cmd/compile: enable constant-time CFG editing

Provide indexes along with block pointers for Preds
and Succs arrays.  This allows us to splice edges in
and out of those arrays in constant time.

Fixes worst-case O(n^2) behavior in deadcode and fuse.

benchmark                     old ns/op      new ns/op     delta
BenchmarkFuse1-8              2065           2057          -0.39%
BenchmarkFuse10-8             9408           9073          -3.56%
BenchmarkFuse100-8            105238         76277         -27.52%
BenchmarkFuse1000-8           3982562        1026750       -74.22%
BenchmarkFuse10000-8          301220329      12824005      -95.74%
BenchmarkDeadCode1-8          1588           1566          -1.39%
BenchmarkDeadCode10-8         4333           4250          -1.92%
BenchmarkDeadCode100-8        32031          32574         +1.70%
BenchmarkDeadCode1000-8       590407         468275        -20.69%
BenchmarkDeadCode10000-8      17822890       5000818       -71.94%
BenchmarkDeadCode100000-8     1388706640     78021127      -94.38%
BenchmarkDeadCode200000-8     5372518479     168598762     -96.86%

Change-Id: Iccabdbb9343fd1c921ba07bbf673330a1c36ee17
Reviewed-on: https://go-review.googlesource.com/22589Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/amd64/ssa.go

@@ -882,7 +882,7 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		// Optimization - if the subsequent block has a load or store
 		// at the same address, we don't need to issue this instruction.
 		mem := v.Args[1]
-		for _, w := range v.Block.Succs[0].Values {
+		for _, w := range v.Block.Succs[0].Block().Values {
 			if w.Op == ssa.OpPhi {
 				if w.Type.IsMemory() {
 					mem = w
@@ -978,10 +978,10 @@ func ssaGenBlock(s *gc.SSAGenState, b, next *ssa.Block) {
 
 	switch b.Kind {
 	case ssa.BlockPlain, ssa.BlockCall, ssa.BlockCheck:
-		if b.Succs[0] != next {
+		if b.Succs[0].Block() != next {
 			p := gc.Prog(obj.AJMP)
 			p.To.Type = obj.TYPE_BRANCH
-			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0]})
+			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0].Block()})
 		}
 	case ssa.BlockDefer:
 		// defer returns in rax:
@@ -994,11 +994,11 @@ func ssaGenBlock(s *gc.SSAGenState, b, next *ssa.Block) {
 		p.To.Reg = x86.REG_AX
 		p = gc.Prog(x86.AJNE)
 		p.To.Type = obj.TYPE_BRANCH
-		s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[1]})
-		if b.Succs[0] != next {
+		s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[1].Block()})
+		if b.Succs[0].Block() != next {
 			p := gc.Prog(obj.AJMP)
 			p.To.Type = obj.TYPE_BRANCH
-			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0]})
+			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0].Block()})
 		}
 	case ssa.BlockExit:
 		gc.Prog(obj.AUNDEF) // tell plive.go that we never reach here
@@ -1025,22 +1025,22 @@ func ssaGenBlock(s *gc.SSAGenState, b, next *ssa.Block) {
 		likely := b.Likely
 		var p *obj.Prog
 		switch next {
-		case b.Succs[0]:
+		case b.Succs[0].Block():
 			p = gc.Prog(jmp.invasm)
 			likely *= -1
 			p.To.Type = obj.TYPE_BRANCH
-			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[1]})
-		case b.Succs[1]:
+			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[1].Block()})
+		case b.Succs[1].Block():
 			p = gc.Prog(jmp.asm)
 			p.To.Type = obj.TYPE_BRANCH
-			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0]})
+			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0].Block()})
 		default:
 			p = gc.Prog(jmp.asm)
 			p.To.Type = obj.TYPE_BRANCH
-			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0]})
+			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0].Block()})
 			q := gc.Prog(obj.AJMP)
 			q.To.Type = obj.TYPE_BRANCH
-			s.Branches = append(s.Branches, gc.Branch{P: q, B: b.Succs[1]})
+			s.Branches = append(s.Branches, gc.Branch{P: q, B: b.Succs[1].Block()})
 		}
 
 		// liblink reorders the instruction stream as it sees fit.

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

@@ -136,19 +136,19 @@ func ssaGenBlock(s *gc.SSAGenState, b, next *ssa.Block) {
 
 	switch b.Kind {
 	case ssa.BlockCall:
-		if b.Succs[0] != next {
+		if b.Succs[0].Block() != next {
 			p := gc.Prog(obj.AJMP)
 			p.To.Type = obj.TYPE_BRANCH
-			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0]})
+			s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0].Block()})
 		}
 	case ssa.BlockRet:
 		gc.Prog(obj.ARET)
 	case ssa.BlockARMLT:
 		p := gc.Prog(arm.ABLT)
 		p.To.Type = obj.TYPE_BRANCH
-		s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0]})
+		s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[0].Block()})
 		p = gc.Prog(obj.AJMP)
 		p.To.Type = obj.TYPE_BRANCH
-		s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[1]})
+		s.Branches = append(s.Branches, gc.Branch{P: p, B: b.Succs[1].Block()})
 	}
 }

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

@@ -3799,7 +3799,8 @@ func (s *state) resolveFwdRef(v *ssa.Value) {
 	// Find variable value on each predecessor.
 	var argstore [4]*ssa.Value
 	args := argstore[:0]
-	for _, p := range b.Preds {
+	for _, e := range b.Preds {
+		p := e.Block()
 		args = append(args, s.lookupVarOutgoing(p, v.Type, name, v.Line))
 	}
 
@@ -4044,7 +4045,7 @@ func oneFPJump(b *ssa.Block, jumps *FloatingEQNEJump, likely ssa.BranchPredictio
 	p := Prog(jumps.Jump)
 	p.To.Type = obj.TYPE_BRANCH
 	to := jumps.Index
-	branches = append(branches, Branch{p, b.Succs[to]})
+	branches = append(branches, Branch{p, b.Succs[to].Block()})
 	if to == 1 {
 		likely = -likely
 	}
@@ -4066,10 +4067,10 @@ func oneFPJump(b *ssa.Block, jumps *FloatingEQNEJump, likely ssa.BranchPredictio
 func SSAGenFPJump(s *SSAGenState, b, next *ssa.Block, jumps *[2][2]FloatingEQNEJump) {
 	likely := b.Likely
 	switch next {
-	case b.Succs[0]:
+	case b.Succs[0].Block():
 		s.Branches = oneFPJump(b, &jumps[0][0], likely, s.Branches)
 		s.Branches = oneFPJump(b, &jumps[0][1], likely, s.Branches)
-	case b.Succs[1]:
+	case b.Succs[1].Block():
 		s.Branches = oneFPJump(b, &jumps[1][0], likely, s.Branches)
 		s.Branches = oneFPJump(b, &jumps[1][1], likely, s.Branches)
 	default:
@@ -4077,7 +4078,7 @@ func SSAGenFPJump(s *SSAGenState, b, next *ssa.Block, jumps *[2][2]FloatingEQNEJ
 		s.Branches = oneFPJump(b, &jumps[1][1], likely, s.Branches)
 		q := Prog(obj.AJMP)
 		q.To.Type = obj.TYPE_BRANCH
-		s.Branches = append(s.Branches, Branch{q, b.Succs[1]})
+		s.Branches = append(s.Branches, Branch{q, b.Succs[1].Block()})
 	}
 }
 

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

@@ -12,18 +12,30 @@ type Block struct {
 	// these IDs densely, but no guarantees.
 	ID ID
 
