Segmented stack space

Author: chfast

lib/fizzy/execute.hpp

@@ -63,7 +63,14 @@ class ExecutionContext
         ~Guard() noexcept
         {
             --m_execution_context.depth;
-            m_execution_context.stack_space_ptr = stack_space;
+
+            m_execution_context.free_stack_space = prev_free_stack_space;
+            if (prev_stack_space_segment != nullptr)
+            {
+                assert(m_execution_context.stack_space_segment == stack_space);
+                delete[] stack_space;
+                m_execution_context.stack_space_segment = prev_stack_space_segment;
+            }
         }
     };
 
@@ -78,24 +85,26 @@ class ExecutionContext
     /// the call depth back to the original value when going out of scope.
     Guard allocate_stack_space(size_t required_stack_space = 0) noexcept
     {
+        ++depth;
         Guard g{*this};
 
+        g.prev_free_stack_space = free_stack_space;
+
         if (required_stack_space <= free_stack_space)
         {
-            const auto offset =
-            g.stack_space =
+            // Must be a segment of default size or required_stack_space is 0.
+            const auto offset = DefaultStackSpaceSegmentSize - free_stack_space;
+            g.stack_space = stack_space_segment + offset;
+            g.prev_free_stack_space = free_stack_space;
+            free_stack_space -= required_stack_space;
+            return g;
         }
 
-        const auto stack_space_size = std::size(stack_space);
-        const auto used_space = static_cast<size_t>(stack_space_ptr - stack_space);
-        [[maybe_unused]] const auto free_space = stack_space_size - used_space;
-
-        assert(required_stack_space <= free_space);
-
-        ++depth;
-
-        g.stack_space = stack_space_ptr;
-        stack_space_ptr += required_stack_space;
+        g.prev_stack_space_segment = stack_space_segment;
+        const auto new_segment_size = std::max(DefaultStackSpaceSegmentSize, required_stack_space);
+        stack_space_segment = new Value[new_segment_size];
+        g.stack_space = stack_space_segment;
+        free_stack_space = new_segment_size - required_stack_space;
         return g;
     }
 };

test/unittests/execute_call_depth_test.cpp

@@ -213,7 +213,7 @@ TEST(execute_call_depth, call_host_function_calling_wasm_function_inclusive)
     constexpr auto host_f = [](std::any&, Instance& instance, const Value*,
                                 ExecutionContext& ctx) noexcept {
         recorded_depth = ctx.depth;
-        const auto ctx_guard = ctx.increment_call_depth();
+        const auto ctx_guard = ctx.allocate_stack_space();
         return fizzy::execute(instance, 2 /* $leaf */, {}, ctx);
     };
 
@@ -304,7 +304,7 @@ TEST(execute_call_depth, call_host_function_calling_another_wasm_module)
                                 ExecutionContext& ctx) noexcept {
         recorded_depth = ctx.depth;
         auto instance = *std::any_cast<Instance*>(&host_context);
-        const auto ctx_guard = ctx.increment_call_depth();
+        const auto ctx_guard = ctx.allocate_stack_space();
         return fizzy::execute(*instance, 0, {}, ctx);
     };
 
@@ -466,7 +466,7 @@ TEST(execute_call_depth, execute_host_function_within_wasm_recursion_limit)
     constexpr auto host_f = [](std::any&, Instance& instance, const Value*,
                                 ExecutionContext& ctx) noexcept {
         max_recorded_wasm_recursion_depth = std::max(max_recorded_wasm_recursion_depth, ctx.depth);
-        const auto ctx_guard = ctx.increment_call_depth();
+        const auto ctx_guard = ctx.allocate_stack_space();
         return fizzy::execute(instance, 0, {}, ctx);
     };
 
@@ -537,7 +537,7 @@ TEST(execute_call, call_host_function_calling_wasm_interleaved_infinite_recursio
                                 ExecutionContext& ctx) noexcept {
         EXPECT_LT(ctx.depth, DepthLimit);
         ++counter;
-        const auto ctx_guard = ctx.increment_call_depth();
+        const auto ctx_guard = ctx.allocate_stack_space();
         return fizzy::execute(instance, 1, {}, ctx);
     };