Further minor optimizations to instantiation (#3791)

* Shrink the size of `FuncData`

Before this commit on a 64-bit system the `FuncData` type had a size of
88 bytes and after this commit it has a size of 32 bytes. A `FuncData`
is required for all host functions in a store, including those inserted
from a `Linker` into a store used during linking. This means that
instantiation ends up creating a nontrivial number of these types and
pushing them into the store. Looking at some profiles there were some
surprisingly expensive movements of `FuncData` from the stack to a
vector for moves-by-value generated by Rust. Shrinking this type enables
more efficient code to be generated and additionally means less storage
is needed in a store's function array.

For instantiating the spidermonkey and rustpython modules this improves
instantiation by 10% since they each import a fair number of host
functions and the speedup here is relative to the number of items
imported.

* Use `ptr::copy_nonoverlapping` during initialization

Prevoiusly `ptr::copy` was used for copying imports into place which
translates to `memmove`, but `ptr::copy_nonoverlapping` can be used here
since it's statically known these areas don't overlap. While this
doesn't end up having a performance difference it's something I kept
noticing while looking at the disassembly of `initialize_vmcontext` so I
figured I'd go ahead and implement.

* Indirect shared signature ids in the VMContext

This commit is a small improvement for the instantiation time of modules
by avoiding copying a list of `VMSharedSignatureIndex` entries into each
`VMContext`, instead building one inside of a module and sharing that
amongst all instances. This involves less lookups at instantiation time
and less movement of data during instantiation. The downside is that
type-checks on `call_indirect` now involve an additionally load, but I'm
assuming that these are somewhat pessimized enough as-is that the
runtime impact won't be much there.

For instantiation performance this is a 5-10% win with
rustpyhon/spidermonky instantiation. This should also reduce the size of
each `VMContext` for an instantiation since signatures are no longer
stored inline but shared amongst all instances with one module.

Note that one subtle change here is that the array of
`VMSharedSignatureIndex` was previously indexed by `TypeIndex`, and now
it's indexed by `SignaturedIndex` which is a deduplicated form of
`TypeIndex`. This is done because we already had a list of those lying
around in `Module`, so it was easier to reuse that than to build a
separate array and store it somewhere.

* Reserve space in `Store<T>` with `InstancePre`

This commit updates the instantiation process to reserve space in a
`Store<T>` for the functions that an `InstancePre<T>`, as part of
instantiation, will insert into it. Using an `InstancePre<T>` to
instantiate allows pre-computing the number of host functions that will
be inserted into a store, and by pre-reserving space we can avoid costly
reallocations during instantiation by ensuring the function vector has
enough space to fit everything during the instantiation process.

Overall this makes instantiation of rustpython/spidermonkey about 8%
faster locally.

* Fix tests

* Use checked arithmetic

crates/environ/src/vmoffsets.rs

@@ -7,7 +7,7 @@
 //      interrupts: *const VMInterrupts,
 //      externref_activations_table: *mut VMExternRefActivationsTable,
 //      store: *mut dyn Store,
-//      signature_ids: [VMSharedSignatureIndex; module.num_signature_ids],
+//      signature_ids: *const VMSharedSignatureIndex,
 //      imported_functions: [VMFunctionImport; module.num_imported_functions],
 //      imported_tables: [VMTableImport; module.num_imported_tables],
 //      imported_memories: [VMMemoryImport; module.num_imported_memories],
@@ -21,7 +21,7 @@
 
 use crate::{
     DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, FuncIndex, GlobalIndex, MemoryIndex,
-    Module, TableIndex, TypeIndex,
+    Module, TableIndex,
 };
 use more_asserts::assert_lt;
 use std::convert::TryFrom;
@@ -52,8 +52,6 @@ fn align(offset: u32, width: u32) -> u32 {
 pub struct VMOffsets<P> {
     /// The size in bytes of a pointer on the target.
     pub ptr: P,
-    /// The number of signature declarations in the module.
-    pub num_signature_ids: u32,
     /// The number of imported functions in the module.
     pub num_imported_functions: u32,
     /// The number of imported tables in the module.
@@ -117,8 +115,6 @@ impl PtrSize for u8 {
 pub struct VMOffsetsFields<P> {
     /// The size in bytes of a pointer on the target.
     pub ptr: P,
-    /// The number of signature declarations in the module.
-    pub num_signature_ids: u32,
     /// The number of imported functions in the module.
     pub num_imported_functions: u32,
     /// The number of imported tables in the module.
@@ -142,7 +138,6 @@ impl<P: PtrSize> VMOffsets<P> {
     pub fn new(ptr: P, module: &Module) -> Self {
         VMOffsets::from(VMOffsetsFields {
             ptr,
-            num_signature_ids: cast_to_u32(module.types.len()),
             num_imported_functions: cast_to_u32(module.num_imported_funcs),
             num_imported_tables: cast_to_u32(module.num_imported_tables),
             num_imported_memories: cast_to_u32(module.num_imported_memories),
@@ -165,7 +160,6 @@ impl<P: PtrSize> From<VMOffsetsFields<P>> for VMOffsets<P> {
     fn from(fields: VMOffsetsFields<P>) -> VMOffsets<P> {
         let mut ret = Self {
             ptr: fields.ptr,
-            num_signature_ids: fields.num_signature_ids,
             num_imported_functions: fields.num_imported_functions,
             num_imported_tables: fields.num_imported_tables,
             num_imported_memories: fields.num_imported_memories,
@@ -210,12 +204,7 @@ impl<P: PtrSize> From<VMOffsetsFields<P>> for VMOffsets<P> {
             .unwrap();
         ret.imported_functions = ret
             .signature_ids
-            .checked_add(
-                fields
-                    .num_signature_ids
-                    .checked_mul(u32::from(ret.size_of_vmshared_signature_index()))
-                    .unwrap(),
-            )
+            .checked_add(u32::from(ret.ptr.size()))
             .unwrap();
         ret.imported_tables = ret
             .imported_functions
@@ -535,9 +524,9 @@ impl<P: PtrSize> VMOffsets<P> {
         self.store
     }
 
-    /// The offset of the `signature_ids` array.
+    /// The offset of the `signature_ids` array pointer.
     #[inline]
-    pub fn vmctx_signature_ids_begin(&self) -> u32 {
+    pub fn vmctx_signature_ids_array(&self) -> u32 {
         self.signature_ids
     }
 
@@ -603,14 +592,6 @@ impl<P: PtrSize> VMOffsets<P> {
         self.size
     }
 
-    /// Return the offset to `VMSharedSignatureId` index `index`.
-    #[inline]
-    pub fn vmctx_vmshared_signature_id(&self, index: TypeIndex) -> u32 {
-        assert_lt!(index.as_u32(), self.num_signature_ids);
-        self.vmctx_signature_ids_begin()
-            + index.as_u32() * u32::from(self.size_of_vmshared_signature_index())
-    }
-
     /// Return the offset to `VMFunctionImport` index `index`.
     #[inline]
     pub fn vmctx_vmfunction_import(&self, index: FuncIndex) -> u32 {