cmd/compile: reuse blocks in critical pass

If a phi has duplicate arguments, then the new block that is constructed
to remove the critical edge can be used for all of the duplicate
arguments.

read-only data = -904 bytes (-0.058308%)
global text (code) = -2240 bytes (-0.060056%)
Total difference -3144 bytes (-0.056218%)

Change-Id: Iee3762744d6a8c9d26cdfa880bb23feb62b03c9c
Reviewed-on: https://go-review.googlesource.com/20746
Run-TryBot: Todd Neal <todd@tneal.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -8,28 +8,85 @@ package ssa
 // more than one outedge to a block with more than one inedge).
 // Regalloc wants a critical-edge-free CFG so it can implement phi values.
 func critical(f *Func) {
-	for _, b := range f.Blocks {
+	// maps from phi arg ID to the new block created for that argument
+	blocks := make([]*Block, f.NumValues())
+	// need to iterate over f.Blocks without range, as we might
+	// need to split critical edges on newly constructed blocks
+	for j := 0; j < len(f.Blocks); j++ {
+		b := f.Blocks[j]
 		if len(b.Preds) <= 1 {
 			continue
 		}
 
+		var phi *Value
+		// determine if we've only got a single phi in this
+		// block, this is easier to handle than the general
+		// case of a block with multiple phi values.
+		for _, v := range b.Values {
+			if v.Op == OpPhi {
+				if phi != nil {
+					phi = nil
+					break
+				}
+				phi = v
+			}
+		}
+
+		// reset our block map
+		if phi != nil {
+			for _, v := range phi.Args {
+				blocks[v.ID] = nil
+			}
+		}
+
 		// split input edges coming from multi-output blocks.
-		for i, c := range b.Preds {
+		for i := 0; i < len(b.Preds); i++ {
+			c := b.Preds[i]
 			if c.Kind == BlockPlain {
 				continue // only single output block
 			}
 
-			// allocate a new block to place on the edge
-			d := f.NewBlock(BlockPlain)
-			d.Line = c.Line
-			if f.pass.debug > 0 {
-				f.Config.Warnl(c.Line, "split critical edge")
+			var d *Block         // new block used to remove critical edge
+			reusedBlock := false // if true, then this is not the first use of this block
+			if phi != nil {
+				argID := phi.Args[i].ID
+				// find or record the block that we used to split
+				// critical edges for this argument
+				if d = blocks[argID]; d == nil {
+					d = f.NewBlock(BlockPlain)
+					d.Line = c.Line
+					blocks[argID] = d
+					if f.pass.debug > 0 {
+						f.Config.Warnl(c.Line, "split critical edge")
+					}
+				} else {
+					reusedBlock = true
+				}
+			} else {
+				// no existing block, so allocate a new block
+				// to place on the edge
+				d = f.NewBlock(BlockPlain)
+				d.Line = c.Line
+				if f.pass.debug > 0 {
+					f.Config.Warnl(c.Line, "split critical edge")
+				}
+			}
+
+			// if this not the first argument for the
+			// block, then we need to remove the
+			// corresponding elements from the block
+			// predecessors and phi args
+			if reusedBlock {
+				d.Preds = append(d.Preds, c)
+				b.Preds[i] = nil
+				phi.Args[i] = nil
+			} else {
+				// splice it in
+				d.Preds = append(d.Preds, c)
+				d.Succs = append(d.Succs, b)
+				b.Preds[i] = d
 			}
 
-			// splice it in
-			d.Preds = append(d.Preds, c)
-			d.Succs = append(d.Succs, b)
-			b.Preds[i] = d
 			// replace b with d in c's successor list.
 			for j, b2 := range c.Succs {
 				if b2 == b {
@@ -38,5 +95,33 @@ func critical(f *Func) {
 				}
 			}
 		}
+
+		// clean up phi's args and b's predecessor list
+		if phi != nil {
+			phi.Args = filterNilValues(phi.Args)
+			b.Preds = filterNilBlocks(b.Preds)
+		}
+	}
+}
+
+// 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
 }