Initial support for function, table, memory, and global imports.

lib/runtime/src/imports.rs

@@ -0,0 +1,46 @@
+use cranelift_entity::{BoxedSlice, PrimaryMap};
+use cranelift_wasm::{FuncIndex, GlobalIndex, MemoryIndex, TableIndex};
+use vmcontext::{VMFunctionBody, VMGlobalImport, VMMemoryImport, VMTableImport};
+
+/// Resolved import pointers.
+#[derive(Debug)]
+pub struct Imports {
+    /// Resolved addresses for imported functions.
+    pub functions: BoxedSlice<FuncIndex, *const VMFunctionBody>,
+
+    /// Resolved addresses for imported tables.
+    pub tables: BoxedSlice<TableIndex, VMTableImport>,
+
+    /// Resolved addresses for imported memories.
+    pub memories: BoxedSlice<MemoryIndex, VMMemoryImport>,
+
+    /// Resolved addresses for imported globals.
+    pub globals: BoxedSlice<GlobalIndex, VMGlobalImport>,
+}
+
+impl Imports {
+    /// Construct a new `Imports` instance.
+    pub fn new(
+        function_imports: PrimaryMap<FuncIndex, *const VMFunctionBody>,
+        table_imports: PrimaryMap<TableIndex, VMTableImport>,
+        memory_imports: PrimaryMap<MemoryIndex, VMMemoryImport>,
+        global_imports: PrimaryMap<GlobalIndex, VMGlobalImport>,
+    ) -> Self {
+        Self {
+            functions: function_imports.into_boxed_slice(),
+            tables: table_imports.into_boxed_slice(),
+            memories: memory_imports.into_boxed_slice(),
+            globals: global_imports.into_boxed_slice(),
+        }
+    }
+
+    /// Construct a new `Imports` instance with no imports.
+    pub fn none() -> Self {
+        Self {
+            functions: PrimaryMap::new().into_boxed_slice(),
+            tables: PrimaryMap::new().into_boxed_slice(),
+            memories: PrimaryMap::new().into_boxed_slice(),
+            globals: PrimaryMap::new().into_boxed_slice(),
+        }
+    }
+}

lib/runtime/src/instance.rs

@@ -0,0 +1,256 @@
+//! An `Instance` contains all the runtime state used by execution of a wasm
+//! module.
+
+use cranelift_entity::EntityRef;
+use cranelift_entity::{BoxedSlice, PrimaryMap};
+use cranelift_wasm::{
+    DefinedFuncIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, FuncIndex,
+};
+use imports::Imports;
+use memory::LinearMemory;
+use sig_registry::SignatureRegistry;
+use std::string::String;
+use table::Table;
+use vmcontext::{
+    VMCallerCheckedAnyfunc, VMContext, VMFunctionBody, VMGlobalDefinition, VMMemoryDefinition,
+    VMTableDefinition,
+};
+use wasmtime_environ::{DataInitializer, Module};
+
+/// An Instance of a WebAssemby module.
+#[derive(Debug)]
+pub struct Instance {
+    /// WebAssembly linear memory data.
+    memories: BoxedSlice<DefinedMemoryIndex, LinearMemory>,
+
+    /// WebAssembly table data.
+    tables: BoxedSlice<DefinedTableIndex, Table>,
+
+    /// Function Signature IDs.
+    /// FIXME: This should be shared across instances rather than per-Instance.
+    sig_registry: SignatureRegistry,
+
+    /// Resolved imports.
+    vmctx_imports: Imports,
+
+    /// Table storage base address vector pointed to by vmctx.
+    vmctx_tables: BoxedSlice<DefinedTableIndex, VMTableDefinition>,
+
+    /// Memory base address vector pointed to by vmctx.
+    vmctx_memories: BoxedSlice<DefinedMemoryIndex, VMMemoryDefinition>,
+
+    /// WebAssembly global variable data.
+    vmctx_globals: BoxedSlice<DefinedGlobalIndex, VMGlobalDefinition>,
+
+    /// Context pointer used by JIT code.
+    vmctx: VMContext,
+}
+
+impl Instance {
+    /// Create a new `Instance`.
+    pub fn new(
+        module: &Module,
+        finished_functions: &BoxedSlice<DefinedFuncIndex, *const VMFunctionBody>,
+        mut vmctx_imports: Imports,
+        data_initializers: &[DataInitializer],
+    ) -> Result<Self, String> {
+        let mut sig_registry = instantiate_signatures(module);
+        let mut memories = instantiate_memories(module, data_initializers)?;
+        let mut tables = instantiate_tables(
+            module,
+            finished_functions,
+            &vmctx_imports.functions,
+            &mut sig_registry,
+        );
+
+        let mut vmctx_memories = memories
+            .values_mut()
+            .map(LinearMemory::vmmemory)
+            .collect::<PrimaryMap<DefinedMemoryIndex, _>>()
+            .into_boxed_slice();
+
+        let mut vmctx_globals = instantiate_globals(module);
+
+        let mut vmctx_tables = tables
+            .values_mut()
+            .map(Table::vmtable)
+            .collect::<PrimaryMap<DefinedTableIndex, _>>()
+            .into_boxed_slice();
+
+        let vmctx_imported_functions_ptr = vmctx_imports
+            .functions
+            .values_mut()
+            .into_slice()
+            .as_mut_ptr();
+        let vmctx_imported_tables_ptr = vmctx_imports.tables.values_mut().into_slice().as_mut_ptr();
+        let vmctx_imported_memories_ptr = vmctx_imports
+            .memories
+            .values_mut()
+            .into_slice()
+            .as_mut_ptr();
+        let vmctx_imported_globals_ptr =
+            vmctx_imports.globals.values_mut().into_slice().as_mut_ptr();
+        let vmctx_memories_ptr = vmctx_memories.values_mut().into_slice().as_mut_ptr();
+        let vmctx_globals_ptr = vmctx_globals.values_mut().into_slice().as_mut_ptr();
+        let vmctx_tables_ptr = vmctx_tables.values_mut().into_slice().as_mut_ptr();
+        let vmctx_shared_signatures_ptr = sig_registry.vmshared_signatures();
+
+        Ok(Self {
+            memories,
+            tables,
+            sig_registry,
+            vmctx_imports,
+            vmctx_memories,
+            vmctx_globals,
+            vmctx_tables,
+            vmctx: VMContext::new(
+                vmctx_imported_functions_ptr,
+                vmctx_imported_tables_ptr,
+                vmctx_imported_memories_ptr,
+                vmctx_imported_globals_ptr,
+                vmctx_tables_ptr,
+                vmctx_memories_ptr,
+                vmctx_globals_ptr,
+                vmctx_shared_signatures_ptr,
+            ),
+        })
+    }
+
+    /// Return a reference to the vmctx used by JIT code.
+    pub fn vmctx(&self) -> &VMContext {
+        &self.vmctx
+    }
+
+    /// Return a mutable reference to the vmctx used by JIT code.
+    pub fn vmctx_mut(&mut self) -> &mut VMContext {
+        &mut self.vmctx
+    }
+
+    /// Return the offset from the vmctx pointer to its containing Instance.
+    pub(crate) fn vmctx_offset() -> isize {
+        offset_of!(Self, vmctx) as isize
+    }
+
+    /// Grow memory by the specified amount of pages.
+    ///
+    /// Returns `None` if memory can't be grown by the specified amount
+    /// of pages.
+    pub fn memory_grow(&mut self, memory_index: DefinedMemoryIndex, delta: u32) -> Option<u32> {
+        let result = self
+            .memories
+            .get_mut(memory_index)
+            .unwrap_or_else(|| panic!("no memory for index {}", memory_index.index()))
+            .grow(delta);
+
+        // Keep current the VMContext pointers used by JIT code.
+        self.vmctx_memories[memory_index] = self.memories[memory_index].vmmemory();
+
+        result
+    }
+
+    /// Returns the number of allocated wasm pages.
+    pub fn memory_size(&mut self, memory_index: DefinedMemoryIndex) -> u32 {
+        self.memories
+            .get(memory_index)
+            .unwrap_or_else(|| panic!("no memory for index {}", memory_index.index()))
+            .size()
+    }
+
+    /// Test whether any of the objects inside this instance require signal
+    /// handlers to catch out of bounds accesses.
+    pub(crate) fn needs_signal_handlers(&self) -> bool {
+        self.memories
+            .values()
+            .any(|memory| memory.needs_signal_handlers)
+    }
+
+    /// Return the number of imported memories.
+    pub(crate) fn num_imported_memories(&self) -> usize {
+        self.vmctx_imports.functions.len()
+    }
+}
+
+fn instantiate_signatures(module: &Module) -> SignatureRegistry {
+    let mut sig_registry = SignatureRegistry::new();
+    for (sig_index, sig) in module.signatures.iter() {
+        sig_registry.register(sig_index, sig);
+    }
+    sig_registry
+}
+
+/// Allocate memory for just the tables of the current module.
+fn instantiate_tables(
+    module: &Module,
+    finished_functions: &BoxedSlice<DefinedFuncIndex, *const VMFunctionBody>,
+    imported_functions: &BoxedSlice<FuncIndex, *const VMFunctionBody>,
+    sig_registry: &mut SignatureRegistry,
+) -> BoxedSlice<DefinedTableIndex, Table> {
+    let num_imports = module.imported_memories.len();
+    let mut tables: PrimaryMap<DefinedTableIndex, _> =
+        PrimaryMap::with_capacity(module.table_plans.len() - num_imports);
+    for table in &module.table_plans.values().as_slice()[num_imports..] {
+        tables.push(Table::new(table));
+    }
+
+    for init in &module.table_elements {
+        debug_assert!(init.base.is_none(), "globalvar base not supported yet");
+        let defined_table_index = module
+            .defined_table_index(init.table_index)
+            .expect("Initializers for imported tables not supported yet");
+        let slice = tables[defined_table_index].as_mut();
+        let subslice = &mut slice[init.offset..init.offset + init.elements.len()];
+        for (i, func_idx) in init.elements.iter().enumerate() {
+            let callee_sig = module.functions[*func_idx];
+            let func_ptr = if let Some(index) = module.defined_func_index(*func_idx) {
+                finished_functions[index]
+            } else {
+                imported_functions[*func_idx]
+            };
+            let type_index = sig_registry.lookup(callee_sig);
+            subslice[i] = VMCallerCheckedAnyfunc {
+                func_ptr,
+                type_index,
+            };
+        }
+    }
+
+    tables.into_boxed_slice()
+}
+
+/// Allocate memory for just the memories of the current module.
+fn instantiate_memories(
+    module: &Module,
+    data_initializers: &[DataInitializer],
+) -> Result<BoxedSlice<DefinedMemoryIndex, LinearMemory>, String> {
+    let num_imports = module.imported_memories.len();
+    let mut memories: PrimaryMap<DefinedMemoryIndex, _> =
+        PrimaryMap::with_capacity(module.memory_plans.len() - num_imports);
+    for plan in &module.memory_plans.values().as_slice()[num_imports..] {
+        memories.push(LinearMemory::new(&plan)?);
+    }
+
+    for init in data_initializers {
+        debug_assert!(init.base.is_none(), "globalvar base not supported yet");
+        let defined_memory_index = module
+            .defined_memory_index(init.memory_index)
+            .expect("Initializers for imported memories not supported yet");
+        let mem_mut = memories[defined_memory_index].as_mut();
+        let to_init = &mut mem_mut[init.offset..init.offset + init.data.len()];
+        to_init.copy_from_slice(init.data);
+    }
+
+    Ok(memories.into_boxed_slice())
+}
+
+/// Allocate memory for just the globals of the current module,
+/// without any initializers applied yet.
+fn instantiate_globals(module: &Module) -> BoxedSlice<DefinedGlobalIndex, VMGlobalDefinition> {
+    let num_imports = module.imported_globals.len();
+    let mut vmctx_globals = PrimaryMap::with_capacity(module.globals.len() - num_imports);
+
+    for global in &module.globals.values().as_slice()[num_imports..] {
+        vmctx_globals.push(VMGlobalDefinition::new(global));
+    }
+
+    vmctx_globals.into_boxed_slice()
+}