+	// Line number for block's control operation
+	Line int32
+
 	// The kind of block this is.
 	Kind BlockKind
 
+	// Likely direction for branches.
+	// If BranchLikely, Succs[0] is the most likely branch taken.
+	// If BranchUnlikely, Succs[1] is the most likely branch taken.
+	// Ignored if len(Succs) < 2.
+	// Fatal if not BranchUnknown and len(Succs) > 2.
+	Likely BranchPrediction
+
+	// After flagalloc, records whether flags are live at the end of the block.
+	FlagsLiveAtEnd bool
+
 	// Subsequent blocks, if any. The number and order depend on the block kind.
-	// All successors must be distinct (to make phi values in successors unambiguous).
-	Succs []*Block
+	Succs []Edge
 
 	// Inverse of successors.
 	// The order is significant to Phi nodes in the block.
-	Preds []*Block
 	// TODO: predecessors is a pain to maintain. Can we somehow order phi
 	// arguments by block id and have this field computed explicitly when needed?
+	Preds []Edge
 
 	// A value that determines how the block is exited. Its value depends on the kind
 	// of the block. For instance, a BlockIf has a boolean control value and BlockExit
@@ -41,23 +53,41 @@ type Block struct {
 	// The containing function
 	Func *Func
 
-	// Line number for block's control operation
-	Line int32
-
-	// Likely direction for branches.
-	// If BranchLikely, Succs[0] is the most likely branch taken.
-	// If BranchUnlikely, Succs[1] is the most likely branch taken.
-	// Ignored if len(Succs) < 2.
-	// Fatal if not BranchUnknown and len(Succs) > 2.
-	Likely BranchPrediction
+	// Storage for Succs, Preds, and Values
+	succstorage [2]Edge
+	predstorage [4]Edge
+	valstorage  [9]*Value
+}
 
-	// After flagalloc, records whether flags are live at the end of the block.
-	FlagsLiveAtEnd bool
+// Edge represents a CFG edge.
+// Example edges for b branching to either c or d.
+// (c and d have other predecessors.)
+//   b.Succs = [{c,3}, {d,1}]
+//   c.Preds = [?, ?, ?, {b,0}]
+//   d.Preds = [?, {b,1}, ?]
+// These indexes allow us to edit the CFG in constant time.
+// In addition, it informs phi ops in degenerate cases like:
+// b:
+//    if k then c else c
+// c:
+//    v = Phi(x, y)
+// Then the indexes tell you whether x is chosen from
+// the if or else branch from b.
+//   b.Succs = [{c,0},{c,1}]
+//   c.Preds = [{b,0},{b,1}]
+// means x is chosen if k is true.
+type Edge struct {
+	// block edge goes to (in a Succs list) or from (in a Preds list)
+	b *Block
+	// index of reverse edge.  Invariant:
+	//   e := x.Succs[idx]
+	//   e.b.Preds[e.i] = Edge{x,idx}
+	// and similarly for predecessors.
+	i int
+}
 
-	// Storage for Succs, Preds, and Values
-	succstorage [2]*Block
-	predstorage [4]*Block
-	valstorage  [8]*Value
+func (e Edge) Block() *Block {
+	return e.b
 }
 
 //     kind           control    successors
@@ -66,7 +96,7 @@ type Block struct {
 //    Plain               nil            [next]
 //       If   a boolean Value      [then, else]
 //     Call               mem  [nopanic, panic]  (control opcode should be OpCall or OpStaticCall)
-type BlockKind int32
+type BlockKind int8
 
 // short form print
 func (b *Block) String() string {
@@ -85,7 +115,7 @@ func (b *Block) LongString() string {
 	if len(b.Succs) > 0 {
 		s += " ->"
 		for _, c := range b.Succs {
-			s += " " + c.String()
+			s += " " + c.b.String()
 		}
 	}
 	switch b.Likely {
@@ -110,8 +140,53 @@ func (b *Block) SetControl(v *Value) {
 // AddEdgeTo adds an edge from block b to block c. Used during building of the
 // SSA graph; do not use on an already-completed SSA graph.
 func (b *Block) AddEdgeTo(c *Block) {
-	b.Succs = append(b.Succs, c)
-	c.Preds = append(c.Preds, b)
+	i := len(b.Succs)
+	j := len(c.Preds)
+	b.Succs = append(b.Succs, Edge{c, j})
+	c.Preds = append(c.Preds, Edge{b, i})
+}
+
+// removePred removes the ith input edge from b.
+// It is the responsibility of the caller to remove
+// the corresponding successor edge.
+func (b *Block) removePred(i int) {
+	n := len(b.Preds) - 1
+	if i != n {
+		e := b.Preds[n]
+		b.Preds[i] = e
+		// Update the other end of the edge we moved.
+		e.b.Succs[e.i].i = i
+	}
+	b.Preds[n] = Edge{}
+	b.Preds = b.Preds[:n]
+}
+
+// removeSucc removes the ith output edge from b.
+// It is the responsibility of the caller to remove
+// the corresponding predecessor edge.
+func (b *Block) removeSucc(i int) {
+	n := len(b.Succs) - 1
+	if i != n {
+		e := b.Succs[n]
+		b.Succs[i] = e
+		// Update the other end of the edge we moved.
+		e.b.Preds[e.i].i = i
+	}
+	b.Succs[n] = Edge{}
+	b.Succs = b.Succs[:n]
+}
+
+func (b *Block) swapSuccessors() {
+	if len(b.Succs) != 2 {
+		b.Fatalf("swapSuccessors with len(Succs)=%d", len(b.Succs))
+	}
+	e0 := b.Succs[0]
+	e1 := b.Succs[1]
+	b.Succs[0] = e1
+	b.Succs[1] = e0
+	e0.b.Preds[e0.i].i = 1
+	e1.b.Preds[e1.i].i = 0
+	b.Likely *= -1
 }
 
 func (b *Block) Logf(msg string, args ...interface{})           { b.Func.Logf(msg, args...) }

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

@@ -18,36 +18,14 @@ func checkFunc(f *Func) {
 			f.Fatalf("%s.Func=%s, want %s", b, b.Func.Name, f.Name)
 		}
 
-		if f.RegAlloc == nil {
-			for i, c := range b.Succs {
-				for j, d := range b.Succs {
-					if i != j && c == d {
-						f.Fatalf("%s.Succs has duplicate block %s", b, c)
-					}
-				}
-			}
-		}
-		// Note: duplicate successors are hard in the following case:
-		//      if(...) goto x else goto x
-		//   x: v = phi(a, b)
-		// If the conditional is true, does v get the value of a or b?
-		// We could solve this other ways, but the easiest is just to
-		// require (by possibly adding empty control-flow blocks) that
-		// all successors are distinct. They will need to be distinct
-		// anyway for register allocation (duplicate successors implies
-		// the existence of critical edges).
-		// After regalloc we can allow non-distinct predecessors.
-
-		for _, p := range b.Preds {
-			var found bool
-			for _, c := range p.Succs {
-				if c == b {
-					found = true
-					break
+		for i, e := range b.Preds {
+			if se := e.b.Succs[e.i]; se.b != b || se.i != i {
+				f.Fatalf("block pred/succ not crosslinked correctly %d:%s %d:%s", i, b, se.i, se.b)
 			}
 		}
-			if !found {
-				f.Fatalf("block %s is not a succ of its pred block %s", b, p)
+		for i, e := range b.Succs {
+			if pe := e.b.Preds[e.i]; pe.b != b || pe.i != i {
+				f.Fatalf("block succ/pred not crosslinked correctly %d:%s %d:%s", i, b, pe.i, pe.b)
 			}
 		}
 
