
+                        i32::from(self.offsets.vmmemory_definition_current_length()),
+                    )
+                } else {
+                    let owned_index = self.module.owned_memory_index(def_index).expect("TODO");
+                    let offset = i32::try_from(
+                        self.offsets
+                            .vmctx_vmmemory_definition_current_length(owned_index),
+                    )
+                    .unwrap();
+                    pos.ins()
+                        .load(pointer_type, ir::MemFlags::trusted(), base, offset)
+                }
             }
             None => {
                 let offset = i32::try_from(self.offsets.vmctx_vmmemory_import_from(index)).unwrap();

crates/cranelift/src/func_environ.rs

@@ -10,15 +10,15 @@ use std::mem;
 use std::ops::Range;
 use wasmtime_types::*;
 
-/// Implemenation styles for WebAssembly linear memory.
+/// Implementation styles for WebAssembly linear memory.
 #[derive(Debug, Clone, Hash, Serialize, Deserialize)]
 pub enum MemoryStyle {
     /// The actual memory can be resized and moved.
     Dynamic {
         /// Extra space to reserve when a memory must be moved due to growth.
         reserve: u64,
     },
-    /// Addresss space is allocated up front.
+    /// Address space is allocated up front.
     Static {
         /// The number of mapped and unmapped pages.
         bound: u64,
@@ -160,7 +160,7 @@ pub enum MemoryInitialization {
     ///   which might reside in a compiled module on disk, available immediately
     ///   in a linear memory's address space.
     ///
-    /// To facilitate the latter fo these techniques the `try_static_init`
+    /// To facilitate the latter of these techniques the `try_static_init`
     /// function below, which creates this variant, takes a host page size
     /// argument which can page-align everything to make mmap-ing possible.
     Static {
@@ -919,6 +919,25 @@ impl Module {
         }
     }
 
+    /// Convert a `DefinedMemoryIndex` into an `OwnedMemoryIndex`. Returns None
+    /// if the index is an imported memory.
+    #[inline]
+    pub fn owned_memory_index(&self, memory: DefinedMemoryIndex) -> Option<OwnedMemoryIndex> {
+        if memory.index() >= self.memory_plans.len() {
+            return None;
+        }
+        // Once we know that the memory index is not greater than the number of
+        // plans, we can iterate through the plans up to the memory index and
+        // count how many are not shared (i.e., owned).
+        let owned_memory_index = self
+            .memory_plans
+            .iter()
+            .take(memory.index())
+            .filter(|(_, mp)| !mp.memory.shared)
+            .count();
+        Some(OwnedMemoryIndex::new(owned_memory_index))
+    }
+
     /// Test whether the given memory index is for an imported memory.
     #[inline]
     pub fn is_imported_memory(&self, index: MemoryIndex) -> bool {

crates/environ/src/module.rs

@@ -240,7 +240,7 @@ impl<'a, 'data> ModuleEnvironment<'a, 'data> {
                             EntityType::Function(sig_index)
                         }
                         TypeRef::Memory(ty) => {
-                            if ty.shared {
+                            if ty.shared && !self.validator.features().threads {
                                 return Err(WasmError::Unsupported("shared memories".to_owned()));
                             }
                             self.result.module.num_imported_memories += 1;
@@ -296,7 +296,7 @@ impl<'a, 'data> ModuleEnvironment<'a, 'data> {
 
                 for entry in memories {
                     let memory = entry?;
-                    if memory.shared {
+                    if memory.shared && !self.validator.features().threads {
                         return Err(WasmError::Unsupported("shared memories".to_owned()));
                     }
                     let plan = MemoryPlan::for_memory(memory.into(), &self.tunables);

crates/environ/src/module_environ.rs

@@ -14,7 +14,8 @@
 //      imported_memories: [VMMemoryImport; module.num_imported_memories],
 //      imported_globals: [VMGlobalImport; module.num_imported_globals],
 //      tables: [VMTableDefinition; module.num_defined_tables],
-//      memories: [VMMemoryDefinition; module.num_defined_memories],
+//      memories: [*mut VMMemoryDefinition; module.num_defined_memories],
+//      owned_memories: [VMMemoryDefinition; module.num_owned_memories],
 //      globals: [VMGlobalDefinition; module.num_defined_globals],
 //      anyfuncs: [VMCallerCheckedAnyfunc; module.num_escaped_funcs],
 // }
@@ -26,6 +27,7 @@ use crate::{
 use cranelift_entity::packed_option::ReservedValue;
 use more_asserts::assert_lt;
 use std::convert::TryFrom;
+use wasmtime_types::OwnedMemoryIndex;
 
 /// Sentinel value indicating that wasm has been interrupted.
 // Note that this has a bit of an odd definition. See the `insert_stack_check`
@@ -67,6 +69,8 @@ pub struct VMOffsets<P> {
     pub num_defined_tables: u32,
     /// The number of defined memories in the module.
     pub num_defined_memories: u32,
+    /// The number of memories owned by the module instance.
+    pub num_owned_memories: u32,
     /// The number of defined globals in the module.
     pub num_defined_globals: u32,
     /// The number of escaped functions in the module, the size of the anyfuncs
@@ -86,6 +90,7 @@ pub struct VMOffsets<P> {
     imported_globals: u32,
     defined_tables: u32,
     defined_memories: u32,
+    owned_memories: u32,
     defined_globals: u32,
     defined_anyfuncs: u32,
     size: u32,
@@ -133,16 +138,23 @@ pub struct VMOffsetsFields<P> {
     pub num_defined_tables: u32,
     /// The number of defined memories in the module.
     pub num_defined_memories: u32,
+    /// The number of memories owned by the module instance.
+    pub num_owned_memories: u32,
     /// The number of defined globals in the module.
     pub num_defined_globals: u32,
-    /// The numbe of escaped functions in the module, the size of the anyfunc
+    /// The number of escaped functions in the module, the size of the anyfunc
     /// array.
     pub num_escaped_funcs: u32,
 }
 
 impl<P: PtrSize> VMOffsets<P> {
     /// Return a new `VMOffsets` instance, for a given pointer size.
     pub fn new(ptr: P, module: &Module) -> Self {
+        let num_shared_memories = module
+            .memory_plans
+            .iter()
+            .filter(|p| p.1.memory.shared)
+            .count();
         VMOffsets::from(VMOffsetsFields {
             ptr,
             num_imported_functions: cast_to_u32(module.num_imported_funcs),
@@ -152,6 +164,7 @@ impl<P: PtrSize> VMOffsets<P> {
             num_defined_functions: cast_to_u32(module.functions.len()),
             num_defined_tables: cast_to_u32(module.table_plans.len()),
             num_defined_memories: cast_to_u32(module.memory_plans.len()),
+            num_owned_memories: cast_to_u32(module.memory_plans.len() - num_shared_memories),
             num_defined_globals: cast_to_u32(module.globals.len()),
             num_escaped_funcs: cast_to_u32(module.num_escaped_funcs),
         })
@@ -181,13 +194,14 @@ impl<P: PtrSize> VMOffsets<P> {
                     num_defined_tables: _,
                     num_defined_globals: _,
                     num_defined_memories: _,
+                    num_owned_memories: _,
                     num_defined_functions: _,
                     num_escaped_funcs: _,
 
                     // used as the initial size below
                     size,
 
-                    // exhaustively match teh rest of the fields with input from
+                    // exhaustively match the rest of the fields with input from
                     // the macro
                     $($name,)*
                 } = *self;
@@ -211,6 +225,7 @@ impl<P: PtrSize> VMOffsets<P> {
             defined_anyfuncs: "module functions",
             defined_globals: "defined globals",
             defined_memories: "defined memories",
+            owned_memories: "owned memories",
             defined_tables: "defined tables",
             imported_globals: "imported globals",
             imported_memories: "imported memories",
@@ -237,6 +252,7 @@ impl<P: PtrSize> From<VMOffsetsFields<P>> for VMOffsets<P> {
             num_defined_functions: fields.num_defined_functions,
             num_defined_tables: fields.num_defined_tables,
             num_defined_memories: fields.num_defined_memories,
+            num_owned_memories: fields.num_owned_memories,
             num_defined_globals: fields.num_defined_globals,
             num_escaped_funcs: fields.num_escaped_funcs,
             runtime_limits: 0,
@@ -251,6 +267,7 @@ impl<P: PtrSize> From<VMOffsetsFields<P>> for VMOffsets<P> {
             imported_globals: 0,
             defined_tables: 0,
             defined_memories: 0,
+            owned_memories: 0,
             defined_globals: 0,
             defined_anyfuncs: 0,
             size: 0,
@@ -303,7 +320,9 @@ impl<P: PtrSize> From<VMOffsetsFields<P>> for VMOffsets<P> {
             size(defined_tables)
                 = cmul(ret.num_defined_tables, ret.size_of_vmtable_definition()),
             size(defined_memories)
-                = cmul(ret.num_defined_memories, ret.size_of_vmmemory_definition()),
+                = cmul(ret.num_defined_memories, ret.size_of_vmmemory_pointer()),
+            size(owned_memories)
+                = cmul(ret.num_owned_memories, ret.size_of_vmmemory_definition()),
             align(16),
             size(defined_globals)
                 = cmul(ret.num_defined_globals, ret.size_of_vmglobal_definition()),
@@ -445,6 +464,12 @@ impl<P: PtrSize> VMOffsets<P> {
     pub fn size_of_vmmemory_definition(&self) -> u8 {
         2 * self.pointer_size()
     }
+
+    /// Return the size of `*mut VMMemoryDefinition`.
+    #[inline]
+    pub fn size_of_vmmemory_pointer(&self) -> u8 {
+        self.pointer_size()
+    }
 }
 