lib/runtime/src/lib.rs

@@ -0,0 +1,70 @@
+//! Runtime library support for Wasmtime.
+
+#![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
+#![warn(unused_import_braces)]
+#![cfg_attr(feature = "std", deny(unstable_features))]
+#![cfg_attr(feature = "clippy", plugin(clippy(conf_file = "../../clippy.toml")))]
+#![cfg_attr(
+    feature = "cargo-clippy",
+    allow(clippy::new_without_default, clippy::new_without_default_derive)
+)]
+#![cfg_attr(
+    feature = "cargo-clippy",
+    warn(
+        clippy::float_arithmetic,
+        clippy::mut_mut,
+        clippy::nonminimal_bool,
+        clippy::option_map_unwrap_or,
+        clippy::option_map_unwrap_or_else,
+        clippy::print_stdout,
+        clippy::unicode_not_nfc,
+        clippy::use_self
+    )
+)]
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(not(feature = "std"), feature(alloc))]
+
+extern crate cranelift_codegen;
+extern crate cranelift_entity;
+extern crate cranelift_wasm;
+extern crate errno;
+extern crate region;
+extern crate wasmtime_environ;
+#[cfg(not(feature = "std"))]
+#[macro_use]
+extern crate alloc;
+#[macro_use]
+extern crate lazy_static;
+extern crate libc;
+#[macro_use]
+extern crate memoffset;
+extern crate cast;
+
+mod imports;
+mod instance;
+mod memory;
+mod mmap;
+mod sig_registry;
+mod signalhandlers;
+mod table;
+mod traphandlers;
+mod vmcontext;
+
+pub mod libcalls;
+
+pub use imports::Imports;
+pub use instance::Instance;
+pub use mmap::Mmap;
+pub use signalhandlers::{wasmtime_init_eager, wasmtime_init_finish};
+pub use traphandlers::wasmtime_call_trampoline;
+pub use vmcontext::{
+    VMContext, VMFunctionBody, VMGlobalDefinition, VMGlobalImport, VMMemoryDefinition,
+    VMMemoryImport, VMTableDefinition, VMTableImport,
+};
+
+#[cfg(not(feature = "std"))]
+mod std {
+    pub use alloc::{string, vec};
+    pub use core::*;
+    pub use core::{i32, str, u32};
+}