@@ -252,12 +230,12 @@ func checkFunc(f *Func) {
 	}
 	for _, b := range f.Blocks {
 		for _, c := range b.Preds {
-			if !blockMark[c.ID] {
+			if !blockMark[c.b.ID] {
 				f.Fatalf("predecessor block %v for %v is missing", c, b)
 			}
 		}
 		for _, c := range b.Succs {
-			if !blockMark[c.ID] {
+			if !blockMark[c.b.ID] {
 				f.Fatalf("successor block %v for %v is missing", c, b)
 			}
 		}
@@ -280,7 +258,7 @@ func checkFunc(f *Func) {
 			f.Fatalf("control value for %s is missing: %v", b, b.Control)
 		}
 	}
-	for b := f.freeBlocks; b != nil; b = b.succstorage[0] {
+	for b := f.freeBlocks; b != nil; b = b.succstorage[0].b {
 		if blockMark[b.ID] {
 			f.Fatalf("used block b%d in free list", b.ID)
 		}
@@ -303,7 +281,7 @@ func checkFunc(f *Func) {
 					x := arg.Block
 					y := b
 					if v.Op == OpPhi {
-						y = b.Preds[i]
+						y = b.Preds[i].b
 					}
 					if !domCheck(f, sdom, x, y) {
 						f.Fatalf("arg %d of value %s does not dominate, arg=%s", i, v.LongString(), arg.LongString())

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

@@ -40,9 +40,12 @@ func critical(f *Func) {
 		}
 
 		// split input edges coming from multi-output blocks.
-		for i := 0; i < len(b.Preds); i++ {
-			c := b.Preds[i]
-			if c.Kind == BlockPlain {
+		for i := 0; i < len(b.Preds); {
+			e := b.Preds[i]
+			p := e.b
+			pi := e.i
+			if p.Kind == BlockPlain {
+				i++
 				continue // only single output block
 			}
 
@@ -57,10 +60,10 @@ func critical(f *Func) {
 					// since we're iterating over len(f.Blocks) above, this forces
 					// the new blocks to be re-examined.
 					d = f.NewBlock(BlockPlain)
-					d.Line = c.Line
+					d.Line = p.Line
 					blocks[argID] = d
 					if f.pass.debug > 0 {
-						f.Config.Warnl(c.Line, "split critical edge")
+						f.Config.Warnl(p.Line, "split critical edge")
 					}
 				} else {
 					reusedBlock = true
@@ -69,9 +72,9 @@ func critical(f *Func) {
 				// no existing block, so allocate a new block
 				// to place on the edge
 				d = f.NewBlock(BlockPlain)
-				d.Line = c.Line
+				d.Line = p.Line
 				if f.pass.debug > 0 {
-					f.Config.Warnl(c.Line, "split critical edge")
+					f.Config.Warnl(p.Line, "split critical edge")
 				}
 			}
 
@@ -80,57 +83,34 @@ func critical(f *Func) {
 			// corresponding elements from the block
 			// predecessors and phi args
 			if reusedBlock {
-				d.Preds = append(d.Preds, c)
-				b.Preds[i] = nil
-				phi.Args[i].Uses--
-				phi.Args[i] = nil
-			} else {
-				// splice it in
-				d.Preds = append(d.Preds, c)
-				d.Succs = append(d.Succs, b)
-				b.Preds[i] = d
-			}
+				// Add p->d edge
+				p.Succs[pi] = Edge{d, len(d.Preds)}
+				d.Preds = append(d.Preds, Edge{p, pi})
 
-			// replace b with d in c's successor list.
-			for j, b2 := range c.Succs {
-				if b2 == b {
-					c.Succs[j] = d
-					break
-				}
-			}
-		}
+				// Remove p as a predecessor from b.
+				b.removePred(i)
 
-		// clean up phi's args and b's predecessor list
-		if phi != nil {
-			phi.Args = filterNilValues(phi.Args)
-			b.Preds = filterNilBlocks(b.Preds)
+				// Update corresponding phi args
+				n := len(b.Preds)
+				phi.Args[i].Uses--
+				phi.Args[i] = phi.Args[n]
+				phi.Args[n] = nil
+				phi.Args = phi.Args[:n]
 				// splitting occasionally leads to a phi having
 				// a single argument (occurs with -N)
-			if len(phi.Args) == 1 {
+				if n == 1 {
 					phi.Op = OpCopy
 				}
+				// Don't increment i in this case because we moved
+				// an unprocessed predecessor down into slot i.
+			} else {
+				// splice it in
+				p.Succs[pi] = Edge{d, 0}
+				b.Preds[i] = Edge{d, 0}
+				d.Preds = append(d.Preds, Edge{p, pi})
+				d.Succs = append(d.Succs, Edge{b, i})
+				i++
 			}
 		}
-}
-
-// filterNilValues preserves the order of v, while filtering out nils.
-func filterNilValues(v []*Value) []*Value {
-	nv := v[:0]
-	for i := range v {
-		if v[i] != nil {
-			nv = append(nv, v[i])
-		}
-	}
-	return nv
-}
-
-// filterNilBlocks preserves the order of b, while filtering out nils.
-func filterNilBlocks(b []*Block) []*Block {
-	nb := b[:0]
-	for i := range b {
-		if b[i] != nil {
-			nb = append(nb, b[i])
-		}
 	}
-	return nb
 }

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

@@ -25,7 +25,8 @@ func reachableBlocks(f *Func) []bool {
 		if b.Kind == BlockFirst {
 			s = s[:1]
 		}
-		for _, c := range s {
+		for _, e := range s {
+			c := e.b
 			if !reachable[c.ID] {
 				reachable[c.ID] = true
 				p = append(p, c) // push
@@ -69,7 +70,7 @@ func liveValues(f *Func, reachable []bool) []bool {
 		v := q[len(q)-1]
 		q = q[:len(q)-1]
 		for i, x := range v.Args {
-			if v.Op == OpPhi && !reachable[v.Block.Preds[i].ID] {
+			if v.Op == OpPhi && !reachable[v.Block.Preds[i].b.ID] {
 				continue
 			}
 			if !live[x.ID] {
@@ -100,9 +101,12 @@ func deadcode(f *Func) {
 		if reachable[b.ID] {
 			continue
 		}
-		for _, c := range b.Succs {
-			if reachable[c.ID] {
-				c.removePred(b)
+		for i := 0; i < len(b.Succs); {
+			e := b.Succs[i]
+			if reachable[e.b.ID] {
+				b.removeEdge(i)
+			} else {
+				i++
 			}
 		}
 	}
@@ -115,16 +119,9 @@ func deadcode(f *Func) {
 		if b.Kind != BlockFirst {
 			continue
 		}
-		c := b.Succs[1]
-		b.Succs[1] = nil
-		b.Succs = b.Succs[:1]
+		b.removeEdge(1)
 		b.Kind = BlockPlain
 		b.Likely = BranchUnknown
-
-		if reachable[c.ID] {
-			// Note: c must be reachable through some other edge.
-			c.removePred(b)
-		}
 	}
 
 	// Splice out any copies introduced during dead block removal.
@@ -217,35 +214,28 @@ func deadcode(f *Func) {
 	f.Blocks = f.Blocks[:i]
 }
 
