cmd/compile: in poset, make constant handling more flexible

Currently, constants in posets, in addition to being stored in
a DAG, are also stored as SSA values in a slice. This allows to
quickly go through all stored constants, but it's not easy to search
for a specific constant.

Following CLs will benefit from being able to quickly find
a constants by value in the poset, so change the constants
structure to a map. Since we're at it, don't store it as
*ssa.Value: poset always uses dense uint32 indices when
referring a node, so just switch to it.

Using a map also forces us to have a single node per
constant value: this is a good thing in the first place,
so this CL also make sure we never create two nodes for
the same constant value.

Change-Id: I099814578af35f935ebf14bc4767d607021f5f8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/196781
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

@@ -48,6 +48,7 @@ const (
 	undoSetChr               // change back right child of undo.idx to undo.edge
 	undoNonEqual             // forget that SSA value undo.ID is non-equal to undo.idx (another ID)
 	undoNewNode              // remove new node created for SSA value undo.ID
+	undoNewConstant          // remove the constant node idx from the constants map
 	undoAliasNode            // unalias SSA value undo.ID so that it points back to node index undo.idx
 	undoNewRoot              // remove node undo.idx from root list
 	undoChangeRoot           // remove node undo.idx from root list, and put back undo.edge.Target instead
@@ -149,7 +150,7 @@ type poset struct {
 	lastidx   uint32           // last generated dense index
 	flags     uint8            // internal flags
 	values    map[ID]uint32    // map SSA values to dense indexes
-	constants []*Value      // record SSA constants together with their value
+	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
@@ -159,7 +160,7 @@ type poset struct {
 func newPoset() *poset {
 	return &poset{
 		values:    make(map[ID]uint32),
-		constants: make([]*Value, 0, 8),
+		constants: make(map[int64]uint32, 8),
 		nodes:     make([]posetNode, 1, 16),
 		roots:     make([]uint32, 0, 4),
 		noneq:     make(map[ID]bitset),
@@ -205,6 +206,11 @@ func (po *poset) upushalias(id ID, i2 uint32) {
 	po.undo = append(po.undo, posetUndo{typ: undoAliasNode, ID: id, idx: i2})
 }
 
+// upushconst pushes a new undo pass for a new constant
+func (po *poset) upushconst(idx uint32, old uint32) {
+	po.undo = append(po.undo, posetUndo{typ: undoNewConstant, idx: idx, ID: ID(old)})
+}
+
 // addchild adds i2 as direct child of i1.
 func (po *poset) addchild(i1, i2 uint32, strict bool) {
 	i1l, i1r := po.children(i1)
@@ -281,18 +287,33 @@ func (po *poset) newconst(n *Value) {
 		panic("newconst on non-constant")
 	}
 
-	// If this is the first constant, put it into a new root, as
+	// If the same constant is already present in the poset through a different
+	// Value, just alias to it without allocating a new node.
+	val := n.AuxInt
+	if po.flags&posetFlagUnsigned != 0 {
+		val = int64(n.AuxUnsigned())
+	}
+	if c, found := po.constants[val]; found {
+		po.values[n.ID] = c
+		po.upushalias(n.ID, 0)
+		return
+	}
+
+	// Create the new node for this constant
+	i := po.newnode(n)
+
+	// If this is the first constant, put it as a new root, as
 	// we can't record an existing connection so we don't have
 	// a specific DAG to add it to. Notice that we want all
 	// constants to be in root #0, so make sure the new root
 	// goes there.
 	if len(po.constants) == 0 {
 		idx := len(po.roots)
-		i := po.newnode(n)
 		po.roots = append(po.roots, i)
 		po.roots[0], po.roots[idx] = po.roots[idx], po.roots[0]
 		po.upush(undoNewRoot, i, 0)
-		po.constants = append(po.constants, n)
+		po.constants[val] = i
+		po.upushconst(i, 0)
 		return
 	}
 
@@ -301,21 +322,20 @@ func (po *poset) newconst(n *Value) {
 	// and the lower constant that is higher.
 	// The loop is duplicated to handle signed and unsigned comparison,
 	// depending on how the poset was configured.
-	var lowerptr, higherptr *Value
+	var lowerptr, higherptr uint32
 
 	if po.flags&posetFlagUnsigned != 0 {
 		var lower, higher uint64
 		val1 := n.AuxUnsigned()
-		for _, ptr := range po.constants {
-			val2 := ptr.AuxUnsigned()
+		for val2, ptr := range po.constants {
+			val2 := uint64(val2)
 			if val1 == val2 {
-				po.aliasnode(ptr, n)
-				return
+				panic("unreachable")
 			}
-			if val2 < val1 && (lowerptr == nil || val2 > lower) {
+			if val2 < val1 && (lowerptr == 0 || val2 > lower) {
 				lower = val2
 				lowerptr = ptr
-			} else if val2 > val1 && (higherptr == nil || val2 < higher) {
+			} else if val2 > val1 && (higherptr == 0 || val2 < higher) {
 				higher = val2
 				higherptr = ptr
 			}
@@ -323,23 +343,21 @@ func (po *poset) newconst(n *Value) {
 	} else {
 		var lower, higher int64
 		val1 := n.AuxInt
-		for _, ptr := range po.constants {
-			val2 := ptr.AuxInt
+		for val2, ptr := range po.constants {
 			if val1 == val2 {
-				po.aliasnode(ptr, n)
-				return
+				panic("unreachable")
 			}
-			if val2 < val1 && (lowerptr == nil || val2 > lower) {
+			if val2 < val1 && (lowerptr == 0 || val2 > lower) {
 				lower = val2
 				lowerptr = ptr
-			} else if val2 > val1 && (higherptr == nil || val2 < higher) {
+			} else if val2 > val1 && (higherptr == 0 || val2 < higher) {
 				higher = val2
 				higherptr = ptr
 			}
 		}
 	}
 