lib/runtime/src/libcalls.rs

@@ -0,0 +1,157 @@
+//! Runtime library calls. Note that the JIT may sometimes perform these inline
+//! rather than calling them, particularly when CPUs have special instructions
+//! which compute them directly.
+
+use cranelift_wasm::{DefinedMemoryIndex, MemoryIndex};
+use vmcontext::VMContext;
+
+/// Implementation of f32.ceil
+pub extern "C" fn wasmtime_f32_ceil(x: f32) -> f32 {
+    x.ceil()
+}
+
+/// Implementation of f32.floor
+pub extern "C" fn wasmtime_f32_floor(x: f32) -> f32 {
+    x.floor()
+}
+
+/// Implementation of f32.trunc
+pub extern "C" fn wasmtime_f32_trunc(x: f32) -> f32 {
+    x.trunc()
+}
+
+/// Implementation of f32.nearest
+#[allow(clippy::float_arithmetic, clippy::float_cmp)]
+pub extern "C" fn wasmtime_f32_nearest(x: f32) -> f32 {
+    // Rust doesn't have a nearest function, so do it manually.
+    if x == 0.0 {
+        // Preserve the sign of zero.
+        x
+    } else {
+        // Nearest is either ceil or floor depending on which is nearest or even.
+        let u = x.ceil();
+        let d = x.floor();
+        let um = (x - u).abs();
+        let dm = (x - d).abs();
+        if um < dm
+            || (um == dm && {
+                let h = u / 2.;
+                h.floor() == h
+            })
+        {
+            u
+        } else {
+            d
+        }
+    }
+}
+
+/// Implementation of f64.ceil
+pub extern "C" fn wasmtime_f64_ceil(x: f64) -> f64 {
+    x.ceil()
+}
+
+/// Implementation of f64.floor
+pub extern "C" fn wasmtime_f64_floor(x: f64) -> f64 {
+    x.floor()
+}
+
+/// Implementation of f64.trunc
+pub extern "C" fn wasmtime_f64_trunc(x: f64) -> f64 {
+    x.trunc()
+}
+
+/// Implementation of f64.nearest
+#[allow(clippy::float_arithmetic, clippy::float_cmp)]
+pub extern "C" fn wasmtime_f64_nearest(x: f64) -> f64 {
+    // Rust doesn't have a nearest function, so do it manually.
+    if x == 0.0 {
+        // Preserve the sign of zero.
+        x
+    } else {
+        // Nearest is either ceil or floor depending on which is nearest or even.
+        let u = x.ceil();
+        let d = x.floor();
+        let um = (x - u).abs();
+        let dm = (x - d).abs();
+        if um < dm
+            || (um == dm && {
+                let h = u / 2.;
+                h.floor() == h
+            })
+        {
+            u
+        } else {
+            d
+        }
+    }
+}
+
+/// Implementation of memory.grow for locally-defined 32-bit memories.
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_memory32_grow(
+    delta: u32,
+    memory_index: u32,
+    vmctx: *mut VMContext,
+) -> u32 {
+    let instance = (&mut *vmctx).instance();
+    let memory_index = DefinedMemoryIndex::from_u32(memory_index);
+
+    instance
+        .memory_grow(memory_index, delta)
+        .unwrap_or(u32::max_value())
+}
+
+/// Implementation of memory.grow for imported 32-bit memories.
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_imported_memory32_grow(
+    delta: u32,
+    memory_index: u32,
+    vmctx: *mut VMContext,
+) -> u32 {
+    let instance = (&mut *vmctx).instance();
+    assert!(
+        (memory_index as usize) < instance.num_imported_memories(),
+        "imported memory index for memory.grow out of bounds"
+    );
+
+    let memory_index = MemoryIndex::from_u32(memory_index);
+    let import = instance.vmctx_mut().imported_memory_mut(memory_index);
+    let foreign_instance = (&mut *import.vmctx).instance();
+    let foreign_memory = &mut *import.from;
+    let foreign_index = foreign_instance.vmctx().memory_index(foreign_memory);
+
+    foreign_instance
+        .memory_grow(foreign_index, delta)
+        .unwrap_or(u32::max_value())
+}
+
+/// Implementation of memory.size for locally-defined 32-bit memories.
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_memory32_size(memory_index: u32, vmctx: *mut VMContext) -> u32 {
+    let instance = (&mut *vmctx).instance();
+    let memory_index = DefinedMemoryIndex::from_u32(memory_index);
+
+    instance.memory_size(memory_index)
+}
+
+/// Implementation of memory.size for imported 32-bit memories.
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_imported_memory32_size(
+    memory_index: u32,
+    vmctx: *mut VMContext,
+) -> u32 {
+    let instance = (&mut *vmctx).instance();
+    assert!(
+        (memory_index as usize) < instance.num_imported_memories(),
+        "imported memory index for memory.grow out of bounds"
+    );
+
+    let memory_index = MemoryIndex::from_u32(memory_index);
+    let import = instance.vmctx_mut().imported_memory_mut(memory_index);
+    let foreign_instance = (&mut *import.vmctx).instance();
+    let foreign_memory = &mut *import.from;
+    let foreign_index = foreign_instance.vmctx().memory_index(foreign_memory);
+
+    foreign_instance.memory_size(foreign_index)
+}

lib/runtime/src/memory.rs

@@ -0,0 +1,168 @@
+//! Memory management for linear memories.
+//!
+//! `LinearMemory` is to WebAssembly linear memories what `Table` is to WebAssembly tables.
+
+use mmap::Mmap;
+use region;
+use std::string::String;
+use vmcontext::VMMemoryDefinition;
+use wasmtime_environ::{MemoryPlan, MemoryStyle, WASM_MAX_PAGES, WASM_PAGE_SIZE};
+
+/// A linear memory instance.
+#[derive(Debug)]
+pub struct LinearMemory {
+    // The underlying allocation.
+    mmap: Mmap,
+
+    // The current logical size in wasm pages of this linear memory.
+    current: u32,
+
+    // The optional maximum size in wasm pages of this linear memory.
+    maximum: Option<u32>,
+
+    // Size in bytes of extra guard pages after the end to optimize loads and stores with
+    // constant offsets.
+    offset_guard_size: usize,
+
+    // Records whether we're using a bounds-checking strategy which requires
+    // handlers to catch trapping accesses.
+    pub(crate) needs_signal_handlers: bool,
+}
+
+impl LinearMemory {
+    /// Create a new linear memory instance with specified minimum and maximum number of wasm pages.
+    pub fn new(plan: &MemoryPlan) -> Result<Self, String> {
+        // `maximum` cannot be set to more than `65536` pages.
+        assert!(plan.memory.minimum <= WASM_MAX_PAGES);
+        assert!(plan.memory.maximum.is_none() || plan.memory.maximum.unwrap() <= WASM_MAX_PAGES);
+
+        let offset_guard_bytes = plan.offset_guard_size as usize;
+
+        // If we have an offset guard, or if we're doing the static memory
+        // allocation strategy, we need signal handlers to catch out of bounds
+        // acceses.
+        let needs_signal_handlers = offset_guard_bytes > 0
+            || match plan.style {
+                MemoryStyle::Dynamic => false,
+                MemoryStyle::Static { .. } => true,
+            };
+
+        let minimum_pages = match plan.style {
+            MemoryStyle::Dynamic => plan.memory.minimum,
+            MemoryStyle::Static { bound } => {
+                assert!(bound >= plan.memory.minimum);
+                bound
+            }
+        } as usize;
+        let minimum_bytes = minimum_pages.checked_mul(WASM_PAGE_SIZE as usize).unwrap();
+        let request_bytes = minimum_bytes.checked_add(offset_guard_bytes).unwrap();
+        let mapped_pages = plan.memory.minimum as usize;
+        let mapped_bytes = mapped_pages * WASM_PAGE_SIZE as usize;
+        let unmapped_pages = minimum_pages - mapped_pages;
+        let unmapped_bytes = unmapped_pages * WASM_PAGE_SIZE as usize;
+        let inaccessible_bytes = unmapped_bytes + offset_guard_bytes;
+
+        let mmap = Mmap::with_size(request_bytes)?;
+
+        // Make the unmapped and offset-guard pages inaccessible.
+        unsafe {
+            region::protect(
+                mmap.as_ptr().add(mapped_bytes),
+                inaccessible_bytes,
+                region::Protection::None,
+            )
+        }
+        .expect("unable to make memory inaccessible");
+
+        Ok(Self {
+            mmap,
+            current: plan.memory.minimum,
+            maximum: plan.memory.maximum,
+            offset_guard_size: offset_guard_bytes,
+            needs_signal_handlers,
+        })
+    }
+
+    /// Returns the number of allocated wasm pages.
+    pub fn size(&self) -> u32 {
+        self.current
+    }
+
+    /// Grow memory by the specified amount of wasm pages.
+    ///
+    /// Returns `None` if memory can't be grown by the specified amount
+    /// of wasm pages.
+    pub fn grow(&mut self, delta: u32) -> Option<u32> {
+        let new_pages = match self.current.checked_add(delta) {
+            Some(new_pages) => new_pages,
+            // Linear memory size overflow.
+            None => return None,
+        };
+        let prev_pages = self.current;
+
+        if let Some(maximum) = self.maximum {
+            if new_pages > maximum {
+                // Linear memory size would exceed the declared maximum.
+                return None;
+            }
+        }
+
+        // Wasm linear memories are never allowed to grow beyond what is
+        // indexable. If the memory has no maximum, enforce the greatest
+        // limit here.
+        if new_pages >= WASM_MAX_PAGES {
+            // Linear memory size would exceed the index range.
+            return None;
+        }
+
+        let new_bytes = new_pages as usize * WASM_PAGE_SIZE as usize;
+
+        if new_bytes > self.mmap.len() - self.offset_guard_size {
+            // If we have no maximum, this is a "dynamic" heap, and it's allowed to move.
+            assert!(self.maximum.is_none());
+            let guard_bytes = self.offset_guard_size;
+            let request_bytes = new_bytes.checked_add(guard_bytes)?;
+
+            let mut new_mmap = Mmap::with_size(request_bytes).ok()?;
+
+            // Make the offset-guard pages inaccessible.
+            unsafe {
+                region::protect(
+                    new_mmap.as_ptr().add(new_bytes),
+                    guard_bytes,
+                    region::Protection::None,
+                )
+            }
+            .expect("unable to make memory inaccessible");
+
+            let copy_len = self.mmap.len() - self.offset_guard_size;
+            new_mmap.as_mut_slice()[..copy_len].copy_from_slice(&self.mmap.as_slice()[..copy_len]);
+
+            self.mmap = new_mmap;
+        }
+
+        self.current = new_pages;
+
+        Some(prev_pages)
+    }
+
+    /// Return a `VMMemoryDefinition` for exposing the memory to JIT code.
+    pub fn vmmemory(&mut self) -> VMMemoryDefinition {
+        VMMemoryDefinition {
+            base: self.mmap.as_mut_ptr(),
+            current_length: self.mmap.len(),
+        }
+    }
+}
+
+impl AsRef<[u8]> for LinearMemory {
+    fn as_ref(&self) -> &[u8] {
+        self.mmap.as_slice()
+    }
+}
+
+impl AsMut<[u8]> for LinearMemory {
+    fn as_mut(&mut self) -> &mut [u8] {
+        self.mmap.as_mut_slice()
+    }
+}

lib/runtime/src/mmap.rs

@@ -0,0 +1,144 @@
+//! Low-level abstraction for allocating and managing zero-filled pages
+//! of memory.
+
+use errno;
+use libc;
+use region;
+use std::ptr;
+use std::slice;
+use std::string::String;
+
+/// Round `size` up to the nearest multiple of `page_size`.
+fn round_up_to_page_size(size: usize, page_size: usize) -> usize {
+    (size + (page_size - 1)) & !(page_size - 1)
+}
+
+/// A simple struct consisting of a page-aligned pointer to page-aligned
+/// and initially-zeroed memory and a length.
+#[derive(Debug)]
+pub struct Mmap {
+    ptr: *mut u8,
+    len: usize,
+}
+
+impl Mmap {
+    /// Construct a new empty instance of `Mmap`.
+    pub fn new() -> Self {
+        Self {
+            ptr: ptr::null_mut(),
+            len: 0,
+        }
+    }
+
+    /// Create a new `Mmap` pointing to at least `size` bytes of memory,
+    /// suitably sized and aligned for memory protection.
+    #[cfg(not(target_os = "windows"))]
+    pub fn with_size(size: usize) -> Result<Self, String> {
+        let page_size = region::page::size();
+        let alloc_size = round_up_to_page_size(size, page_size);
+        let ptr = unsafe {
+            libc::mmap(
+                ptr::null_mut(),
+                alloc_size,
+                libc::PROT_READ | libc::PROT_WRITE,
+                libc::MAP_PRIVATE | libc::MAP_ANON,
+                -1,
+                0,
+            )
+        };
+        if ptr as isize == -1isize {
+            Err(errno::errno().to_string())
+        } else {
+            Ok(Self {
+                ptr: ptr as *mut u8,
+                len: alloc_size,
+            })
+        }
+    }
+
+    /// Create a new `Mmap` pointing to at least `size` bytes of memory,
+    /// suitably sized and aligned for memory protection.
+    #[cfg(target_os = "windows")]
+    pub fn with_size(size: usize) -> Result<Self, String> {
+        use winapi::um::memoryapi::VirtualAlloc;
+        use winapi::um::winnt::{MEM_COMMIT, MEM_RESERVE, PAGE_READWRITE};
+
+        let page_size = region::page::size();
+
+        // VirtualAlloc always rounds up to the next multiple of the page size
+        let ptr = unsafe {
+            VirtualAlloc(
+                ptr::null_mut(),
+                size,
+                MEM_COMMIT | MEM_RESERVE,
+                PAGE_READWRITE,
+            )
+        };
+        if !ptr.is_null() {
+            Ok(Self {
+                ptr: ptr as *mut u8,
+                len: round_up_to_page_size(size, page_size),
+            })
+        } else {
+            Err(errno::errno().to_string())
+        }
+    }
+
+    /// Return the allocated memory as a slice of u8.
+    pub fn as_slice(&self) -> &[u8] {
+        unsafe { slice::from_raw_parts(self.ptr, self.len) }
+    }
+
+    /// Return the allocated memory as a mutable slice of u8.
+    pub fn as_mut_slice(&mut self) -> &mut [u8] {
+        unsafe { slice::from_raw_parts_mut(self.ptr, self.len) }
+    }
+
+    /// Return the allocated memory as a pointer to u8.
+    pub fn as_ptr(&self) -> *const u8 {
+        self.ptr
+    }
+
+    /// Return the allocated memory as a mutable pointer to u8.
+    pub fn as_mut_ptr(&mut self) -> *mut u8 {
+        self.ptr
+    }
+
+    /// Return the lengthof the allocated memory.
+    pub fn len(&self) -> usize {
+        self.len
+    }
+}
+
+impl Drop for Mmap {
+    #[cfg(not(target_os = "windows"))]
+    fn drop(&mut self) {
+        if !self.ptr.is_null() {
+            let r = unsafe { libc::munmap(self.ptr as *mut libc::c_void, self.len) };
+            assert_eq!(r, 0, "munmap failed: {}", errno::errno());
+        }
+    }
+
+    #[cfg(target_os = "windows")]
+    fn drop(&mut self) {
+        if !self.ptr.is_null() {
+            use winapi::um::memoryapi::VirtualFree;
+            use winapi::um::winnt::MEM_RELEASE;
+            let r = unsafe { VirtualFree(self.ptr, self.len, MEM_RELEASE) };
+            assert_eq!(r, 0);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_round_up_to_page_size() {
+        assert_eq!(round_up_to_page_size(0, 4096), 0);
+        assert_eq!(round_up_to_page_size(1, 4096), 4096);
+        assert_eq!(round_up_to_page_size(4096, 4096), 4096);
+        assert_eq!(round_up_to_page_size(4097, 4096), 8192);
+    }
+}

lib/runtime/src/sig_registry.rs

@@ -0,0 +1,55 @@
+//! Implement a registry of function signatures, for fast indirect call
+//! signature checking.
+
+use cast;
+use cranelift_codegen::ir;
+use cranelift_entity::PrimaryMap;
+use cranelift_wasm::SignatureIndex;
+use std::collections::{hash_map, HashMap};
+use vmcontext::VMSharedSignatureIndex;
+
+#[derive(Debug)]
+pub struct SignatureRegistry {
+    signature_hash: HashMap<ir::Signature, VMSharedSignatureIndex>,
+    shared_signatures: PrimaryMap<SignatureIndex, VMSharedSignatureIndex>,
+}
+
+impl SignatureRegistry {
+    pub fn new() -> Self {
+        Self {
+            signature_hash: HashMap::new(),
+            shared_signatures: PrimaryMap::new(),
+        }
+    }
+
+    pub fn vmshared_signatures(&mut self) -> *mut VMSharedSignatureIndex {
+        self.shared_signatures
+            .values_mut()
+            .into_slice()
+            .as_mut_ptr()
+    }
+
+    /// Register the given signature.
+    pub fn register(&mut self, sig_index: SignatureIndex, sig: &ir::Signature) {
+        // TODO: Refactor this interface so that we're not passing in redundant
+        // information.
+        debug_assert_eq!(sig_index.index(), self.shared_signatures.len());
+        use cranelift_entity::EntityRef;
+
+        let len = self.signature_hash.len();
+        let sig_id = match self.signature_hash.entry(sig.clone()) {
+            hash_map::Entry::Occupied(entry) => *entry.get(),
+            hash_map::Entry::Vacant(entry) => {
+                let sig_id = VMSharedSignatureIndex::new(cast::u32(len).unwrap());
+                entry.insert(sig_id);
+                sig_id
+            }
+        };
+        self.shared_signatures.push(sig_id);
+    }
+
+    /// Return the identifying runtime index for the given signature.
+    pub fn lookup(&mut self, sig_index: SignatureIndex) -> VMSharedSignatureIndex {
+        self.shared_signatures[sig_index]
+    }
+}

lib/runtime/src/signalhandlers.rs

@@ -0,0 +1,120 @@
+//! Interface to low-level signal-handling mechanisms.
+
+#![allow(non_upper_case_globals)]
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+
+use std::borrow::{Borrow, BorrowMut};
+use std::cell::RefCell;
+use std::sync::RwLock;
+use vmcontext::VMContext;
+
+include!(concat!(env!("OUT_DIR"), "/signalhandlers.rs"));
+
+struct InstallState {
+    tried: bool,
+    success: bool,
+}
+
+impl InstallState {
+    fn new() -> Self {
+        Self {
+            tried: false,
+            success: false,
+        }
+    }
+}
+
+lazy_static! {
+    static ref EAGER_INSTALL_STATE: RwLock<InstallState> = RwLock::new(InstallState::new());
+    static ref LAZY_INSTALL_STATE: RwLock<InstallState> = RwLock::new(InstallState::new());
+}
+
+/// This function performs the low-overhead signal handler initialization that we
+/// want to do eagerly to ensure a more-deterministic global process state. This
+/// is especially relevant for signal handlers since handler ordering depends on
+/// installation order: the wasm signal handler must run *before* the other crash
+/// handlers and since POSIX signal handlers work LIFO, this function needs to be
+/// called at the end of the startup process, after other handlers have been
+/// installed. This function can thus be called multiple times, having no effect
+/// after the first call.
+#[no_mangle]
+pub extern "C" fn wasmtime_init_eager() {
+    let mut locked = EAGER_INSTALL_STATE.write().unwrap();
+    let state = locked.borrow_mut();
+
+    if state.tried {
+        return;
+    }
+
+    state.tried = true;
+    assert!(!state.success);
+
+    if !unsafe { EnsureEagerSignalHandlers() } {
+        return;
+    }
+
+    state.success = true;
+}
+
+thread_local! {
+    static TRAP_CONTEXT: RefCell<TrapContext> = RefCell::new(TrapContext { triedToInstallSignalHandlers: false, haveSignalHandlers: false });
+}
+
+/// Assuming `EnsureEagerProcessSignalHandlers` has already been called,
+/// this function performs the full installation of signal handlers which must
+/// be performed per-thread. This operation may incur some overhead and
+/// so should be done only when needed to use wasm.
+#[no_mangle]
+pub extern "C" fn wasmtime_init_finish(vmctx: &mut VMContext) {
+    if !TRAP_CONTEXT.with(|cx| cx.borrow().triedToInstallSignalHandlers) {
+        TRAP_CONTEXT.with(|cx| {
+            cx.borrow_mut().triedToInstallSignalHandlers = true;
+            assert!(!cx.borrow().haveSignalHandlers);
+        });
+
+        {
+            let locked = EAGER_INSTALL_STATE.read().unwrap();
+            let state = locked.borrow();
+            assert!(
+                state.tried,
+                "call wasmtime_init_eager before calling wasmtime_init_finish"
+            );
+            if !state.success {
+                return;
+            }
+        }
+
+        #[cfg(any(target_os = "macos", target_os = "ios"))]
+        ensure_darwin_mach_ports();
+
+        TRAP_CONTEXT.with(|cx| {
+            cx.borrow_mut().haveSignalHandlers = true;
+        })
+    }
+
+    let instance = unsafe { vmctx.instance() };
+    let have_signal_handlers = TRAP_CONTEXT.with(|cx| cx.borrow().haveSignalHandlers);
+    if !have_signal_handlers && instance.needs_signal_handlers() {
+        panic!("failed to install signal handlers");
+    }
+}
+
+#[cfg(any(target_os = "macos", target_os = "ios"))]
+fn ensure_darwin_mach_ports() {
+    let mut locked = LAZY_INSTALL_STATE.write().unwrap();
+    let state = locked.borrow_mut();
+
+    if state.tried {
+        return;
+    }
+
+    state.tried = true;
+    assert!(!state.success);
+
+    if !unsafe { EnsureDarwinMachPorts() } {
+        return;
+    }
+
+    state.success = true;
+}

lib/runtime/src/table.rs

@@ -0,0 +1,61 @@
+//! Memory management for tables.
+//!
+//! `Table` is to WebAssembly tables what `LinearMemory` is to WebAssembly linear memories.
+
+use cranelift_wasm::TableElementType;
+use vmcontext::{VMCallerCheckedAnyfunc, VMTableDefinition};
+use wasmtime_environ::{TablePlan, TableStyle};
+
+/// A table instance.
+#[derive(Debug)]
+pub struct Table {
+    vec: Vec<VMCallerCheckedAnyfunc>,
+    maximum: Option<u32>,
+}
+
+impl Table {
+    /// Create a new table instance with specified minimum and maximum number of elements.
+    pub fn new(plan: &TablePlan) -> Self {
+        match plan.table.ty {
+            TableElementType::Func => (),
+            TableElementType::Val(ty) => {
+                unimplemented!("tables of types other than anyfunc ({})", ty)
+            }
+        };
+
+        match plan.style {
+            TableStyle::CallerChecksSignature => {
+                let mut vec = Vec::new();
+                vec.resize(
+                    plan.table.minimum as usize,
+                    VMCallerCheckedAnyfunc::default(),
+                );
+
+                Self {
+                    vec,
+                    maximum: plan.table.maximum,
+                }
+            }
+        }
+    }
+
+    /// Return a `VMTableDefinition` for exposing the table to JIT code.
+    pub fn vmtable(&mut self) -> VMTableDefinition {
+        VMTableDefinition {
+            base: self.vec.as_mut_ptr() as *mut u8,
+            current_elements: self.vec.len(),
+        }
+    }
+}
+
+impl AsRef<[VMCallerCheckedAnyfunc]> for Table {
+    fn as_ref(&self) -> &[VMCallerCheckedAnyfunc] {
+        self.vec.as_slice()
+    }
+}
+
+impl AsMut<[VMCallerCheckedAnyfunc]> for Table {
+    fn as_mut(&mut self) -> &mut [VMCallerCheckedAnyfunc] {
+        self.vec.as_mut_slice()
+    }
+}

