Shrink the size of the anyfunc table in VMContext (#3850)

* Shrink the size of the anyfunc table in `VMContext`

This commit shrinks the size of the `VMCallerCheckedAnyfunc` table
allocated into a `VMContext` to be the size of the number of "escaped"
functions in a module rather than the number of functions in a module.
Escaped functions include exports, table elements, etc, and are
typically an order of magnitude smaller than the number of functions in
general. This should greatly shrink the `VMContext` for some modules
which while we aren't necessarily having any problems with that today
shouldn't cause any problems in the future.

The original motivation for this was that this came up during the recent
lazy-table-initialization work and while it no longer has a direct
performance benefit since tables aren't initialized at all on
instantiation it should still improve long-running instances
theoretically with smaller `VMContext` allocations as well as better
locality between anyfuncs.

* Fix some tests

* Remove redundant hash set

* Use a helper for pushing function type information

* Use a more descriptive `is_escaping` method

* Clarify a comment

* Fix condition

crates/environ/src/vmoffsets.rs

@@ -16,13 +16,14 @@
 //      tables: [VMTableDefinition; module.num_defined_tables],
 //      memories: [VMMemoryDefinition; module.num_defined_memories],
 //      globals: [VMGlobalDefinition; module.num_defined_globals],
-//      anyfuncs: [VMCallerCheckedAnyfunc; module.num_imported_functions + module.num_defined_functions],
+//      anyfuncs: [VMCallerCheckedAnyfunc; module.num_escaped_funcs],
 // }
 
 use crate::{
-    DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, FuncIndex, GlobalIndex, MemoryIndex,
-    Module, TableIndex,
+    AnyfuncIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, FuncIndex,
+    GlobalIndex, MemoryIndex, Module, TableIndex,
 };
+use cranelift_entity::packed_option::ReservedValue;
 use more_asserts::assert_lt;
 use std::convert::TryFrom;
 
@@ -68,6 +69,9 @@ pub struct VMOffsets<P> {
     pub num_defined_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
+    /// array.
+    pub num_escaped_funcs: u32,
 
     // precalculated offsets of various member fields
     interrupts: u32,
@@ -131,6 +135,9 @@ pub struct VMOffsetsFields<P> {
     pub num_defined_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
+    /// array.
+    pub num_escaped_funcs: u32,
 }
 
 impl<P: PtrSize> VMOffsets<P> {
@@ -146,6 +153,7 @@ impl<P: PtrSize> VMOffsets<P> {
             num_defined_tables: cast_to_u32(module.table_plans.len()),
             num_defined_memories: cast_to_u32(module.memory_plans.len()),
             num_defined_globals: cast_to_u32(module.globals.len()),
+            num_escaped_funcs: cast_to_u32(module.num_escaped_funcs),
         })
     }
 
@@ -174,6 +182,7 @@ impl<P: PtrSize> VMOffsets<P> {
                     num_defined_globals: _,
                     num_defined_memories: _,
                     num_defined_functions: _,
+                    num_escaped_funcs: _,
 
                     // used as the initial size below
                     size,
@@ -229,6 +238,7 @@ impl<P: PtrSize> From<VMOffsetsFields<P>> for VMOffsets<P> {
             num_defined_tables: fields.num_defined_tables,
             num_defined_memories: fields.num_defined_memories,
             num_defined_globals: fields.num_defined_globals,
+            num_escaped_funcs: fields.num_escaped_funcs,
             interrupts: 0,
             epoch_ptr: 0,
             externref_activations_table: 0,
@@ -298,7 +308,7 @@ impl<P: PtrSize> From<VMOffsetsFields<P>> for VMOffsets<P> {
             size(defined_globals)
                 = cmul(ret.num_defined_globals, ret.size_of_vmglobal_definition()),
             size(defined_anyfuncs) = cmul(
-                cadd(ret.num_imported_functions, ret.num_defined_functions),
+                ret.num_escaped_funcs,
                 ret.size_of_vmcaller_checked_anyfunc(),
             ),
         }
@@ -674,11 +684,9 @@ impl<P: PtrSize> VMOffsets<P> {
     /// Return the offset to the `VMCallerCheckedAnyfunc` for the given function
     /// index (either imported or defined).
     #[inline]
-    pub fn vmctx_anyfunc(&self, index: FuncIndex) -> u32 {
-        assert_lt!(
-            index.as_u32(),
-            self.num_imported_functions + self.num_defined_functions
-        );
+    pub fn vmctx_anyfunc(&self, index: AnyfuncIndex) -> u32 {
+        assert!(!index.is_reserved_value());
+        assert_lt!(index.as_u32(), self.num_escaped_funcs);
         self.vmctx_anyfuncs_begin()
             + index.as_u32() * u32::from(self.size_of_vmcaller_checked_anyfunc())
     }