-// removePred removes the predecessor p from b's predecessor list.
-func (b *Block) removePred(p *Block) {
-	var i int
-	found := false
-	for j, q := range b.Preds {
-		if q == p {
-			i = j
-			found = true
-			break
-		}
-	}
-	// TODO: the above loop could make the deadcode pass take quadratic time
-	if !found {
-		b.Fatalf("can't find predecessor %v of %v\n", p, b)
-	}
+// removeEdge removes the i'th outgoing edge from b (and
+// the corresponding incoming edge from b.Succs[i].b).
+func (b *Block) removeEdge(i int) {
+	e := b.Succs[i]
+	c := e.b
+	j := e.i
 
-	n := len(b.Preds) - 1
-	b.Preds[i] = b.Preds[n]
-	b.Preds[n] = nil // aid GC
-	b.Preds = b.Preds[:n]
+	// Adjust b.Succs
+	b.removeSucc(i)
 
-	// rewrite phi ops to match the new predecessor list
-	for _, v := range b.Values {
+	// Adjust c.Preds
+	c.removePred(j)
+
+	// Remove phi args from c's phis.
+	n := len(c.Preds)
+	for _, v := range c.Values {
 		if v.Op != OpPhi {
 			continue
 		}
-		v.Args[i].Uses--
-		v.Args[i] = v.Args[n]
-		v.Args[n] = nil // aid GC
+		v.Args[j].Uses--
+		v.Args[j] = v.Args[n]
+		v.Args[n] = nil
 		v.Args = v.Args[:n]
 		phielimValue(v)
 		// Note: this is trickier than it looks. Replacing

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

@@ -4,7 +4,11 @@
 
 package ssa
 
-import "testing"
+import (
+	"fmt"
+	"strconv"
+	"testing"
+)
 
 func TestDeadLoop(t *testing.T) {
 	c := testConfig(t)
@@ -132,3 +136,26 @@ func TestNestedDeadBlocks(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkDeadCode(b *testing.B) {
+	for _, n := range [...]int{1, 10, 100, 1000, 10000, 100000, 200000} {
+		b.Run(strconv.Itoa(n), func(b *testing.B) {
+			c := testConfig(b)
+			blocks := make([]bloc, 0, n+2)
+			blocks = append(blocks,
+				Bloc("entry",
+					Valu("mem", OpInitMem, TypeMem, 0, nil),
+					Goto("exit")))
+			blocks = append(blocks, Bloc("exit", Exit("mem")))
+			for i := 0; i < n; i++ {
+				blocks = append(blocks, Bloc(fmt.Sprintf("dead%d", i), Goto("exit")))
+			}
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				fun := Fun(c, "entry", blocks...)
+				Deadcode(fun.f)
+				fun.f.Free()
+			}
+		})
+	}
+}

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

@@ -47,7 +47,8 @@ func postorderWithNumbering(f *Func, ponums []int) []*Block {
 			// Children have not been visited yet. Mark as explored
 			// and queue any children we haven't seen yet.
 			mark[b.ID] = explored
-			for _, c := range b.Succs {
+			for _, e := range b.Succs {
+				c := e.b
 				if mark[c.ID] == notFound {
 					mark[c.ID] = notExplored
 					s = append(s, c)
@@ -60,7 +61,7 @@ func postorderWithNumbering(f *Func, ponums []int) []*Block {
 	return order
 }
 
-type linkedBlocks func(*Block) []*Block
+type linkedBlocks func(*Block) []Edge
 
 const nscratchslices = 7
 
@@ -101,8 +102,8 @@ func (cfg *Config) scratchBlocksForDom(maxBlockID int) (a, b, c, d, e, f, g []ID
 }
 
 func dominators(f *Func) []*Block {
-	preds := func(b *Block) []*Block { return b.Preds }
-	succs := func(b *Block) []*Block { return b.Succs }
+	preds := func(b *Block) []Edge { return b.Preds }
+	succs := func(b *Block) []Edge { return b.Succs }
 
 	//TODO: benchmark and try to find criteria for swapping between
 	// dominatorsSimple and dominatorsLT
@@ -135,7 +136,8 @@ func (f *Func) dominatorsLTOrig(entry *Block, predFn linkedBlocks, succFn linked
 		w := vertex[i]
 
 		// step2 in TOPLAS paper
-		for _, v := range predFn(fromID[w]) {
+		for _, e := range predFn(fromID[w]) {
+			v := e.b
 			if semi[v.ID] == 0 {
 				// skip unreachable predecessor
 				// not in original, but we're using existing pred instead of building one.
@@ -199,7 +201,8 @@ func (f *Func) dfsOrig(entry *Block, succFn linkedBlocks, semi, vertex, label, p
 		vertex[n] = v.ID
 		label[v.ID] = v.ID
 		// ancestor[v] already zero
-		for _, w := range succFn(v) {
+		for _, e := range succFn(v) {
+			w := e.b
 			// if it has a dfnum, we've already visited it
 			if semi[w.ID] == 0 {
 				// yes, w can be pushed multiple times.
@@ -265,7 +268,8 @@ func dominatorsSimple(f *Func) []*Block {
 		for i := len(post) - 2; i >= 0; i-- {
 			b := post[i]
 			var d *Block
-			for _, p := range b.Preds {
+			for _, e := range b.Preds {
+				p := e.b
 				if idom[p.ID] == nil {
 					continue
 				}

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

@@ -43,7 +43,8 @@ func flagalloc(f *Func) {
 				}
 			}
 			if flag != nil {
-				for _, p := range b.Preds {
+				for _, e := range b.Preds {
+					p := e.b
 					end[p.ID] = flag
 				}
 			}
@@ -73,9 +74,10 @@ func flagalloc(f *Func) {
 		// The current live flag value the pre-flagalloc copy).
 		var flag *Value
 		if len(b.Preds) > 0 {
-			flag = end[b.Preds[0].ID]
+			flag = end[b.Preds[0].b.ID]
 			// Note: the following condition depends on the lack of critical edges.
-			for _, p := range b.Preds[1:] {
+			for _, e := range b.Preds[1:] {
+				p := e.b
 				if end[p.ID] != flag {
 					f.Fatalf("live flag in %s's predecessors not consistent", b)
 				}

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

@@ -34,7 +34,7 @@ type Func struct {
 	Names []LocalSlot
 
 	freeValues *Value // free Values linked by argstorage[0].  All other fields except ID are 0/nil.
-	freeBlocks *Block // free Blocks linked by succstorage[0].  All other fields except ID are 0/nil.
+	freeBlocks *Block // free Blocks linked by succstorage[0].b.  All other fields except ID are 0/nil.
 
 	idom []*Block   // precomputed immediate dominators
 	sdom sparseTree // precomputed dominator tree
@@ -146,8 +146,8 @@ func (f *Func) NewBlock(kind BlockKind) *Block {
 	var b *Block
 	if f.freeBlocks != nil {
 		b = f.freeBlocks
-		f.freeBlocks = b.succstorage[0]
-		b.succstorage[0] = nil
+		f.freeBlocks = b.succstorage[0].b
+		b.succstorage[0].b = nil
 	} else {
 		ID := f.bid.get()
 		if int(ID) < len(f.Config.blocks) {
@@ -173,7 +173,7 @@ func (f *Func) freeBlock(b *Block) {
 	id := b.ID
 	*b = Block{}
 	b.ID = id
-	b.succstorage[0] = f.freeBlocks
+	b.succstorage[0].b = f.freeBlocks
 	f.freeBlocks = b
 }
 

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

@@ -104,7 +104,7 @@ func Equiv(f, g *Func) bool {
 				return false
 			}
 			for i := range fb.Succs {
-				if !checkBlk(fb.Succs[i], gb.Succs[i]) {
+				if !checkBlk(fb.Succs[i].b, gb.Succs[i].b) {
 					return false
 				}
 			}
@@ -112,7 +112,7 @@ func Equiv(f, g *Func) bool {
 				return false
 			}
 			for i := range fb.Preds {
-				if !checkBlk(fb.Preds[i], gb.Preds[i]) {
+				if !checkBlk(fb.Preds[i].b, gb.Preds[i].b) {
 					return false
 				}
 			}