lib/runtime/src/traphandlers.rs

@@ -0,0 +1,109 @@
+//! WebAssembly trap handling, which is built on top of the lower-level
+//! signalhandling mechanisms.
+
+use libc::c_int;
+use signalhandlers::{jmp_buf, CodeSegment};
+use std::cell::{Cell, RefCell};
+use std::mem;
+use std::ptr;
+use std::string::String;
+use vmcontext::{VMContext, VMFunctionBody};
+
+// Currently we uset setjmp/longjmp to unwind out of a signal handler
+// and back to the point where WebAssembly was called (via `call_wasm`).
+// This works because WebAssembly code currently does not use any EH
+// or require any cleanups, and we never unwind through non-wasm frames.
+// In the future, we'll likely replace this with fancier stack unwinding.
+extern "C" {
+    fn setjmp(env: *mut jmp_buf) -> c_int;
+    fn longjmp(env: *const jmp_buf, val: c_int) -> !;
+}
+
+#[derive(Copy, Clone, Debug)]
+struct TrapData {
+    pc: *const u8,
+}
+
+thread_local! {
+    static TRAP_DATA: Cell<TrapData> = Cell::new(TrapData { pc: ptr::null() });
+    static JMP_BUFS: RefCell<Vec<jmp_buf>> = RefCell::new(Vec::new());
+}
+
+/// Record the Trap code and wasm bytecode offset in TLS somewhere
+#[doc(hidden)]
+#[allow(non_snake_case)]
+#[no_mangle]
+pub extern "C" fn RecordTrap(pc: *const u8, _codeSegment: *const CodeSegment) {
+    // TODO: Look up the wasm bytecode offset and trap code and record them instead.
+    TRAP_DATA.with(|data| data.set(TrapData { pc }));
+}
+
+/// Initiate an unwind.
+#[doc(hidden)]
+#[allow(non_snake_case)]
+#[no_mangle]
+pub extern "C" fn Unwind() {
+    JMP_BUFS.with(|bufs| {
+        let buf = bufs.borrow_mut().pop().unwrap();
+        unsafe { longjmp(&buf, 1) };
+    })
+}
+
+/// Return the CodeSegment containing the given pc, if any exist in the process.
+/// This method does not take a lock.
+#[doc(hidden)]
+#[allow(non_snake_case)]
+#[no_mangle]
+pub extern "C" fn LookupCodeSegment(_pc: *const ::std::os::raw::c_void) -> *const CodeSegment {
+    // TODO: Implement this.
+    -1isize as *const CodeSegment
+}
+
+/// A simple guard to ensure that `JMP_BUFS` is reset when we're done.
+struct ScopeGuard {
+    orig_num_bufs: usize,
+}
+
+impl ScopeGuard {
+    fn new() -> Self {
+        Self {
+            orig_num_bufs: JMP_BUFS.with(|bufs| bufs.borrow().len()),
+        }
+    }
+}
+
+impl Drop for ScopeGuard {
+    fn drop(&mut self) {
+        let orig_num_bufs = self.orig_num_bufs;
+        JMP_BUFS.with(|bufs| {
+            bufs.borrow_mut()
+                .resize(orig_num_bufs, unsafe { mem::zeroed() })
+        });
+    }
+}
+
+/// Call the wasm function pointed to by `callee`. `values_vec` points to
+/// a buffer which holds the incoming arguments, and to which the outgoing
+/// return values will be written.
+#[no_mangle]
+pub unsafe extern "C" fn wasmtime_call_trampoline(
+    callee: *const VMFunctionBody,
+    values_vec: *mut u8,
+    vmctx: *mut VMContext,
+) -> Result<(), String> {
+    // In case wasm code calls Rust that panics and unwinds past this point,
+    // ensure that JMP_BUFS is unwound to its incoming state.
+    let _guard = ScopeGuard::new();
+
+    let func: fn(*mut u8, *mut VMContext) = mem::transmute(callee);
+
+    JMP_BUFS.with(|bufs| {
+        let mut buf = mem::uninitialized();
+        if setjmp(&mut buf) != 0 {
+            return TRAP_DATA.with(|data| Err(format!("wasm trap at {:?}", data.get().pc)));
+        }
+        bufs.borrow_mut().push(buf);
+        func(values_vec, vmctx);
+        Ok(())
+    })
+}

