slides for GC in spasm

1 files changed, 150 insertions, 0 deletions

diff --git a/website/content/slides/10_vm_gc/_index.md b/website/content/slides/10_vm_gc/_index.md
new file mode 100644
index 0000000..09453e9
--- /dev/null
+++ b/website/content/slides/10_vm_gc/_index.md
@@ -0,0 +1,150 @@
+---
+title: "Garbage collection in the VM"
+date: 2018-12-05T17:45:53+03:00
+draft: false
+outputs: ["Reveal"]
+---
+# Garbage Collection in the VM
+
+---
+## Contents
+
+1. Allocation
+2. Mark and sweep
+3. Embedability
+4. Incremental 
+5. Optimizations
+
+---
+## Disclaimer
+
+> This is how GC can be implemented. Everything is subject to change.
+
+---
+## Allocation
+
+Having a custom allocator is a *must* have for most VM
+
+- decent performance
+- optimizations
+
+---
+### Allocators and C++
+
+[Allocators in
+C++](https://sofiacpp.github.io/advanced-cpp/slides/17_allocators.html)
+
+---
+### Simplicity matters
+
+For simplicity, we are going to not use a special allocator.
+
+---
+## Mark and sweep
+
+
+- mark reacheable objects starting from variables (roots)
+- sweep
+    - unmarked objects are returned to the free heap
+    - marked objects are unmarked (for the next cycle)
+
+---
+### Mark
+
+- every object starts as dead
+- reacheable objects are marked as alive
+
+---
+#### Reachable objects
+
+
+---
+#### Roots
+
+- registers
+- stack
+- global object / environment
+
+Where are these stored?
+
+---
+#### Roots
+
+- The `data_stack`
+    - everything upto `m_SP`
+- The global object
+
+---
+#### Marking
+
+    void Spasm::Mark() {
+        for (auto cell = &data_stack[0]; cell != m_SP; ++cell) {
+            Mark(cell); // ?
+        }
+        Mark(m_Global);
+    }
+
+---
+#### Visit every object in the heap?
+
+- Visitor pattern again
+
+---
+
+    struct MarkVisitor
+    {
+        typedef void ResultType;
+
+        void Visit(double d) const {}
+        void VisitUndefined() const {}
+        void VisitNull() const {}
+        void Visit(StringValue& value) const;
+        void Visit(ArrayValue& value) const;
+        void Visit(ObjectValue& value) const;
+        void Visit(FunctionValue& value) const;
+    };
+
+---
+
+    void Visit(StringValue& value) const {
+        if (Dead(value)) {
+            SetAlive(value);
+        }
+    }
+
+---
+
+    void Visit(ArrayValue& value) const {
+        if (Dead(value)) {
+            SetAlive(value);
+            for (auto i = 0u; i < value.length(); ++i) {
+                value.item(i).Visit(*this);
+            }
+        }
+    }
+
+---
+
+    void Visit(ObjectValue& value) const {
+        if (Dead(value)) {
+            SetAlive(value);
+            for (auto* p : value.m_Properties) {
+                p->first.Visit(*this);
+                p->second.Visit(*this);
+            }
+            value.m_Prototype->Visit(*this);
+        }
+    }
+
+---
+#### State
+
+Where to store whether an object is *dead* or *alive*
+
+- `bool` per object, string, array, function
+- `bitmap` that holds one bit for a range of objects
+- `bit` per pointer
+
+
+---
+# ?