 /// Offsets for `VMGlobalImport`.
@@ -604,6 +629,12 @@ impl<P: PtrSize> VMOffsets<P> {
         self.defined_memories
     }
 
+    /// The offset of the `owned_memories` array.
+    #[inline]
+    pub fn vmctx_owned_memories_begin(&self) -> u32 {
+        self.owned_memories
+    }
+
     /// The offset of the `globals` array.
     #[inline]
     pub fn vmctx_globals_begin(&self) -> u32 {
@@ -667,11 +698,19 @@ impl<P: PtrSize> VMOffsets<P> {
         self.vmctx_tables_begin() + index.as_u32() * u32::from(self.size_of_vmtable_definition())
     }
 
-    /// Return the offset to `VMMemoryDefinition` index `index`.
+    /// Return the offset to the `*mut VMMemoryDefinition` at index `index`.
     #[inline]
-    pub fn vmctx_vmmemory_definition(&self, index: DefinedMemoryIndex) -> u32 {
+    pub fn vmctx_vmmemory_pointer(&self, index: DefinedMemoryIndex) -> u32 {
         assert_lt!(index.as_u32(), self.num_defined_memories);
-        self.vmctx_memories_begin() + index.as_u32() * u32::from(self.size_of_vmmemory_definition())
+        self.vmctx_memories_begin() + index.as_u32() * u32::from(self.size_of_vmmemory_pointer())
+    }
+
+    /// Return the offset to the owned `VMMemoryDefinition` at index `index`.
+    #[inline]
+    pub fn vmctx_vmmemory_definition(&self, index: OwnedMemoryIndex) -> u32 {
+        assert_lt!(index.as_u32(), self.num_owned_memories);
+        self.vmctx_owned_memories_begin()
+            + index.as_u32() * u32::from(self.size_of_vmmemory_definition())
     }
 
     /// Return the offset to the `VMGlobalDefinition` index `index`.
@@ -735,13 +774,13 @@ impl<P: PtrSize> VMOffsets<P> {
 
     /// Return the offset to the `base` field in `VMMemoryDefinition` index `index`.
     #[inline]
-    pub fn vmctx_vmmemory_definition_base(&self, index: DefinedMemoryIndex) -> u32 {
+    pub fn vmctx_vmmemory_definition_base(&self, index: OwnedMemoryIndex) -> u32 {
         self.vmctx_vmmemory_definition(index) + u32::from(self.vmmemory_definition_base())
     }
 
     /// Return the offset to the `current_length` field in `VMMemoryDefinition` index `index`.
     #[inline]
-    pub fn vmctx_vmmemory_definition_current_length(&self, index: DefinedMemoryIndex) -> u32 {
+    pub fn vmctx_vmmemory_definition_current_length(&self, index: OwnedMemoryIndex) -> u32 {
         self.vmctx_vmmemory_definition(index) + u32::from(self.vmmemory_definition_current_length())
     }
 

crates/environ/src/vmoffsets.rs

@@ -14,6 +14,7 @@ edition = "2021"
 wasmtime-environ = { path = "../environ", version = "=0.38.0" }
 wasmtime-fiber = { path = "../fiber", version = "=0.38.0", optional = true }
 wasmtime-jit-debug = { path = "../jit-debug", version = "=0.38.0", features = ["gdb_jit_int"] }
+wasmtime-types = { path = "../types", version = "=0.38.0" }
 region = "2.1.0"
 libc = { version = "0.2.112", default-features = false }
 log = "0.4.8"