lib/runtime/src/vmcontext.rs

@@ -0,0 +1,577 @@
+//! This file declares `VMContext` and several related structs which contain
+//! fields that JIT code accesses directly.
+
+use cranelift_entity::EntityRef;
+use cranelift_wasm::{
+    DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, FuncIndex, Global, GlobalIndex,
+    GlobalInit, MemoryIndex, TableIndex,
+};
+use instance::Instance;
+use std::{mem, ptr, u32};
+
+/// A placeholder byte-sized type which is just used to provide some amount of type
+/// safety when dealing with pointers to JIT-compiled function bodies. Note that it's
+/// deliberately not Copy, as we shouldn't be carelessly copying function body bytes
+/// around.
+#[repr(C)]
+pub struct VMFunctionBody(u8);
+
+#[cfg(test)]
+mod test_vmfunction_body {
+    use super::VMFunctionBody;
+    use std::mem::size_of;
+
+    #[test]
+    fn check_vmfunction_body_offsets() {
+        assert_eq!(size_of::<VMFunctionBody>(), 1);
+    }
+}
+
+/// The fields a JIT needs to access to utilize a WebAssembly table
+/// imported from another instance.
+#[derive(Debug, Copy, Clone)]
+#[repr(C)]
+pub struct VMTableImport {
+    /// A pointer to the imported table description.
+    pub from: *mut VMTableDefinition,
+
+    /// A pointer to the VMContext that owns the table description.
+    pub vmctx: *mut VMContext,
+}
+
+#[cfg(test)]
+mod test_vmtable_import {
+    use super::VMTableImport;
+    use std::mem::size_of;
+    use wasmtime_environ::VMOffsets;
+
+    #[test]
+    fn check_vmtable_import_offsets() {
+        let offsets = VMOffsets::new(size_of::<*mut u8>() as u8);
+        assert_eq!(
+            size_of::<VMTableImport>(),
+            usize::from(offsets.size_of_vmtable_import())
+        );
+        assert_eq!(
+            offset_of!(VMTableImport, from),
+            usize::from(offsets.vmtable_import_from())
+        );
+        assert_eq!(
+            offset_of!(VMTableImport, vmctx),
+            usize::from(offsets.vmtable_import_vmctx())
+        );
+    }
+}
+
+/// The fields a JIT needs to access to utilize a WebAssembly linear
+/// memory imported from another instance.
+#[derive(Debug, Copy, Clone)]
+#[repr(C)]
+pub struct VMMemoryImport {
+    /// A pointer to the imported memory description.
+    pub from: *mut VMMemoryDefinition,
+
+    /// A pointer to the VMContext that owns the memory description.
+    pub vmctx: *mut VMContext,
+}
+
+#[cfg(test)]
+mod test_vmmemory_import {
+    use super::VMMemoryImport;
+    use std::mem::size_of;
+    use wasmtime_environ::VMOffsets;
+
+    #[test]
+    fn check_vmmemory_import_offsets() {
+        let offsets = VMOffsets::new(size_of::<*mut u8>() as u8);
+        assert_eq!(
+            size_of::<VMMemoryImport>(),
+            usize::from(offsets.size_of_vmmemory_import())
+        );
+        assert_eq!(
+            offset_of!(VMMemoryImport, from),
+            usize::from(offsets.vmmemory_import_from())
+        );
+        assert_eq!(
+            offset_of!(VMMemoryImport, vmctx),
+            usize::from(offsets.vmmemory_import_vmctx())
+        );
+    }
+}
+
+/// The fields a JIT needs to access to utilize a WebAssembly global
+/// variable imported from another instance.
+#[derive(Debug, Copy, Clone)]
+#[repr(C)]
+pub struct VMGlobalImport {
+    /// A pointer to the imported global variable description.
+    pub from: *mut VMGlobalDefinition,
+}
+
+#[cfg(test)]
+mod test_vmglobal_import {
+    use super::VMGlobalImport;
+    use std::mem::size_of;
+    use wasmtime_environ::VMOffsets;
+
+    #[test]
+    fn check_vmglobal_import_offsets() {
+        let offsets = VMOffsets::new(size_of::<*mut u8>() as u8);
+        assert_eq!(
+            size_of::<VMGlobalImport>(),
+            usize::from(offsets.size_of_vmglobal_import())
+        );
+        assert_eq!(
+            offset_of!(VMGlobalImport, from),
+            usize::from(offsets.vmglobal_import_from())
+        );
+    }
+}
+
+/// The fields a JIT needs to access to utilize a WebAssembly linear
+/// memory defined within the instance, namely the start address and the
+/// size in bytes.
+#[derive(Debug, Copy, Clone)]
+#[repr(C)]
+pub struct VMMemoryDefinition {
+    /// The start address.
+    pub base: *mut u8,
+
+    /// The current logical size of this linear memory in bytes.
+    pub current_length: usize,
+}
+
+#[cfg(test)]
+mod test_vmmemory_definition {
+    use super::VMMemoryDefinition;
+    use std::mem::size_of;
+    use wasmtime_environ::VMOffsets;
+
+    #[test]
+    fn check_vmmemory_definition_offsets() {
+        let offsets = VMOffsets::new(size_of::<*mut u8>() as u8);
+        assert_eq!(
+            size_of::<VMMemoryDefinition>(),
+            usize::from(offsets.size_of_vmmemory_definition())
+        );
+        assert_eq!(
+            offset_of!(VMMemoryDefinition, base),
+            usize::from(offsets.vmmemory_definition_base())
+        );
+        assert_eq!(
+            offset_of!(VMMemoryDefinition, current_length),
+            usize::from(offsets.vmmemory_definition_current_length())
+        );
+        /* TODO: Assert that the size of `current_length` matches.
+        assert_eq!(
+            size_of::<VMMemoryDefinition::current_length>(),
+            usize::from(offsets.size_of_vmmemory_definition_current_length())
+        );
+        */
+    }
+}
+
+/// The fields a JIT needs to access to utilize a WebAssembly table
+/// defined within the instance.
+#[derive(Debug, Copy, Clone)]
+#[repr(C)]
+pub struct VMTableDefinition {
+    /// Pointer to the table data.
+    pub base: *mut u8,
+
+    /// The current number of elements in the table.
+    pub current_elements: usize,
+}
+
+#[cfg(test)]
+mod test_vmtable_definition {
+    use super::VMTableDefinition;
+    use std::mem::size_of;
+    use wasmtime_environ::VMOffsets;
+
+    #[test]
+    fn check_vmtable_definition_offsets() {
+        let offsets = VMOffsets::new(size_of::<*mut u8>() as u8);
+        assert_eq!(
+            size_of::<VMTableDefinition>(),
+            usize::from(offsets.size_of_vmtable_definition())
+        );
+        assert_eq!(
+            offset_of!(VMTableDefinition, base),
+            usize::from(offsets.vmtable_definition_base())
+        );
+        assert_eq!(
+            offset_of!(VMTableDefinition, current_elements),
+            usize::from(offsets.vmtable_definition_current_elements())
+        );
+    }
+}
+
+/// The storage for a WebAssembly global defined within the instance.
+///
+/// TODO: Pack the globals more densely, rather than using the same size
+/// for every type.
+#[derive(Debug, Copy, Clone)]
+#[repr(C, align(8))]
+pub struct VMGlobalDefinition {
+    storage: [u8; 8],
+    // If more elements are added here, remember to add offset_of tests below!
+}
+
+#[cfg(test)]
+mod test_vmglobal_definition {
+    use super::VMGlobalDefinition;
+    use std::mem::{align_of, size_of};
+    use wasmtime_environ::VMOffsets;
+
+    #[test]
+    fn check_vmglobal_definition_alignment() {
+        assert!(align_of::<VMGlobalDefinition>() >= align_of::<i32>());
+        assert!(align_of::<VMGlobalDefinition>() >= align_of::<i64>());
+        assert!(align_of::<VMGlobalDefinition>() >= align_of::<f32>());
+        assert!(align_of::<VMGlobalDefinition>() >= align_of::<f64>());
+    }
+
+    #[test]
+    fn check_vmglobal_definition_offsets() {
+        let offsets = VMOffsets::new(size_of::<*mut u8>() as u8);
+        assert_eq!(
+            size_of::<VMGlobalDefinition>(),
+            usize::from(offsets.size_of_vmglobal_definition())
+        );
+    }
+}
+
+impl VMGlobalDefinition {
+    /// Construct a `VMGlobalDefinition`.
+    pub fn new(global: &Global) -> Self {
+        let mut result = Self { storage: [0; 8] };
+        match global.initializer {
+            GlobalInit::I32Const(x) => *unsafe { result.as_i32_mut() } = x,
+            GlobalInit::I64Const(x) => *unsafe { result.as_i64_mut() } = x,
+            GlobalInit::F32Const(x) => *unsafe { result.as_f32_bits_mut() } = x,
+            GlobalInit::F64Const(x) => *unsafe { result.as_f64_bits_mut() } = x,
+            GlobalInit::GetGlobal(_x) => unimplemented!("globals init with get_global"),
+            GlobalInit::Import => panic!("attempting to initialize imported global"),
+        }
+        result
+    }
+
+    /// Return a reference to the value as an i32.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_i32(&self) -> &i32 {
+        &*(self.storage.as_ref().as_ptr() as *const u8 as *const i32)
+    }
+
+    /// Return a mutable reference to the value as an i32.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_i32_mut(&mut self) -> &mut i32 {
+        &mut *(self.storage.as_mut().as_mut_ptr() as *mut u8 as *mut i32)
+    }
+
+    /// Return a reference to the value as an i64.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_i64(&self) -> &i64 {
+        &*(self.storage.as_ref().as_ptr() as *const u8 as *const i64)
+    }
+
+    /// Return a mutable reference to the value as an i64.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_i64_mut(&mut self) -> &mut i64 {
+        &mut *(self.storage.as_mut().as_mut_ptr() as *mut u8 as *mut i64)
+    }
+
+    /// Return a reference to the value as an f32.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_f32(&self) -> &f32 {
+        &*(self.storage.as_ref().as_ptr() as *const u8 as *const f32)
+    }
+
+    /// Return a mutable reference to the value as an f32.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_f32_mut(&mut self) -> &mut f32 {
+        &mut *(self.storage.as_mut().as_mut_ptr() as *mut u8 as *mut f32)
+    }
+
+    /// Return a reference to the value as f32 bits.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_f32_bits(&self) -> &u32 {
+        &*(self.storage.as_ref().as_ptr() as *const u8 as *const u32)
+    }
+
+    /// Return a mutable reference to the value as f32 bits.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_f32_bits_mut(&mut self) -> &mut u32 {
