runtime: add pallocbits and tests

This change adds a per-chunk bitmap for page allocation called
pallocBits with algorithms for allocating and freeing pages out of the
bitmap. This change also adds tests for pallocBits, but does not yet
integrate it into the runtime.

Updates #35112.

Change-Id: I479006ed9f1609c80eedfff0580d5426b064b0ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/190620
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/export_test.go

@@ -730,3 +730,66 @@ func RunGetgThreadSwitchTest() {
 		panic("g1 != g3")
 	}
 }
+
+const (
+	PallocChunkPages = pallocChunkPages
+)
+
+// Expose pallocBits for testing.
+type PallocBits pallocBits
+
+func (b *PallocBits) Find(npages uintptr, searchIdx uint) (uint, uint) {
+	return (*pallocBits)(b).find(npages, searchIdx)
+}
+func (b *PallocBits) AllocRange(i, n uint) { (*pallocBits)(b).allocRange(i, n) }
+func (b *PallocBits) Free(i, n uint)       { (*pallocBits)(b).free(i, n) }
+
+// Expose non-trivial helpers for testing.
+func FindBitRange64(c uint64, n uint) uint { return findBitRange64(c, n) }
+
+// Given two PallocBits, returns a set of bit ranges where
+// they differ.
+func DiffPallocBits(a, b *PallocBits) []BitRange {
+	ba := (*pageBits)(a)
+	bb := (*pageBits)(b)
+
+	var d []BitRange
+	base, size := uint(0), uint(0)
+	for i := uint(0); i < uint(len(ba))*64; i++ {
+		if ba.get(i) != bb.get(i) {
+			if size == 0 {
+				base = i
+			}
+			size++
+		} else {
+			if size != 0 {
+				d = append(d, BitRange{base, size})
+			}
+			size = 0
+		}
+	}
+	if size != 0 {
+		d = append(d, BitRange{base, size})
+	}
+	return d
+}
+
+// StringifyPallocBits gets the bits in the bit range r from b,
+// and returns a string containing the bits as ASCII 0 and 1
+// characters.
+func StringifyPallocBits(b *PallocBits, r BitRange) string {
+	str := ""
+	for j := r.I; j < r.I+r.N; j++ {
+		if (*pageBits)(b).get(j) != 0 {
+			str += "1"
+		} else {
+			str += "0"
+		}
+	}
+	return str
+}
+
+// BitRange represents a range over a bitmap.
+type BitRange struct {
+	I, N uint // bit index and length in bits
+}