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

@@ -25,6 +25,11 @@ func fuse(f *Func) {
 //
 // If all Phi ops in ss have identical variables for slots corresponding to
 // s0, s1 and b then the branch can be dropped.
+// This optimization often comes up in switch statements with multiple
+// expressions in a case clause:
+//   switch n {
+//     case 1,2,3: return 4
+//   }
 // TODO: If ss doesn't contain any OpPhis, are s0 and s1 dead code anyway.
 func fuseBlockIf(b *Block) bool {
 	if b.Kind != BlockIf {
@@ -32,17 +37,21 @@ func fuseBlockIf(b *Block) bool {
 	}
 
 	var ss0, ss1 *Block
-	s0 := b.Succs[0]
+	s0 := b.Succs[0].b
+	i0 := b.Succs[0].i
 	if s0.Kind != BlockPlain || len(s0.Preds) != 1 || len(s0.Values) != 0 {
 		s0, ss0 = b, s0
 	} else {
-		ss0 = s0.Succs[0]
+		ss0 = s0.Succs[0].b
+		i0 = s0.Succs[0].i
 	}
-	s1 := b.Succs[1]
+	s1 := b.Succs[1].b
+	i1 := b.Succs[1].i
 	if s1.Kind != BlockPlain || len(s1.Preds) != 1 || len(s1.Values) != 0 {
 		s1, ss1 = b, s1
 	} else {
-		ss1 = s1.Succs[0]
+		ss1 = s1.Succs[0].b
+		i1 = s1.Succs[0].i
 	}
 
 	if ss0 != ss1 {
@@ -52,18 +61,7 @@ func fuseBlockIf(b *Block) bool {
 
 	// s0 and s1 are equal with b if the corresponding block is missing
 	// (2nd, 3rd and 4th case in the figure).
-	i0, i1 := -1, -1
-	for i, p := range ss.Preds {
-		if p == s0 {
-			i0 = i
-		}
-		if p == s1 {
-			i1 = i
-		}
-	}
-	if i0 == -1 || i1 == -1 {
-		b.Fatalf("invalid predecessors")
-	}
+
 	for _, v := range ss.Values {
 		if v.Op == OpPhi && v.Uses > 0 && v.Args[i0] != v.Args[i1] {
 			return false
@@ -76,28 +74,23 @@ func fuseBlockIf(b *Block) bool {
 	// have identical predecessors (verified above).
 	// No critical edge is introduced because b will have one successor.
 	if s0 != b && s1 != b {
-		ss.removePred(s0)
-
-		// Replace edge b->s1->ss with b->ss.
+		// Replace edge b->s0->ss with b->ss.
 		// We need to keep a slot for Phis corresponding to b.
-		for i := range b.Succs {
-			if b.Succs[i] == s1 {
-				b.Succs[i] = ss
-			}
-		}
-		for i := range ss.Preds {
-			if ss.Preds[i] == s1 {
-				ss.Preds[i] = b
-			}
-		}
+		b.Succs[0] = Edge{ss, i0}
+		ss.Preds[i0] = Edge{b, 0}
+		b.removeEdge(1)
+		s1.removeEdge(0)
 	} else if s0 != b {
-		ss.removePred(s0)
+		b.removeEdge(0)
+		s0.removeEdge(0)
 	} else if s1 != b {
-		ss.removePred(s1)
+		b.removeEdge(1)
+		s1.removeEdge(0)
+	} else {
+		b.removeEdge(1)
 	}
 	b.Kind = BlockPlain
 	b.SetControl(nil)
-	b.Succs = append(b.Succs[:0], ss)
 
 	// Trash the empty blocks s0 & s1.
 	if s0 != b {
@@ -120,30 +113,27 @@ func fuseBlockPlain(b *Block) bool {
 		return false
 	}
 
-	c := b.Succs[0]
+	c := b.Succs[0].b
 	if len(c.Preds) != 1 {
 		return false
 	}
 
-	// move all of b'c values to c.
+	// move all of b's values to c.
 	for _, v := range b.Values {
 		v.Block = c
 		c.Values = append(c.Values, v)
 	}
 
 	// replace b->c edge with preds(b) -> c
-	c.predstorage[0] = nil
+	c.predstorage[0] = Edge{}
 	if len(b.Preds) > len(b.predstorage) {
 		c.Preds = b.Preds
 	} else {
 		c.Preds = append(c.predstorage[:0], b.Preds...)
 	}
-	for _, p := range c.Preds {
-		for i, q := range p.Succs {
-			if q == b {
-				p.Succs[i] = c
-			}
-		}
+	for i, e := range c.Preds {
+		p := e.b
+		p.Succs[e.i] = Edge{c, i}
 	}
 	if f := b.Func; f.Entry == b {
 		f.Entry = c

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

 package ssa
 
 import (
+	"fmt"
+	"strconv"
 	"testing"
 )
 
@@ -127,3 +129,41 @@ func TestFuseEliminatesEmptyBlocks(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkFuse(b *testing.B) {
+	for _, n := range [...]int{1, 10, 100, 1000, 10000} {
+		b.Run(strconv.Itoa(n), func(b *testing.B) {
+			c := testConfig(b)
+
+			blocks := make([]bloc, 0, 2*n+3)
+			blocks = append(blocks,
+				Bloc("entry",
+					Valu("mem", OpInitMem, TypeMem, 0, nil),
+					Valu("cond", OpArg, TypeBool, 0, nil),
+					Valu("x", OpArg, TypeInt64, 0, nil),
+					Goto("exit")))
+
+			phiArgs := make([]string, 0, 2*n)
+			for i := 0; i < n; i++ {
+				cname := fmt.Sprintf("c%d", i)
+				blocks = append(blocks,
+					Bloc(fmt.Sprintf("b%d", i), If("cond", cname, "merge")),
+					Bloc(cname, Goto("merge")))
+				phiArgs = append(phiArgs, "x", "x")
+			}
+			blocks = append(blocks,
+				Bloc("merge",
+					Valu("phi", OpPhi, TypeMem, 0, nil, phiArgs...),
+					Goto("exit")),
+				Bloc("exit",
+					Exit("mem")))
+
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				fun := Fun(c, "entry", blocks...)
+				fuse(fun.f)
+				fun.f.Free()
+			}
+		})
+	}
+}

src/cmd/compile/internal/ssa/gen/rulegen.go

@@ -273,35 +273,30 @@ func genRules(arch arch) {
 				log.Fatalf("unmatched successors %v in %s", m, rule)
 			}
 