+        &mut *(self.storage.as_mut().as_mut_ptr() as *mut u8 as *mut u32)
+    }
+
+    /// Return a reference to the value as an f64.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_f64(&self) -> &f64 {
+        &*(self.storage.as_ref().as_ptr() as *const u8 as *const f64)
+    }
+
+    /// Return a mutable reference to the value as an f64.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_f64_mut(&mut self) -> &mut f64 {
+        &mut *(self.storage.as_mut().as_mut_ptr() as *mut u8 as *mut f64)
+    }
+
+    /// Return a reference to the value as f64 bits.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_f64_bits(&self) -> &u64 {
+        &*(self.storage.as_ref().as_ptr() as *const u8 as *const u64)
+    }
+
+    /// Return a mutable reference to the value as f64 bits.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn as_f64_bits_mut(&mut self) -> &mut u64 {
+        &mut *(self.storage.as_mut().as_mut_ptr() as *mut u8 as *mut u64)
+    }
+}
+
+/// An index into the shared signature registry, usable for checking signatures
+/// at indirect calls.
+#[repr(C)]
+#[derive(Debug, Eq, PartialEq, Clone, Copy)]
+pub struct VMSharedSignatureIndex(u32);
+
+#[cfg(test)]
+mod test_vmshared_signature_index {
+    use super::VMSharedSignatureIndex;
+    use std::mem::size_of;
+    use wasmtime_environ::VMOffsets;
+
+    #[test]
+    fn check_vmshared_signature_index() {
+        let offsets = VMOffsets::new(size_of::<*mut u8>() as u8);
+        assert_eq!(
+            size_of::<VMSharedSignatureIndex>(),
+            usize::from(offsets.size_of_vmshared_signature_index())
+        );
+    }
+}
+
+impl VMSharedSignatureIndex {
+    pub fn new(value: u32) -> Self {
+        VMSharedSignatureIndex(value)
+    }
+}
+
+/// The VM caller-checked "anyfunc" record, for caller-side signature checking.
+/// It consists of the actual function pointer and a signature id to be checked
+/// by the caller.
+#[derive(Debug, Clone)]
+#[repr(C)]
+pub struct VMCallerCheckedAnyfunc {
+    pub func_ptr: *const VMFunctionBody,
+    pub type_index: VMSharedSignatureIndex,
+    // If more elements are added here, remember to add offset_of tests below!
+}
+
+#[cfg(test)]
+mod test_vmcaller_checked_anyfunc {
+    use super::VMCallerCheckedAnyfunc;
+    use std::mem::size_of;
+    use wasmtime_environ::VMOffsets;
+
+    #[test]
+    fn check_vmcaller_checked_anyfunc_offsets() {
+        let offsets = VMOffsets::new(size_of::<*mut u8>() as u8);
+        assert_eq!(
+            size_of::<VMCallerCheckedAnyfunc>(),
+            usize::from(offsets.size_of_vmcaller_checked_anyfunc())
+        );
+        assert_eq!(
+            offset_of!(VMCallerCheckedAnyfunc, func_ptr),
+            usize::from(offsets.vmcaller_checked_anyfunc_func_ptr())
+        );
+        assert_eq!(
+            offset_of!(VMCallerCheckedAnyfunc, type_index),
+            usize::from(offsets.vmcaller_checked_anyfunc_type_index())
+        );
+    }
+}
+
+impl Default for VMCallerCheckedAnyfunc {
+    fn default() -> Self {
+        Self {
+            func_ptr: ptr::null_mut(),
+            type_index: VMSharedSignatureIndex::new(u32::MAX),
+        }
+    }
+}
+
+/// The VM "context", which is pointed to by the `vmctx` arg in Cranelift.
+/// This has pointers to the globals, memories, tables, and other runtime
+/// state associated with the current instance.
+///
+/// TODO: The number of memories, globals, tables, and signature IDs does
+/// not change dynamically, and pointer arrays are not indexed dynamically,
+/// so these fields could all be contiguously allocated.
+#[derive(Debug)]
+#[repr(C)]
+pub struct VMContext {
+    /// A pointer to an array of `*const VMFunctionBody` instances, indexed by `FuncIndex`.
+    imported_functions: *const *const VMFunctionBody,
+
+    /// A pointer to an array of `VMTableImport` instances, indexed by `TableIndex`.
+    imported_tables: *mut VMTableImport,
+
+    /// A pointer to an array of `VMMemoryImport` instances, indexed by `MemoryIndex`.
+    imported_memories: *mut VMMemoryImport,
+
+    /// A pointer to an array of `VMGlobalImport` instances, indexed by `GlobalIndex`.
+    imported_globals: *mut VMGlobalImport,
+
+    /// A pointer to an array of locally-defined `VMTableDefinition` instances,
+    /// indexed by `DefinedTableIndex`.
+    tables: *mut VMTableDefinition,
+
+    /// A pointer to an array of locally-defined `VMMemoryDefinition` instances,
+    /// indexed by `DefinedMemoryIndex`.
+    memories: *mut VMMemoryDefinition,
+
+    /// A pointer to an array of locally-defined `VMGlobalDefinition` instances,
+    /// indexed by `DefinedGlobalIndex`.
+    globals: *mut VMGlobalDefinition,
+
+    /// Signature identifiers for signature-checking indirect calls.
+    signature_ids: *mut VMSharedSignatureIndex,
+    // If more elements are added here, remember to add offset_of tests below!
+}
+
+#[cfg(test)]
+mod test {
+    use super::VMContext;
+    use std::mem::size_of;
+    use wasmtime_environ::VMOffsets;
+
+    #[test]
+    fn check_vmctx_offsets() {
+        let offsets = VMOffsets::new(size_of::<*mut u8>() as u8);
+        assert_eq!(size_of::<VMContext>(), usize::from(offsets.size_of_vmctx()));
+        assert_eq!(
+            offset_of!(VMContext, memories),
+            usize::from(offsets.vmctx_memories())
+        );
+        assert_eq!(
+            offset_of!(VMContext, globals),
+            usize::from(offsets.vmctx_globals())
+        );
+        assert_eq!(
+            offset_of!(VMContext, tables),
+            usize::from(offsets.vmctx_tables())
+        );
+        assert_eq!(
+            offset_of!(VMContext, signature_ids),
+            usize::from(offsets.vmctx_signature_ids())
+        );
+    }
+}
+
+impl VMContext {
+    /// Create a new `VMContext` instance.
+    pub fn new(
+        imported_functions: *const *const VMFunctionBody,
+        imported_tables: *mut VMTableImport,
+        imported_memories: *mut VMMemoryImport,
+        imported_globals: *mut VMGlobalImport,
+        tables: *mut VMTableDefinition,
+        memories: *mut VMMemoryDefinition,
+        globals: *mut VMGlobalDefinition,
+        signature_ids: *mut VMSharedSignatureIndex,
+    ) -> Self {
+        Self {
+            imported_functions,
+            imported_tables,
+            imported_memories,
+            imported_globals,
+            tables,
+            memories,
+            globals,
+            signature_ids,
+        }
+    }
+
+    /// Return a reference to imported function `index`.
+    pub unsafe fn imported_function(&self, index: FuncIndex) -> *const VMFunctionBody {
+        *self.imported_functions.add(index.index())
+    }
+
+    /// Return a reference to imported table `index`.
+    pub unsafe fn imported_table(&self, index: TableIndex) -> &VMTableImport {
+        &*self.imported_tables.add(index.index())
+    }
+
+    /// Return a mutable reference to imported table `index`.
+    pub unsafe fn imported_table_mut(&mut self, index: TableIndex) -> &mut VMTableImport {
+        &mut *self.imported_tables.add(index.index())
+    }
+
+    /// Return a reference to imported memory `index`.
+    pub unsafe fn imported_memory(&self, index: MemoryIndex) -> &VMMemoryImport {
+        &*self.imported_memories.add(index.index())
+    }
+
+    /// Return a mutable reference to imported memory `index`.
+    pub unsafe fn imported_memory_mut(&mut self, index: MemoryIndex) -> &mut VMMemoryImport {
+        &mut *self.imported_memories.add(index.index())
+    }
+
+    /// Return a reference to imported global `index`.
+    pub unsafe fn imported_global(&self, index: GlobalIndex) -> &VMGlobalImport {
+        &*self.imported_globals.add(index.index())
+    }
+
+    /// Return a mutable reference to imported global `index`.
+    pub unsafe fn imported_global_mut(&mut self, index: GlobalIndex) -> &mut VMGlobalImport {
+        &mut *self.imported_globals.add(index.index())
+    }
+
+    /// Return a reference to locally-defined table `index`.
+    pub unsafe fn table(&self, index: DefinedTableIndex) -> &VMTableDefinition {
+        &*self.tables.add(index.index())
+    }
+
+    /// Return a mutable reference to locally-defined table `index`.
+    pub unsafe fn table_mut(&mut self, index: DefinedTableIndex) -> &mut VMTableDefinition {
+        &mut *self.tables.add(index.index())
+    }
+
+    /// Return a reference to locally-defined linear memory `index`.
+    pub unsafe fn memory(&self, index: DefinedMemoryIndex) -> &VMMemoryDefinition {
+        &*self.memories.add(index.index())
+    }
+
+    /// Return a mutable reference to locally-defined linear memory `index`.
+    pub unsafe fn memory_mut(&mut self, index: DefinedMemoryIndex) -> &mut VMMemoryDefinition {
+        &mut *self.memories.add(index.index())
+    }
+
+    /// Return a reference to locally-defined global variable `index`.
+    pub unsafe fn global(&self, index: DefinedGlobalIndex) -> &VMGlobalDefinition {
+        &*self.globals.add(index.index())
+    }
+
+    /// Return a mutable reference to locally-defined global variable `index`.
+    pub unsafe fn global_mut(&mut self, index: DefinedGlobalIndex) -> &mut VMGlobalDefinition {
+        &mut *self.globals.add(index.index())
+    }
+
+    /// Return a mutable reference to the associated `Instance`.
+    #[allow(clippy::cast_ptr_alignment)]
+    pub unsafe fn instance(&mut self) -> &mut Instance {
+        &mut *((self as *mut Self as *mut u8).offset(-Instance::vmctx_offset()) as *mut Instance)
+    }
+
+    /// Return the memory index for the given `VMMemoryDefinition`.
+    pub fn memory_index(&self, memory: &mut VMMemoryDefinition) -> DefinedMemoryIndex {
+        // TODO: Use `offset_from` once it stablizes.
+        let begin = self.memories;
+        let end: *mut VMMemoryDefinition = memory;
+        DefinedMemoryIndex::new(
+            (end as usize - begin as usize) / mem::size_of::<VMMemoryDefinition>(),
+        )
+    }
+}