cmd/compile: in poset, change the way inequality is recorded

Before this CL, inequality was recorded in a bit matrix using
SSA IDs. This allowed to record inequality for SSA values that
we didn't know any relation in the partial order of. Unfortunately,
this also means that inequality is harder to use within the poset
itself as there is not fast way to map from internal poset indices
and SSA values.

Since we will need to check for inequality in following CLs within
code that lost track of SSA values, switch to use a bit matrix
of poset indices instead. This requires always allocate a poset
node (as a new root) for values that are first seen in a SetNonEqual
call, but it doesn't sound like a big problem. The other solution
(creating and maintaining a reverse map from poset indices to SSA
values) seem more complicated and memory hungry.

Change-Id: Ic917485abbe70aef7ad6fa98408e5430328b6cd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/196782
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -153,7 +153,7 @@ type poset struct {
 	constants map[int64]uint32  // record SSA constants together with their value
 	nodes     []posetNode       // nodes (in all DAGs)
 	roots     []uint32          // list of root nodes (forest)
-	noneq     map[ID]bitset    // non-equal relations
+	noneq     map[uint32]bitset // non-equal relations
 	undo      []posetUndo       // undo chain
 }
 
@@ -163,7 +163,7 @@ func newPoset() *poset {
 		constants: make(map[int64]uint32, 8),
 		nodes:     make([]posetNode, 1, 16),
 		roots:     make([]uint32, 0, 4),
-		noneq:     make(map[ID]bitset),
+		noneq:     make(map[uint32]bitset),
 		undo:      make([]posetUndo, 0, 4),
 	}
 }
@@ -197,8 +197,8 @@ func (po *poset) upushnew(id ID, idx uint32) {
 }
 
 // upushneq pushes a new undo pass for a nonequal relation
-func (po *poset) upushneq(id1 ID, id2 ID) {
-	po.undo = append(po.undo, posetUndo{typ: undoNonEqual, ID: id1, idx: uint32(id2)})
+func (po *poset) upushneq(idx1 uint32, idx2 uint32) {
+	po.undo = append(po.undo, posetUndo{typ: undoNonEqual, ID: ID(idx1), idx: idx2})
 }
 
 // upushalias pushes a new undo pass for aliasing two nodes
@@ -622,38 +622,61 @@ func (po *poset) collapsepath(n1, n2 *Value) bool {
 	return true
 }
 
-// Check whether it is recorded that id1!=id2
-func (po *poset) isnoneq(id1, id2 ID) bool {
-	if id1 < id2 {
-		id1, id2 = id2, id1
+// Check whether it is recorded that i1!=i2
+func (po *poset) isnoneq(i1, i2 uint32) bool {
+	if i1 == i2 {
+		return false
+	}
+	if i1 < i2 {
+		i1, i2 = i2, i1
 	}
 
 	// Check if we recorded a non-equal relation before
-	if bs, ok := po.noneq[id1]; ok && bs.Test(uint32(id2)) {
+	if bs, ok := po.noneq[i1]; ok && bs.Test(i2) {
 		return true
 	}
 	return false
 }
 
-// Record that id1!=id2
-func (po *poset) setnoneq(id1, id2 ID) {
-	if id1 < id2 {
-		id1, id2 = id2, id1
+// Record that i1!=i2
+func (po *poset) setnoneq(n1, n2 *Value) {
+	i1, f1 := po.lookup(n1)
+	i2, f2 := po.lookup(n2)
+
+	// If any of the nodes do not exist in the poset, allocate them. Since
+	// we don't know any relation (in the partial order) about them, they must
+	// become independent roots.
+	if !f1 {
+		i1 = po.newnode(n1)
+		po.roots = append(po.roots, i1)
+		po.upush(undoNewRoot, i1, 0)
+	}
+	if !f2 {
+		i2 = po.newnode(n2)
+		po.roots = append(po.roots, i2)
+		po.upush(undoNewRoot, i2, 0)
 	}
-	bs := po.noneq[id1]
+
+	if i1 == i2 {
+		panic("setnoneq on same node")
+	}
+	if i1 < i2 {
+		i1, i2 = i2, i1
+	}
+	bs := po.noneq[i1]
 	if bs == nil {
 		// Given that we record non-equality relations using the
-		// higher ID as a key, the bitsize will never change size.
+		// higher index as a key, the bitsize will never change size.
 		// TODO(rasky): if memory is a problem, consider allocating
-		// a small bitset and lazily grow it when higher IDs arrive.
-		bs = newBitset(int(id1))
-		po.noneq[id1] = bs
-	} else if bs.Test(uint32(id2)) {
+		// a small bitset and lazily grow it when higher indices arrive.
+		bs = newBitset(int(i1))
+		po.noneq[i1] = bs
+	} else if bs.Test(i2) {
 		// Already recorded
 		return
 	}
-	bs.Set(uint32(id2))
-	po.upushneq(id1, id2)
+	bs.Set(i2)
+	po.upushneq(i1, i2)
 }
 
