[dev.ssa] cmd/compile/internal/ssa: transform degenerate control blocks

* In cases where we end up with empty branches like in
if a then jmp b else jmp b;
the flow can be replaced by a; jmp b.

The following functions is optimized as follows:
func f(a bool, x int) int {
        v := 0
        if a {
                v = -1
        } else {
                v = -1
        }
        return x | v
}

Before this change:
02819 (arith_ssa.go:362)  VARDEF "".~r2+16(FP)
02820 (arith_ssa.go:362)  MOVQ  $0, "".~r2+16(FP)
02821 (arith_ssa.go:362)  MOVB  "".a(FP), AX
02822 (arith_ssa.go:362)  TESTB AX, AX
02823 (arith_ssa.go:364)  JEQ 2824
02824 (arith_ssa.go:369)  VARDEF "".~r2+16(FP)
02825 (arith_ssa.go:369)  MOVQ  $-1, "".~r2+16(FP)
02826 (arith_ssa.go:369)  RET

After this change:
02819 (arith_ssa.go:362)  VARDEF "".~r2+16(FP)
02820 (arith_ssa.go:369)  VARDEF "".~r2+16(FP)
02821 (arith_ssa.go:369)  MOVQ  $-1, "".~r2+16(FP)
02822 (arith_ssa.go:369)  RET

Updates #14277

Change-Id: Ibe7d284f43406c704903632a4fcf2a4a64059686
Reviewed-on: https://go-review.googlesource.com/19464
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -6,43 +6,131 @@ package ssa
 
 // fuse simplifies control flow by joining basic blocks.
 func fuse(f *Func) {
-	for _, b := range f.Blocks {
-		if b.Kind != BlockPlain {
-			continue
+	for changed := true; changed; {
+		changed = false
+		for _, b := range f.Blocks {
+			changed = fuseBlockIf(b) || changed
+			changed = fuseBlockPlain(b) || changed
 		}
-		c := b.Succs[0]
-		if len(c.Preds) != 1 {
-			continue
-		}
-
-		// 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
-		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
-				}
-			}
-		}
-		if f.Entry == b {
-			f.Entry = c
-		}
-
-		// trash b, just in case
-		b.Kind = BlockInvalid
-		b.Values = nil
-		b.Preds = nil
-		b.Succs = nil
 	}
 }
+
+// fuseBlockIf handles the following cases where s0 and s1 are empty blocks.
+//
+//   b       b          b
+//  / \      | \      / |
+// s0  s1    |  s1   s0 |
+//  \ /      | /      \ |
+//   ss      ss        ss
+//
+// If ss doesn't contain any Phi ops and s0 & s1 are empty then the branch
+// can be dropped.
+// TODO: If ss doesn't contain any Phi ops, are s0 and s1 dead code anyway?
+func fuseBlockIf(b *Block) bool {
+	if b.Kind != BlockIf {
+		return false
+	}
+
+	var ss0, ss1 *Block
+	s0 := b.Succs[0]
+	if s0.Kind != BlockPlain || len(s0.Preds) != 1 || len(s0.Values) != 0 {
+		s0, ss0 = nil, s0
+	} else {
+		ss0 = s0.Succs[0]
+	}
+	s1 := b.Succs[1]
+	if s1.Kind != BlockPlain || len(s1.Preds) != 1 || len(s1.Values) != 0 {
+		s1, ss1 = nil, s1
+	} else {
+		ss1 = s1.Succs[0]
+	}
+
+	if ss0 != ss1 {
+		return false
+	}
+	ss := ss0
+
+	// TODO: Handle OpPhi operations. We can still replace OpPhi if the
+	// slots corresponding to b, s0 and s1 point to the same variable.
+	for _, v := range ss.Values {
+		if v.Op == OpPhi {
+			return false
+		}
+	}
+
+	// Now we have two following b->ss, b->s0->ss and b->s1->ss,
+	// with s0 and s1 empty if exist.
+	// We can replace it with b->ss without if ss has no phis
+	// which is checked above.
+	// No critical edge is introduced because b will have one successor.
+	if s0 != nil {
+		ss.removePred(s0)
+	}
+	if s1 != nil {
+		ss.removePred(s1)
+	}
+	if s0 != nil && s1 != nil {
+		// Add an edge if both edges are removed, otherwise b is no longer connected to ss.
+		ss.Preds = append(ss.Preds, b)
+	}
+	b.Kind = BlockPlain
+	b.Control = nil
+	b.Succs = append(b.Succs[:0], ss)
+
+	// Trash the empty blocks s0 & s1.
+	if s0 != nil {
+		s0.Kind = BlockInvalid
+		s0.Values = nil
+		s0.Succs = nil
+		s0.Preds = nil
+	}
+	if s1 != nil {
+		s1.Kind = BlockInvalid
+		s1.Values = nil
+		s1.Succs = nil
+		s1.Preds = nil
+	}
+	return true
+}
+
+func fuseBlockPlain(b *Block) bool {
+	if b.Kind != BlockPlain {
+		return false
+	}
+
+	c := b.Succs[0]
+	if len(c.Preds) != 1 {
+		return false
+	}
+
+	// move all of b'c 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
+	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
+			}
+		}
+	}
+	if f := b.Func; f.Entry == b {
+		f.Entry = c
+	}
+
+	// trash b, just in case
+	b.Kind = BlockInvalid
+	b.Values = nil
+	b.Preds = nil
+	b.Succs = nil
+	return true
+}