Add ExecutionContext documentation

Author: chfast

lib/fizzy/execution_context.hpp

@@ -5,13 +5,32 @@
 #pragma once
 
 #include "value.hpp"
+#include <algorithm>
 #include <cassert>
 #include <cstddef>
 
 namespace fizzy
 {
 /// The storage for information shared by calls in the same execution "thread".
 /// Users may decide how to allocate the execution context, but some good defaults are available.
+///
+/// The ExecutionContext manages WebAssembly stack space shared between calls in the same execution
+/// thread. The shared stack space is allocated and managed by create_local_context() and
+/// LocalContext objects.
+///
+/// The shared stack space is conceptually implemented as linked list of stack space segments.
+/// If the required stack space for a new call fits in the current segment no new
+/// allocation is needed. Otherwise new segment is allocated. The size of the new segment is
+/// at least DefaultStackSpaceSegmentSize but can be larger if the call's requires stack space
+/// exceeds the default size (in this case the call occupies the segment exclusively).
+///
+/// When the LocalContext which allocated new stack segment is being destroyed (i.e. when the first
+/// call occupying this stack segment ends) this segment is freed. This may not be the optimal
+/// strategy in case the same segment is going to be allocated multiple times.
+/// There is alternative design when segments are not freed when not used any more and can be reused
+/// when more stack space is needed. However, this requires additional housekeeping (e.g. having
+/// forward pointer to the next segment) and handling some additional edge-cases (e.g. reallocate
+/// an unused segment in case it is smaller then the required stack space).
 class ExecutionContext
 {
     static constexpr size_t DefaultStackSpaceSegmentSize = 100;
@@ -21,11 +40,20 @@ class ExecutionContext
     /// when going out of scope.
     class [[nodiscard]] LocalContext
     {
-        ExecutionContext& m_shared_ctx;  ///< Reference to the shared execution context.
+        /// Reference to the shared execution context.
+        ExecutionContext& m_shared_ctx;
 
     public:
+        /// Pointer to the reserved "required" stack space.
         Value* stack_space = nullptr;
+
+        /// Pointer to the previous segment.
+        /// This is not null only for LocalContexts which allocated new segment.
         Value* prev_stack_space_segment = nullptr;
+
+        /// Amount of free stack space before this LocalContext has been created.
+        /// This is used to restore "free" space information in ExecutionContext (m_shared_ctx)
+        /// when this LocalContext object is destroyed.
         size_t prev_free_stack_space = 0;
 
         LocalContext(const LocalContext&) = delete;
@@ -73,14 +101,25 @@ class ExecutionContext
     };
 
 public:
+    /// Pre-allocated first segment of the shared stack space.
     Value first_stack_space_segment[DefaultStackSpaceSegmentSize];
+
+    /// Point to the current stack space segment.
     Value* stack_space_segment = first_stack_space_segment;
+
+    /// Amount of free stack space remaining in the current segment.
+    /// It is better to keep information about "free" than "used" space
+    /// because then we don't need to know the current segment size.
     size_t free_stack_space = DefaultStackSpaceSegmentSize;
 
-    int depth = 0;  ///< Current call depth.
+    /// Current call depth.
+    int depth = 0;
 
     /// Increments the call depth and returns the local call context which
     /// decrements the call depth back to the original value when going out of scope.
+    /// This also allocates and manages the shared stack space.
+    /// @param  required_stack_space    Size of the required stack space in bytes.
+    /// @see ExecutionContext
     LocalContext create_local_context(size_t required_stack_space = 0)
     {
         return LocalContext{*this, required_stack_space};