cmd/compile: don't compact liveness maps in place

Currently Liveness.compact rewrites the Liveness.livevars slice in
place. However, we're about to add register maps, which we'll want to
track in livevars, but compact independently from the stack maps.
Hence, this CL modifies Liveness.compact to consume Liveness.livevars
and produce a new slice of deduplicated stack maps. This is somewhat
clearer anyway because it avoids potential confusion over how
Liveness.livevars is indexed.

Passes toolstash -cmp.

For #24543.

Change-Id: I7093fbc71143f8a29e677aa30c96e501f953ca2b
Reviewed-on: https://go-review.googlesource.com/108498
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/gc/plive.go

@@ -122,13 +122,15 @@ type Liveness struct {
 
 	be []BlockEffects
 
-	// stackMapIndex maps from safe points (i.e., CALLs) to their
-	// index within the stack maps.
-	stackMapIndex map[*ssa.Value]int
-
 	// An array with a bit vector for each safe point tracking live variables.
+	// Indexed sequentially by safe points in Block and Value order.
 	livevars []bvec
 
+	// stackMapIndex maps from safe points (i.e., CALLs) to their
+	// index within stackMaps.
+	stackMapIndex map[*ssa.Value]int
+	stackMaps     []bvec
+
 	cache progeffectscache
 }
 
@@ -766,7 +768,7 @@ func (lv *Liveness) clobber() {
 	for _, n := range lv.vars {
 		varSize += n.Type.Size()
 	}
-	if len(lv.livevars) > 1000 || varSize > 10000 {
+	if len(lv.stackMaps) > 1000 || varSize > 10000 {
 		// Be careful to avoid doing too much work.
 		// Bail if >1000 safepoints or >10000 bytes of variables.
 		// Otherwise, giant functions make this experiment generate too much code.
@@ -810,7 +812,7 @@ func (lv *Liveness) clobber() {
 				b.Values = append(b.Values, oldSched[0])
 				oldSched = oldSched[1:]
 			}
-			clobber(lv, b, lv.livevars[0])
+			clobber(lv, b, lv.stackMaps[0])
 		}
 
 		// Copy values into schedule, adding clobbering around safepoints.
@@ -831,10 +833,10 @@ func (lv *Liveness) clobber() {
 				before = false
 			}
 			if before {
-				clobber(lv, b, lv.livevars[lv.stackMapIndex[v]])
+				clobber(lv, b, lv.stackMaps[lv.stackMapIndex[v]])
 			}
 			b.Values = append(b.Values, v)
-			clobber(lv, b, lv.livevars[lv.stackMapIndex[v]])
+			clobber(lv, b, lv.stackMaps[lv.stackMapIndex[v]])
 		}
 	}
 }
@@ -980,7 +982,6 @@ func (lv *Liveness) compact() {
 	for i := range remap {
 		remap[i] = -1
 	}
-	uniq := 0 // unique tables found so far
 
 	// Consider bit vectors in turn.
 	// If new, assign next number using uniq,
@@ -996,7 +997,7 @@ Outer:
 			if j < 0 {
 				break
 			}
-			jlive := lv.livevars[j]
+			jlive := lv.stackMaps[j]
 			if live.Eq(jlive) {
 				remap[i] = j
 				continue Outer
@@ -1008,29 +1009,24 @@ Outer:
 			}
 		}
 
-		table[h] = uniq
-		remap[i] = uniq
-		lv.livevars[uniq] = live
-		uniq++
+		table[h] = len(lv.stackMaps)
+		remap[i] = len(lv.stackMaps)
+		lv.stackMaps = append(lv.stackMaps, live)
 	}
 
-	// We've already reordered lv.livevars[0:uniq]. Clear the
-	// pointers later in the array so they can be GC'd.
-	tail := lv.livevars[uniq:]
-	for i := range tail { // memclr loop pattern
-		tail[i] = bvec{}
-	}
-	lv.livevars = lv.livevars[:uniq]
+	// Clear lv.livevars to allow GC of duplicate maps and to
+	// prevent accidental use.
+	lv.livevars = nil
 
 	// Record compacted stack map indexes for each value.
 	// These will later become PCDATA instructions.
-	lv.showlive(nil, lv.livevars[0])
+	lv.showlive(nil, lv.stackMaps[0])
 	pos := 1
 	lv.stackMapIndex = make(map[*ssa.Value]int)
 	for _, b := range lv.f.Blocks {
 		for _, v := range b.Values {
 			if issafepoint(v) {
-				lv.showlive(v, lv.livevars[remap[pos]])
+				lv.showlive(v, lv.stackMaps[remap[pos]])
 				lv.stackMapIndex[v] = remap[pos]
 				pos++
 			}
@@ -1153,7 +1149,7 @@ func (lv *Liveness) printDebug() {
 		// program listing, with individual effects listed
 
 		if b == lv.f.Entry {
-			live := lv.livevars[pcdata]
+			live := lv.stackMaps[pcdata]
 			fmt.Printf("(%s) function entry\n", linestr(lv.fn.Func.Nname.Pos))
 			fmt.Printf("\tlive=")
 			printed = false
@@ -1190,7 +1186,7 @@ func (lv *Liveness) printDebug() {
 				continue
 			}
 
-			live := lv.livevars[pcdata]
+			live := lv.stackMaps[pcdata]
 			fmt.Printf("\tlive=")
 			printed = false
 			for j, n := range lv.vars {
@@ -1228,14 +1224,14 @@ func (lv *Liveness) printDebug() {
 // remaining bytes are the raw bitmaps.
 func (lv *Liveness) emit(argssym, livesym *obj.LSym) {
 	args := bvalloc(lv.argWords())
-	aoff := duint32(argssym, 0, uint32(len(lv.livevars))) // number of bitmaps
-	aoff = duint32(argssym, aoff, uint32(args.n))         // number of bits in each bitmap
+	aoff := duint32(argssym, 0, uint32(len(lv.stackMaps))) // number of bitmaps
+	aoff = duint32(argssym, aoff, uint32(args.n))          // number of bits in each bitmap
 
 	locals := bvalloc(lv.localWords())
-	loff := duint32(livesym, 0, uint32(len(lv.livevars))) // number of bitmaps
-	loff = duint32(livesym, loff, uint32(locals.n))       // number of bits in each bitmap
+	loff := duint32(livesym, 0, uint32(len(lv.stackMaps))) // number of bitmaps
+	loff = duint32(livesym, loff, uint32(locals.n))        // number of bits in each bitmap
 
-	for _, live := range lv.livevars {
+	for _, live := range lv.stackMaps {
 		args.Clear()
 		locals.Clear()