-			// Modify predecessor lists for no-longer-reachable blocks
-			for succ := range m {
-				fmt.Fprintf(w, "b.Func.removePredecessor(b, %s)\n", succ)
-			}
-
 			fmt.Fprintf(w, "b.Kind = %s\n", blockName(t[0], arch))
 			if t[1] == "nil" {
 				fmt.Fprintf(w, "b.SetControl(nil)\n")
 			} else {
 				fmt.Fprintf(w, "b.SetControl(%s)\n", genResult0(w, arch, t[1], new(int), false, false, rule.loc))
 			}
-			if len(newsuccs) < len(succs) {
-				fmt.Fprintf(w, "b.Succs = b.Succs[:%d]\n", len(newsuccs))
+
+			succChanged := false
+			for i := 0; i < len(succs); i++ {
+				if succs[i] != newsuccs[i] {
+					succChanged = true
 				}
-			for i, a := range newsuccs {
-				fmt.Fprintf(w, "b.Succs[%d] = %s\n", i, a)
 			}
-			// Update branch prediction
-			switch {
-			case len(newsuccs) != 2:
-				fmt.Fprintln(w, "b.Likely = BranchUnknown")
-			case newsuccs[0] == succs[0] && newsuccs[1] == succs[1]:
-				// unchanged
-			case newsuccs[0] == succs[1] && newsuccs[1] == succs[0]:
-				// flipped
-				fmt.Fprintln(w, "b.Likely *= -1")
-			default:
-				// unknown
-				fmt.Fprintln(w, "b.Likely = BranchUnknown")
+			if succChanged {
+				if len(succs) != 2 {
+					log.Fatalf("changed successors, len!=2 in %s", rule)
+				}
+				if succs[0] != newsuccs[1] || succs[1] != newsuccs[0] {
+					log.Fatalf("can only handle swapped successors in %s", rule)
+				}
+				fmt.Fprintln(w, "b.swapSuccessors()")
+			}
+			for i := 0; i < len(succs); i++ {
+				fmt.Fprintf(w, "_ = %s\n", newsuccs[i])
 			}
 
 			if *genLog {

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

@@ -383,7 +383,8 @@ func (b *Block) LongHTML() string {
 	}
 	if len(b.Succs) > 0 {
 		s += " →" // right arrow
-		for _, c := range b.Succs {
+		for _, e := range b.Succs {
+			c := e.b
 			s += " " + c.HTML()
 		}
 	}
@@ -423,7 +424,8 @@ func (p htmlFuncPrinter) startBlock(b *Block, reachable bool) {
 	fmt.Fprintf(p.w, "<li class=\"ssa-start-block\">%s:", b.HTML())
 	if len(b.Preds) > 0 {
 		io.WriteString(p.w, " &#8592;") // left arrow
-		for _, pred := range b.Preds {
+		for _, e := range b.Preds {
+			pred := e.b
 			fmt.Fprintf(p.w, " %s", pred.HTML())
 		}
 	}

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

@@ -39,7 +39,8 @@ blockloop:
 			break
 		}
 
-		for _, c := range b.Succs {
+		for _, e := range b.Succs {
+			c := e.b
 			indegree[c.ID]--
 			if indegree[c.ID] == 0 {
 				posdegree.remove(c.ID)
@@ -54,9 +55,9 @@ blockloop:
 		var likely *Block
 		switch b.Likely {
 		case BranchLikely:
-			likely = b.Succs[0]
+			likely = b.Succs[0].b
 		case BranchUnlikely:
-			likely = b.Succs[1]
+			likely = b.Succs[1].b
 		}
 		if likely != nil && !scheduled[likely.ID] {
 			bid = likely.ID
@@ -66,7 +67,8 @@ blockloop:
 		// Use degree for now.
 		bid = 0
 		mindegree := f.NumBlocks()
-		for _, c := range order[len(order)-1].Succs {
+		for _, e := range order[len(order)-1].Succs {
+			c := e.b
 			if scheduled[c.ID] {
 				continue
 			}

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

@@ -134,11 +134,11 @@ func likelyadjust(f *Func) {
 			// and less influential than inferences from loop structure.
 		case BlockCall, BlockDefer:
 			local[b.ID] = blCALL
-			certain[b.ID] = max8(blCALL, certain[b.Succs[0].ID])
+			certain[b.ID] = max8(blCALL, certain[b.Succs[0].b.ID])
 
 		default:
 			if len(b.Succs) == 1 {
-				certain[b.ID] = certain[b.Succs[0].ID]
+				certain[b.ID] = certain[b.Succs[0].b.ID]
 			} else if len(b.Succs) == 2 {
 				// If successor is an unvisited backedge, it's in loop and we don't care.
 				// Its default unlikely is also zero which is consistent with favoring loop edges.
@@ -146,8 +146,8 @@ func likelyadjust(f *Func) {
 				// default "everything returns" unlikeliness is erased by min with the
 				// backedge likeliness; however a loop with calls on every path will be
 				// tagged with call cost. Net effect is that loop entry is favored.
-				b0 := b.Succs[0].ID
-				b1 := b.Succs[1].ID
+				b0 := b.Succs[0].b.ID
+				b1 := b.Succs[1].b.ID
 				certain[b.ID] = min8(certain[b0], certain[b1])
 
 				l := b2l[b.ID]
@@ -270,7 +270,8 @@ func loopnestfor(f *Func) *loopnest {
 		// and there may be more than one such s.
 		// Since there's at most 2 successors, the inner/outer ordering
 		// between them can be established with simple comparisons.
-		for _, bb := range b.Succs {
+		for _, e := range b.Succs {
+			bb := e.b
 			l := b2l[bb.ID]
 
 			if sdom.isAncestorEq(bb, b) { // Found a loop header
@@ -405,12 +406,12 @@ func (ln *loopnest) findExits() {
 	for _, b := range ln.po {
 		l := b2l[b.ID]
 		if l != nil && len(b.Succs) == 2 {
-			sl := b2l[b.Succs[0].ID]
-			if recordIfExit(l, sl, b.Succs[0]) {
+			sl := b2l[b.Succs[0].b.ID]
+			if recordIfExit(l, sl, b.Succs[0].b) {
 				continue
 			}
-			sl = b2l[b.Succs[1].ID]
-			if recordIfExit(l, sl, b.Succs[1]) {
+			sl = b2l[b.Succs[1].b.ID]
+			if recordIfExit(l, sl, b.Succs[1].b) {
 				continue
 			}
 		}

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

@@ -103,14 +103,14 @@ nextb:
 		// First condition: loop entry has a single predecessor, which
 		// is the header block.  This implies that b.Succs[entry] is
 		// reached iff ind < max.
-		if len(b.Succs[entry].Preds) != 1 {
+		if len(b.Succs[entry].b.Preds) != 1 {
 			// b.Succs[1-entry] must exit the loop.
 			continue
 		}
 
 		// Second condition: b.Succs[entry] dominates nxt so that
 		// nxt is computed when inc < max, meaning nxt <= max.
-		if !f.sdom.isAncestorEq(b.Succs[entry], nxt.Block) {
+		if !f.sdom.isAncestorEq(b.Succs[entry].b, nxt.Block) {
 			// inc+ind can only be reached through the branch that enters the loop.
 			continue
 		}
@@ -150,7 +150,7 @@ nextb:
 			nxt:   nxt,
 			min:   min,
 			max:   max,
-			entry: b.Succs[entry],
+			entry: b.Succs[entry].b,
 		})
 		b.Logf("found induction variable %v (inc = %v, min = %v, max = %v)\n", ind, inc, min, max)
 	}

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

@@ -149,11 +149,11 @@ func checkedptr(b *Block) *Value {
 // predecessor.
 func nonnilptr(b *Block) *Value {
 	if len(b.Preds) == 1 {
-		bp := b.Preds[0]
+		bp := b.Preds[0].b
 		if bp.Kind == BlockCheck {
 			return bp.Control.Args[0]
 		}
-		if bp.Kind == BlockIf && bp.Control.Op == OpIsNonNil && bp.Succs[0] == b {
+		if bp.Kind == BlockIf && bp.Control.Op == OpIsNonNil && bp.Succs[0].b == b {
 			return bp.Control.Args[0]
 		}
 	}

