cmd/compile: improve undo of poset

prove uses the poset datastructure in a DFS walk, and always undoes
it back to its pristine status. Before this CL, poset's undo of
a new node creation didn't fully deallocate the node, which means
that at the end of prove there was still some allocated memory pending.

This was not a problem until now because the posets used by prove
were discarded after each function, but it would prevent recycling
them between functions (as a followup CL does).

Change-Id: I1c1c99c03fe19ad765395a43958cb256f686765a
Reviewed-on: https://go-review.googlesource.com/112976
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/fmt_test.go

@@ -608,6 +608,8 @@ var knownFormats = map[string]string{
 	"[]byte %x":                                       "",
 	"[]cmd/compile/internal/ssa.Edge %v":              "",
 	"[]cmd/compile/internal/ssa.ID %v":                "",
+	"[]cmd/compile/internal/ssa.posetNode %v":         "",
+	"[]cmd/compile/internal/ssa.posetUndo %v":         "",
 	"[]cmd/compile/internal/syntax.token %s":          "",
 	"[]string %v":                                     "",
 	"[]uint32 %v":                                     "",
@@ -718,6 +720,7 @@ var knownFormats = map[string]string{
 	"uint16 %v":                                                            "",
 	"uint16 %x":                                                            "",
 	"uint32 %d":                                                            "",
+	"uint32 %v":                                                            "",
 	"uint32 %x":                                                            "",
 	"uint64 %08x":                                                          "",
 	"uint64 %d":                                                            "",

src/cmd/compile/internal/ssa/poset.go

@@ -679,13 +679,24 @@ func (po *poset) CheckIntegrity() (err error) {
 // It can be used for debugging purposes, as a poset is supposed to
 // be empty after it's fully rolled back through Undo.
 func (po *poset) CheckEmpty() error {
-	// Check that the poset is completely empty
+	if len(po.nodes) != 1 {
+		return fmt.Errorf("non-empty nodes list: %v", po.nodes)
+	}
 	if len(po.values) != 0 {
 		return fmt.Errorf("non-empty value map: %v", po.values)
 	}
 	if len(po.roots) != 0 {
 		return fmt.Errorf("non-empty root list: %v", po.roots)
 	}
+	if len(po.constants) != 0 {
+		return fmt.Errorf("non-empty constants: %v", po.constants)
+	}
+	if len(po.undo) != 0 {
+		return fmt.Errorf("non-empty undo list: %v", po.undo)
+	}
+	if po.lastidx != 0 {
+		return fmt.Errorf("lastidx index is not zero: %v", po.lastidx)
+	}
 	for _, bs := range po.noneq {
 		for _, x := range bs {
 			if x != 0 {
@@ -693,14 +704,6 @@ func (po *poset) CheckEmpty() error {
 			}
 		}
 	}
-	for idx, n := range po.nodes {
-		if n.l|n.r != 0 {
-			return fmt.Errorf("non-empty node %v->[%d,%d]", idx, n.l.Target(), n.r.Target())
-		}
-	}
-	if len(po.constants) != 0 {
-		return fmt.Errorf("non-empty constant")
-	}
 	return nil
 }
 
@@ -1123,6 +1126,9 @@ func (po *poset) Undo() {
 			po.noneq[pass.ID].Clear(pass.idx)
 
 		case undoNewNode:
+			if pass.idx != po.lastidx {
+				panic("invalid newnode index")
+			}
 			if pass.ID != 0 {
 				if po.values[pass.ID] != pass.idx {
 					panic("invalid newnode undo pass")
@@ -1131,6 +1137,8 @@ func (po *poset) Undo() {
 			}
 			po.setchl(pass.idx, 0)
 			po.setchr(pass.idx, 0)
+			po.nodes = po.nodes[:pass.idx]
+			po.lastidx--
 
 			// If it was the last inserted constant, remove it
 			nc := len(po.constants)

src/cmd/compile/internal/ssa/poset_test.go

@@ -626,7 +626,6 @@ func TestPosetConst(t *testing.T) {
 
 func TestPosetNonEqual(t *testing.T) {
 	testPosetOps(t, false, []posetTestOp{
-		{Checkpoint, 0, 0},
 		{Equal_Fail, 10, 20},
 		{NonEqual_Fail, 10, 20},