 // CheckIntegrity verifies internal integrity of a poset. It is intended
@@ -876,7 +899,17 @@ func (po *poset) NonEqual(n1, n2 *Value) bool {
 	if n1.ID == n2.ID {
 		panic("should not call NonEqual with n1==n2")
 	}
-	if po.isnoneq(n1.ID, n2.ID) {
+
+	// If we never saw the nodes before, we don't
+	// have a recorded non-equality.
+	i1, f1 := po.lookup(n1)
+	i2, f2 := po.lookup(n2)
+	if !f1 || !f2 {
+		return false
+	}
+
+	// Check if we recored inequality
+	if po.isnoneq(i1, i2) {
 		return true
 	}
 
@@ -892,12 +925,6 @@ func (po *poset) NonEqual(n1, n2 *Value) bool {
 // if this is a contradiction.
 // Implements SetOrder() and SetOrderOrEqual()
 func (po *poset) setOrder(n1, n2 *Value, strict bool) bool {
-	// If we are trying to record n1<=n2 but we learned that n1!=n2,
-	// record n1<n2, as it provides more information.
-	if !strict && po.isnoneq(n1.ID, n2.ID) {
-		strict = true
-	}
-
 	i1, f1 := po.lookup(n1)
 	i2, f2 := po.lookup(n2)
 
@@ -956,6 +983,12 @@ func (po *poset) setOrder(n1, n2 *Value, strict bool) bool {
 			return !strict
 		}
 
+		// If we are trying to record n1<=n2 but we learned that n1!=n2,
+		// record n1<n2, as it provides more information.
+		if !strict && po.isnoneq(i1, i2) {
+			strict = true
+		}
+
 		// Both n1 and n2 are in the poset. This is the complex part of the algorithm
 		// as we need to find many different cases and DAG shapes.
 
@@ -1052,11 +1085,6 @@ func (po *poset) SetEqual(n1, n2 *Value) bool {
 		panic("should not call Add with n1==n2")
 	}
 
-	// If we recorded that n1!=n2, this is a contradiction.
-	if po.isnoneq(n1.ID, n2.ID) {
-		return false
-	}
-
 	i1, f1 := po.lookup(n1)
 	i2, f2 := po.lookup(n2)
 
@@ -1076,6 +1104,11 @@ func (po *poset) SetEqual(n1, n2 *Value) bool {
 			return true
 		}
 
+		// If we recorded that n1!=n2, this is a contradiction.
+		if po.isnoneq(i1, i2) {
+			return false
+		}
+
 		// If we already knew that n1<=n2, we can collapse the path to
 		// record n1==n2 (and viceversa).
 		if po.reaches(i1, i2, false) {
@@ -1114,31 +1147,40 @@ func (po *poset) SetNonEqual(n1, n2 *Value) bool {
 		panic("should not call SetNonEqual with n1==n2")
 	}
 
-	// See if we already know this
-	if po.isnoneq(n1.ID, n2.ID) {
+	// Check whether the nodes are already in the poset
+	i1, f1 := po.lookup(n1)
+	i2, f2 := po.lookup(n2)
+
+	// If either node wasn't present, we just record the new relation
+	// and exit.
+	if !f1 || !f2 {
+		po.setnoneq(n1, n2)
+		return true
+	}
+
+	// See if we already know this, in which case there's nothing to do.
+	if po.isnoneq(i1, i2) {
 		return true
 	}
 
-	// Check if we're contradicting an existing relation
+	// Check if we're contradicting an existing equality relation
 	if po.Equal(n1, n2) {
 		return false
 	}
 
 	// Record non-equality
-	po.setnoneq(n1.ID, n2.ID)
+	po.setnoneq(n1, n2)
 
 	// If we know that i1<=i2 but not i1<i2, learn that as we
 	// now know that they are not equal. Do the same for i2<=i1.
-	i1, f1 := po.lookup(n1)
-	i2, f2 := po.lookup(n2)
-	if f1 && f2 {
+	// Do this check only if both nodes were already in the DAG,
+	// otherwise there cannot be an existing relation.
 	if po.reaches(i1, i2, false) && !po.reaches(i1, i2, true) {
 		po.addchild(i1, i2, true)
 	}
 	if po.reaches(i2, i1, false) && !po.reaches(i2, i1, true) {
 		po.addchild(i2, i1, true)
 	}
-	}
 
 	return true
 }
@@ -1177,7 +1219,7 @@ func (po *poset) Undo() {
 			po.setchr(pass.idx, pass.edge)
 
 		case undoNonEqual:
-			po.noneq[pass.ID].Clear(pass.idx)
+			po.noneq[uint32(pass.ID)].Clear(pass.idx)
 
 		case undoNewNode:
 			if pass.idx != po.lastidx {