runtime: implement a simple mark/sweep garbage collector

testdata/gc.go

@@ -0,0 +1,59 @@
+package main
+
+var xorshift32State uint32 = 1
+
+func xorshift32(x uint32) uint32 {
+	// Algorithm "xor" from p. 4 of Marsaglia, "Xorshift RNGs"
+	x ^= x << 13
+	x ^= x >> 17
+	x ^= x << 5
+	return x
+}
+
+func randuint32() uint32 {
+	xorshift32State = xorshift32(xorshift32State)
+	return xorshift32State
+}
+
+func main() {
+	testNonPointerHeap()
+}
+
+var scalarSlices [4][]byte
+var randSeeds [4]uint32
+
+func testNonPointerHeap() {
+	// Allocate roughly 0.5MB of memory.
+	for i := 0; i < 1000; i++ {
+		// Pick a random index that the optimizer can't predict.
+		index := randuint32() % 4
+
+		// Check whether the contents of the previous allocation was correct.
+		rand := randSeeds[index]
+		for _, b := range scalarSlices[index] {
+			rand = xorshift32(rand)
+			if b != byte(rand) {
+				panic("memory was overwritten!")
+			}
+		}
+
+		// Allocate a randomly-sized slice, randomly sliced to be smaller.
+		sliceLen := randuint32() % 1024
+		slice := make([]byte, sliceLen)
+		cutLen := randuint32() % 1024
+		if cutLen < sliceLen {
+			slice = slice[cutLen:]
+		}
+		scalarSlices[index] = slice
+
+		// Fill the slice with a pattern that looks random but is easily
+		// calculated and verified.
+		rand = randuint32() + 1
+		randSeeds[index] = rand
+		for i := 0; i < len(slice); i++ {
+			rand = xorshift32(rand)
+			slice[i] = byte(rand)
+		}
+	}
+	println("ok")
+}

testdata/gc.txt

@@ -0,0 +1 @@
+ok