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

@@ -31,6 +31,7 @@ func phielim(f *Func) {
 	}
 }
 
+// phielimValue tries to convert the phi v to a copy.
 func phielimValue(v *Value) bool {
 	if v.Op != OpPhi {
 		return false

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

@@ -30,16 +30,16 @@ func phiopt(f *Func) {
 			continue
 		}
 
-		pb0, b0 := b, b.Preds[0]
+		pb0, b0 := b, b.Preds[0].b
 		for len(b0.Succs) == 1 && len(b0.Preds) == 1 {
-			pb0, b0 = b0, b0.Preds[0]
+			pb0, b0 = b0, b0.Preds[0].b
 		}
 		if b0.Kind != BlockIf {
 			continue
 		}
-		pb1, b1 := b, b.Preds[1]
+		pb1, b1 := b, b.Preds[1].b
 		for len(b1.Succs) == 1 && len(b1.Preds) == 1 {
-			pb1, b1 = b1, b1.Preds[0]
+			pb1, b1 = b1, b1.Preds[0].b
 		}
 		if b1 != b0 {
 			continue
@@ -48,9 +48,9 @@ func phiopt(f *Func) {
 
 		// reverse is the predecessor from which the truth value comes.
 		var reverse int
-		if b0.Succs[0] == pb0 && b0.Succs[1] == pb1 {
+		if b0.Succs[0].b == pb0 && b0.Succs[1].b == pb1 {
 			reverse = 0
-		} else if b0.Succs[0] == pb1 && b0.Succs[1] == pb0 {
+		} else if b0.Succs[0].b == pb1 && b0.Succs[1].b == pb0 {
 			reverse = 1
 		} else {
 			b.Fatalf("invalid predecessors\n")

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

@@ -45,7 +45,8 @@ func (p stringFuncPrinter) startBlock(b *Block, reachable bool) {
 	fmt.Fprintf(p.w, "  b%d:", b.ID)
 	if len(b.Preds) > 0 {
 		io.WriteString(p.w, " <-")
-		for _, pred := range b.Preds {
+		for _, e := range b.Preds {
+			pred := e.b
 			fmt.Fprintf(p.w, " b%d", pred.ID)
 		}
 	}

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

@@ -502,7 +502,7 @@ func prove(f *Func) {
 				b.Kind = BlockFirst
 				b.SetControl(nil)
 				if succ == negative {
-					b.Succs[0], b.Succs[1] = b.Succs[1], b.Succs[0]
+					b.swapSuccessors()
 				}
 			}
 
@@ -525,10 +525,10 @@ func getBranch(sdom sparseTree, p *Block, b *Block) branch {
 	// has one predecessor then (apart from the degenerate case),
 	// there is no path from entry that can reach b through p.Succs[1].
 	// TODO: how about p->yes->b->yes, i.e. a loop in yes.
-	if sdom.isAncestorEq(p.Succs[0], b) && len(p.Succs[0].Preds) == 1 {
+	if sdom.isAncestorEq(p.Succs[0].b, b) && len(p.Succs[0].b.Preds) == 1 {
 		return positive
 	}
-	if sdom.isAncestorEq(p.Succs[1], b) && len(p.Succs[1].Preds) == 1 {
+	if sdom.isAncestorEq(p.Succs[1].b, b) && len(p.Succs[1].b.Preds) == 1 {
 		return negative
 	}
 	return unknown

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

@@ -488,11 +488,12 @@ func (s *regAllocState) init(f *Func) {
 	s.primary = make([]int32, f.NumBlocks())
 	for _, b := range f.Blocks {
 		best := -1
-		for i, p := range b.Preds {
+		for i, e := range b.Preds {
+			p := e.b
 			if blockOrder[p.ID] >= blockOrder[b.ID] {
 				continue // backward edge
 			}
-			if best == -1 || blockOrder[p.ID] > blockOrder[b.Preds[best].ID] {
+			if best == -1 || blockOrder[p.ID] > blockOrder[b.Preds[best].b.ID] {
 				best = i
 			}
 		}
@@ -706,7 +707,7 @@ func (s *regAllocState) regalloc(f *Func) {
 			}
 		} else if len(b.Preds) == 1 {
 			// Start regalloc state with the end state of the previous block.
-			s.setState(s.endRegs[b.Preds[0].ID])
+			s.setState(s.endRegs[b.Preds[0].b.ID])
 			if nphi > 0 {
 				f.Fatalf("phis in single-predecessor block")
 			}
@@ -731,7 +732,7 @@ func (s *regAllocState) regalloc(f *Func) {
 			if idx < 0 {
 				f.Fatalf("block with no primary predecessor %s", b)
 			}
-			p := b.Preds[idx]
+			p := b.Preds[idx].b
 			s.setState(s.endRegs[p.ID])
 
 			if s.f.pass.debug > regDebug {
@@ -859,13 +860,14 @@ func (s *regAllocState) regalloc(f *Func) {
 		// desired registers computed during liveness analysis.
 		// Note that we do this phase after startRegs is set above, so that
 		// we get the right behavior for a block which branches to itself.
-		for _, succ := range b.Succs {
+		for _, e := range b.Succs {
+			succ := e.b
 			// TODO: prioritize likely successor?
 			for _, x := range s.startRegs[succ.ID] {
 				desired.add(x.vid, x.r)
 			}
 			// Process phi ops in succ.
-			pidx := predIdx(succ, b)
+			pidx := e.i
 			for _, v := range succ.Values {
 				if v.Op != OpPhi {
 					break
@@ -1194,7 +1196,7 @@ func (s *regAllocState) regalloc(f *Func) {
 		// the merge point and promote them to registers now.
 		if len(b.Succs) == 1 {
 			// For this to be worthwhile, the loop must have no calls in it.
-			top := b.Succs[0]
+			top := b.Succs[0].b
 			loop := s.loopnest.b2l[top.ID]
 			if loop == nil || loop.header != top || loop.containsCall {
 				goto badloop
@@ -1452,7 +1454,7 @@ sinking:
 			if len(d.Preds) > 1 {
 				panic("Should be impossible given critical edges removed")
 			}
-			p := d.Preds[0] // block in loop exiting to d.
+			p := d.Preds[0].b // block in loop exiting to d.
 
 			endregs := s.endRegs[p.ID]
 			for _, regrec := range endregs {
@@ -1570,7 +1572,8 @@ func (s *regAllocState) shuffle(stacklive [][]ID) {
 			continue
 		}
 		e.b = b
-		for i, p := range b.Preds {
+		for i, edge := range b.Preds {
+			p := edge.b
 			e.p = p
 			e.setup(i, s.endRegs[p.ID], s.startRegs[b.ID], stacklive[p.ID])
 			e.process()
@@ -2112,18 +2115,19 @@ func (s *regAllocState) computeLive() {
 
 			// For each predecessor of b, expand its list of live-at-end values.
 			// invariant: live contains the values live at the start of b (excluding phi inputs)
-			for i, p := range b.Preds {
+			for i, e := range b.Preds {
+				p := e.b
 				// Compute additional distance for the edge.
 				// Note: delta must be at least 1 to distinguish the control
 				// value use from the first user in a successor block.
 				delta := int32(normalDistance)
 				if len(p.Succs) == 2 {
-					if p.Succs[0] == b && p.Likely == BranchLikely ||
-						p.Succs[1] == b && p.Likely == BranchUnlikely {
+					if p.Succs[0].b == b && p.Likely == BranchLikely ||
+						p.Succs[1].b == b && p.Likely == BranchUnlikely {
 						delta = likelyDistance
 					}
-					if p.Succs[0] == b && p.Likely == BranchUnlikely ||
-						p.Succs[1] == b && p.Likely == BranchLikely {
+					if p.Succs[0].b == b && p.Likely == BranchUnlikely ||
+						p.Succs[1].b == b && p.Likely == BranchLikely {
 						delta = unlikelyDistance
 					}
 				}