-	if lowerptr == nil && higherptr == nil {
+	if lowerptr == 0 && higherptr == 0 {
 		// This should not happen, as at least one
 		// other constant must exist if we get here.
 		panic("no constant found")
@@ -350,18 +368,17 @@ func (po *poset) newconst(n *Value) {
 	// of them, depending on what other constants are present in the poset.
 	// Notice that we always link constants together, so they
 	// are always part of the same DAG.
-	i := po.newnode(n)
 	switch {
-	case lowerptr != nil && higherptr != nil:
+	case lowerptr != 0 && higherptr != 0:
 		// Both bounds are present, record lower < n < higher.
-		po.addchild(po.values[lowerptr.ID], i, true)
-		po.addchild(i, po.values[higherptr.ID], true)
+		po.addchild(lowerptr, i, true)
+		po.addchild(i, higherptr, true)
 
-	case lowerptr != nil:
+	case lowerptr != 0:
 		// Lower bound only, record lower < n.
-		po.addchild(po.values[lowerptr.ID], i, true)
+		po.addchild(lowerptr, i, true)
 
-	case higherptr != nil:
+	case higherptr != 0:
 		// Higher bound only. To record n < higher, we need
 		// a dummy root:
 		//
@@ -373,7 +390,7 @@ func (po *poset) newconst(n *Value) {
 		//       \    /
 		//       higher
 		//
-		i2 := po.values[higherptr.ID]
+		i2 := higherptr
 		r2 := po.findroot(i2)
 		if r2 != po.roots[0] { // all constants should be in root #0
 			panic("constant not in root #0")
@@ -386,7 +403,8 @@ func (po *poset) newconst(n *Value) {
 		po.addchild(i, i2, true)
 	}
 
-	po.constants = append(po.constants, n)
+	po.constants[val] = i
+	po.upushconst(i, 0)
 }
 
 // aliasnode records that n2 is an alias of n1
@@ -422,6 +440,19 @@ func (po *poset) aliasnode(n1, n2 *Value) {
 				po.upushalias(k, i2)
 			}
 		}
+
+		if n2.isGenericIntConst() {
+			val := n2.AuxInt
+			if po.flags&posetFlagUnsigned != 0 {
+				val = int64(n2.AuxUnsigned())
+			}
+			if po.constants[val] != i2 {
+				panic("aliasing constant which is not registered")
+			}
+			po.constants[val] = i1
+			po.upushconst(i1, i2)
+		}
+
 	} else {
 		// n2.ID wasn't seen before, so record it as alias to i1
 		po.values[n2.ID] = i1
@@ -631,11 +662,7 @@ func (po *poset) CheckIntegrity() {
 	// Record which index is a constant
 	constants := newBitset(int(po.lastidx + 1))
 	for _, c := range po.constants {
-		if idx, ok := po.values[c.ID]; !ok {
-			panic("node missing for constant")
-		} else {
-			constants.Set(idx)
-		}
+		constants.Set(c)
 	}
 
 	// Verify that each node appears in a single DAG, and that
@@ -732,15 +759,10 @@ func (po *poset) DotDump(fn string, title string) error {
 		names[i] = s
 	}
 
-	// Create constant mapping
+	// Create reverse constant mapping
 	consts := make(map[uint32]int64)
-	for _, v := range po.constants {
-		idx := po.values[v.ID]
-		if po.flags&posetFlagUnsigned != 0 {
-			consts[idx] = int64(v.AuxUnsigned())
-		} else {
-			consts[idx] = v.AuxInt
-		}
+	for val, idx := range po.constants {
+		consts[idx] = val
 	}
 
 	fmt.Fprintf(f, "digraph poset {\n")
@@ -1171,10 +1193,25 @@ func (po *poset) Undo() {
 			po.nodes = po.nodes[:pass.idx]
 			po.lastidx--
 
-			// If it was the last inserted constant, remove it
-			nc := len(po.constants)
-			if nc > 0 && po.constants[nc-1].ID == pass.ID {
-				po.constants = po.constants[:nc-1]
+		case undoNewConstant:
+			// FIXME: remove this O(n) loop
+			var val int64
+			var i uint32
+			for val, i = range po.constants {
+				if i == pass.idx {
+					break
+				}
+			}
+			if i != pass.idx {
+				panic("constant not found in undo pass")
+			}
+			if pass.ID == 0 {
+				delete(po.constants, val)
+			} else {
+				// Restore previous index as constant node
+				// (also restoring the invariant on correct bounds)
+				oldidx := uint32(pass.ID)
+				po.constants[val] = oldidx
 			}
 
 		case undoAliasNode: