cmd/viewcore: handle 1.14 free spans

Free spans are now implicit. Derive the free span information
by finding all the memory in the inuse address ranges that isn't
attributed to any span.

Fixes viewcore for all versions of 1.14.
Also works for tip.

Fixes golang/go#38638

Change-Id: Ied283f59391aad8ed5b1b657d2fd6e89e9e9ae6c

internal/gocore/gocore_test.go

@@ -270,4 +270,5 @@ func TestVersions(t *testing.T) {
 	loadExampleVersion(t, "1.12.zip")
 	loadExampleVersion(t, "1.13.zip")
 	loadExampleVersion(t, "1.13.3.zip")
+	loadExampleVersion(t, "1.14.zip")
 }

internal/gocore/process.go

@@ -306,7 +306,7 @@ func (p *Process) readSpans(mheap region, arenas []arena) {
 			panic("weird mapping " + m.Perm().String())
 		}
 	}
-	if mheap.HasField("curArena") {
+	if mheap.HasField("curArena") { // go1.13.3 <= x < go1.14
 		// Subtract from the heap unallocated space
 		// in the current arena.
 		ca := mheap.Field("curArena")
@@ -431,6 +431,52 @@ func (p *Process) readSpans(mheap region, arenas []arena) {
 			}
 		}
 	}
+	if mheap.HasField("pages") { // go1.14+
+		// There are no longer "free" mspans to represent unused pages.
+		// Instead, there are just holes in the pagemap into which we can allocate.
+		// Look through the in-use ranges, and figure out how much is before and
+		// after the scavenge address.
+		pages := mheap.Field("pages")
+		// Regions below scavAddr are unscavenged.
+		// Regions above scavAddr are scavenged.
+		scavAddr := core.Address(pages.Field("scavAddr").Uintptr())
+		inuse := pages.Field("inUse")
+		ranges := inuse.Field("ranges")
+		scavenged := int64(0)
+		unscavenged := int64(0)
+		for i := int64(0); i < ranges.SliceLen(); i++ {
+			r := ranges.SliceIndex(i)
+			base := core.Address(r.Field("base").Uintptr())
+			limit := core.Address(r.Field("limit").Uintptr())
+			if limit <= scavAddr {
+				unscavenged += int64(limit - base)
+			} else if scavAddr <= base {
+				scavenged += int64(limit - base)
+			} else {
+				unscavenged += int64(scavAddr - base)
+				scavenged += int64(limit - scavAddr)
+			}
+		}
+		// Go through all the spans, subtracting the memory they use.
+		// Whatever remains are free spans.
+		for i := int64(0); i < n; i++ {
+			s := allspans.SliceIndex(i).Deref()
+			min := core.Address(s.Field("startAddr").Uintptr())
+			nPages := int64(s.Field("npages").Uintptr())
+			spanSize := nPages * pageSize
+			max := min.Add(spanSize)
+			if max <= scavAddr {
+				unscavenged -= int64(max - min)
+			} else if scavAddr <= min {
+				scavenged -= int64(max - min)
+			} else {
+				unscavenged -= int64(scavAddr - min)
+				scavenged -= int64(max - scavAddr)
+			}
+		}
+		freeSpanSize = scavenged + unscavenged
+		releasedSpanSize = scavenged
+	}
 
 	p.stats = &Stats{"all", all, []*Stats{
 		&Stats{"text", text, nil},