runtime (gc): seperate AVR GC

Author: niaow

src/runtime/gc_avr.go

@@ -0,0 +1,225 @@
+// +build gc.conservative
+// +build avr
+
+package runtime
+
+// This memory manager is partially inspired by the memory allocator included in avr-libc.
+// Unlike avr-libc malloc, this memory manager stores an allocation list instead of a free list.
+// This gives an overhead of 4 bytes/allocation (up from avr-libc's 2 bytes/allocation).
+// The allocation list is stored in ascending address order.
+// The set of free spans is implicitly derived from the gaps between allocations.
+// This allocator has an effective allocation complexity of O(n) and worst-case GC complexity of O(n^2).
+// Due to architectural quirks of AVR, as well as tiny heaps, this should almost always be faster than the standard conservative collector.
+
+import "unsafe"
+
+// isInHeap checks if an address is inside the heap.
+func isInHeap(addr uintptr) bool {
+	return addr >= heapStart && addr < heapEnd
+}
+
+// allocNode is a node in the allocations list.
+// It is prepended to every allocation, resulting in an overhead of 4 bytes per allocation.
+type allocNode struct {
+	// next is a pointer to the next allocation node.
+	next *allocNode
+
+	// len is the length of the body of this node in bytes.
+	len uintptr
+
+	// base is the start of the body of this node.
+	base struct{}
+}
+
+// allocList is a singly linked list of allocations.
+// It is stored in ascending address order.
+var allocList *allocNode
+
+// scanList is a stack of allocations to scan.
+var scanList *allocNode
+
+func markRoot(addr, root uintptr) {
+	markAddr(root)
+}
+
+func markRoots(start, end uintptr) {
+	markMem(start, end)
+}
+
+// markAddr marks the allocation containing the specified address.
+func markAddr(addr uintptr) {
+	if !isInHeap(addr) {
+		// The address is not in the heap.
+		return
+	}
+
+	// Search the allocation list for the address.
+	for node, prev := allocList, &allocList; node != nil; node, prev = node.next, &node.next {
+		baseAddr := uintptr(unsafe.Pointer(&node.base))
+		if addr < baseAddr {
+			// The address comes before this node.
+			// Therefore the address must either be that of a node header or a free span.
+			return
+		}
+
+		endAddr := baseAddr + node.len
+		if addr < endAddr {
+			// The address is included in this allocation.
+			// Move the allocation to the scan stack.
+			*prev = node.next
+			scanList, node.next = node, scanList
+			return
+		}
+	}
+}
+
+// markMem scans a memory region for pointers and marks anything that is pointed to.
+func markMem(start, end uintptr) {
+	if start >= end {
+		return
+	}
+	prevByte := *(*byte)(unsafe.Pointer(start))
+	for ; start != end; start++ {
+		b := *(*byte)(unsafe.Pointer(start))
+		addr := (uintptr(b) << 8) | uintptr(prevByte)
+		markAddr(addr)
+		prevByte = b
+	}
+}
+
+// GC runs a garbage collection cycle.
+func GC() {
+	// Mark phase: mark all reachable objects, recursively.
+	markGlobals()
+	markStack()
+	var keep *allocNode
+	for scanList != nil {
+		// Pop a node off of the scan list.
+		node := scanList
+		scanList = node.next
+
+		// Scan the node.
+		baseAddr := uintptr(unsafe.Pointer(&node.base))
+		endAddr := baseAddr + node.len
+		markMem(baseAddr, endAddr)
+
+		// Insert the node into the output heap.
+		var prev *allocNode
+		keepNode := keep
+		for keepNode != nil && uintptr(unsafe.Pointer(node)) > uintptr(unsafe.Pointer(keepNode)) {
+			// Move onto the next node.
+			prev, keepNode = keepNode, keepNode.next
+		}
+		if prev == nil {
+			keep, node.next = node, keep
+		} else {
+			prev.next, node.next = node, keepNode
+		}
+	}
+
+	// Sweep phase: replace the heap.
+	allocList = keep
+}
+
+// findMem searches the heap for a free span large enough to contain an allocation of the specified size.
+// If there are no sufficiently large free spans available, this returns nil.
+func findMem(size uintptr) *allocNode {
+	// This memory allocator implementation is effectively the same algorithm applied by avr-libc.
+	// It loops through the set of all free spans, and selects the smallest span that is large enough to fit the allocation.
+
+	nodeSize := unsafe.Sizeof(allocNode{}) + size
+
+	// best* store the best-fit free span.
+	var bestDst **allocNode
+	var bestStart uintptr
+	var bestSize uintptr
+
+	start := heapStart
+	dst := &allocList
+searchLoop:
+	for {
+		// Find the allocation node after this free span.
+		node := *dst
+
+		// Find the end of this free span.
+		var end uintptr
+		if node != nil {
+			// The node terminates the free span.
+			end = uintptr(unsafe.Pointer(node))
+		} else {
+			// The free span ends at the end of the heap.
+			end = heapEnd
+		}
+
+		// Calculate the size of the free span.
+		freeSpanSize := end - start
+
+		switch {
+		case freeSpanSize == nodeSize:
+			// This span is a perfect fit.
+			bestDst = dst
+			bestStart = start
+			break searchLoop
+		case freeSpanSize > nodeSize && (bestDst == nil || bestSize > nodeSize):
+			// This span is a better fit than the previous best.
+			bestDst = dst
+			bestStart = start
+			bestSize = freeSpanSize
+		}
+
+		// Move to the next free span.
+		if node == nil {
+			// That was the last free region.
+			break searchLoop
+		}
+		start = uintptr(unsafe.Pointer(&node.base)) + node.len
+		dst = &node.next
+	}
+
+	if bestDst == nil {
+		// There is no suitable allocation.
+		return nil
+	}
+
+	mem := (*allocNode)(unsafe.Pointer(bestStart))
+	*bestDst, mem.next = mem, *bestDst
+	mem.len = size
+	return mem
+}
+
+// alloc tries to find some free space on the heap, possibly doing a garbage
+// collection cycle if needed. If no space is free, it panics.
+//go:noinline
+func alloc(size uintptr) unsafe.Pointer {
+	var ranGC bool
+tryAlloc:
+	// Search for available memory.
+	node := findMem(size)
+	if node == nil {
+		// There is no free span large enough for the allocation.
+
+		if ranGC {
+			// Even after running the GC, there is not enough memory.
+			runtimePanic("out of memory")
+		}
+
+		// Run the garbage collector and try again.
+		GC()
+		ranGC = true
+		goto tryAlloc
+	}
+
+	// Zero the allocation and return it.
+	ptr := unsafe.Pointer(&node.base)
+	memzero(ptr, size)
+	return ptr
+}
+
+func free(ptr unsafe.Pointer) {
+	// TODO: free memory on request, when the compiler knows it is unused.
+}
+
+func initHeap() {
+	// This memory manager requires no initialization other than the zeroing of globals.
+	// This function is provided for compatability with other memory managers.
+}

src/runtime/gc_conservative.go

@@ -1,4 +1,5 @@
 // +build gc.conservative
+// +build !avr
 
 package runtime