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

@@ -317,7 +317,7 @@ func mergePoint(b *Block, a ...*Value) *Block {
 			// Don't know which way to go back. Abort.
 			return nil
 		}
-		b = b.Preds[0]
+		b = b.Preds[0].b
 		d--
 	}
 	return nil // too far away
@@ -341,7 +341,7 @@ found:
 		if len(b.Preds) > 1 {
 			return nil
 		}
-		b = b.Preds[0]
+		b = b.Preds[0].b
 		d--
 
 	}

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

@@ -279,8 +279,8 @@ func rewriteBlockARM(b *Block) bool {
 			no := b.Succs[1]
 			b.Kind = BlockARMLT
 			b.SetControl(cc)
-			b.Succs[0] = yes
-			b.Succs[1] = no
+			_ = yes
+			_ = no
 			return true
 		}
 	}

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

@@ -9777,8 +9777,7 @@ func rewriteBlockgeneric(b *Block) bool {
 			next := b.Succs[0]
 			b.Kind = BlockPlain
 			b.SetControl(nil)
-			b.Succs[0] = next
-			b.Likely = BranchUnknown
+			_ = next
 			return true
 		}
 	case BlockIf:
@@ -9795,9 +9794,9 @@ func rewriteBlockgeneric(b *Block) bool {
 			no := b.Succs[1]
 			b.Kind = BlockIf
 			b.SetControl(cond)
-			b.Succs[0] = no
-			b.Succs[1] = yes
-			b.Likely *= -1
+			b.swapSuccessors()
+			_ = no
+			_ = yes
 			return true
 		}
 		// match: (If (ConstBool [c]) yes no)
@@ -9816,8 +9815,8 @@ func rewriteBlockgeneric(b *Block) bool {
 			}
 			b.Kind = BlockFirst
 			b.SetControl(nil)
-			b.Succs[0] = yes
-			b.Succs[1] = no
+			_ = yes
+			_ = no
 			return true
 		}
 		// match: (If (ConstBool [c]) yes no)
@@ -9836,9 +9835,9 @@ func rewriteBlockgeneric(b *Block) bool {
 			}
 			b.Kind = BlockFirst
 			b.SetControl(nil)
-			b.Succs[0] = no
-			b.Succs[1] = yes
-			b.Likely *= -1
+			b.swapSuccessors()
+			_ = no
+			_ = yes
 			return true
 		}
 	}

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

@@ -28,14 +28,15 @@ func shortcircuit(f *Func) {
 				continue
 			}
 			for i, a := range v.Args {
-				p := b.Preds[i]
+				e := b.Preds[i]
+				p := e.b
 				if p.Kind != BlockIf {
 					continue
 				}
 				if p.Control != a {
 					continue
 				}
-				if p.Succs[0] == b {
+				if e.i == 0 {
 					v.SetArg(i, ct)
 				} else {
 					v.SetArg(i, cf)
@@ -91,39 +92,37 @@ func shortcircuit(f *Func) {
 			}
 
 			// The predecessor we come in from.
-			p := b.Preds[i]
+			e1 := b.Preds[i]
+			p := e1.b
+			pi := e1.i
+
 			// The successor we always go to when coming in
 			// from that predecessor.
-			t := b.Succs[1-a.AuxInt]
+			e2 := b.Succs[1-a.AuxInt]
+			t := e2.b
+			ti := e2.i
 
-			// Change the edge p->b to p->t.
-			for j, x := range p.Succs {
-				if x == b {
-					p.Succs[j] = t
-					break
-				}
-			}
+			// Remove b's incoming edge from p.
+			b.removePred(i)
+			n := len(b.Preds)
+			v.Args[i].Uses--
+			v.Args[i] = v.Args[n]
+			v.Args[n] = nil
+			v.Args = v.Args[:n]
+
+			// Redirect p's outgoing edge to t.
+			p.Succs[pi] = Edge{t, len(t.Preds)}
 
 			// Fix up t to have one more predecessor.
-			j := predIdx(t, b)
-			t.Preds = append(t.Preds, p)
+			t.Preds = append(t.Preds, Edge{p, pi})
 			for _, w := range t.Values {
 				if w.Op != OpPhi {
 					continue
 				}
-				w.AddArg(w.Args[j])
+				w.AddArg(w.Args[ti])
 			}
 
-			// Fix up b to have one less predecessor.
-			n := len(b.Preds) - 1
-			b.Preds[i] = b.Preds[n]
-			b.Preds[n] = nil
-			b.Preds = b.Preds[:n]
-			v.Args[i].Uses--
-			v.Args[i] = v.Args[n]
-			v.Args[n] = nil
-			v.Args = v.Args[:n]
-			if n == 1 {
+			if len(b.Preds) == 1 {
 				v.Op = OpCopy
 				// No longer a phi, stop optimizing here.
 				break
@@ -132,14 +131,3 @@ func shortcircuit(f *Func) {
 		}
 	}
 }
-
-// predIdx returns the index where p appears in the predecessor list of b.
-// p must be in the predecessor list of b.
-func predIdx(b, p *Block) int {
-	for i, x := range b.Preds {
-		if x == p {
-			return i
-		}
-	}
-	panic("predecessor not found")
-}

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

@@ -23,7 +23,7 @@ func TestSizeof(t *testing.T) {
 		_64bit uintptr     // size on 64bit platforms
 	}{
 		{Value{}, 68, 112},
-		{Block{}, 124, 232},
+		{Block{}, 148, 288},
 	}
 
 	for _, tt := range tests {

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

@@ -304,7 +304,8 @@ func (s *stackAllocState) computeLive(spillLive [][]ID) {
 
 			// for each predecessor of b, expand its list of live-at-end values
 			// invariant: s contains the values live at the start of b (excluding phi inputs)
-			for i, p := range b.Preds {
+			for i, e := range b.Preds {
+				p := e.b
 				t.clear()
 				t.addAll(s.live[p.ID])
 				t.addAll(live.contents())

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

@@ -7,31 +7,26 @@ package ssa
 // trim removes blocks with no code in them.
 // These blocks were inserted to remove critical edges.
 func trim(f *Func) {
-	i := 0
+	n := 0
 	for _, b := range f.Blocks {
 		if b.Kind != BlockPlain || len(b.Values) != 0 || len(b.Preds) != 1 {
-			f.Blocks[i] = b
-			i++
+			f.Blocks[n] = b
+			n++
 			continue
 		}
 		// TODO: handle len(b.Preds)>1 case.
 
 		// Splice b out of the graph.
-		pred := b.Preds[0]
-		succ := b.Succs[0]
-		for j, s := range pred.Succs {
-			if s == b {
-				pred.Succs[j] = succ
+		p := b.Preds[0].b
+		i := b.Preds[0].i
+		s := b.Succs[0].b
+		j := b.Succs[0].i
+		p.Succs[i] = Edge{s, j}
+		s.Preds[j] = Edge{p, i}
 	}
+	tail := f.Blocks[n:]
+	for i := range tail {
+		tail[i] = nil
 	}
-		for j, p := range succ.Preds {
-			if p == b {
-				succ.Preds[j] = pred
-			}
-		}
-	}
-	for j := i; j < len(f.Blocks); j++ {
-		f.Blocks[j] = nil
-	}
-	f.Blocks = f.Blocks[:i]
+	f.Blocks = f.Blocks[:n]
 }