[dev.ssa] cmd/compile: optimization for && and || expressions

Compiling && and || expressions often leads to control
flow of the following form:

p:
  If a goto b else c
b: <- p ...
  x = phi(a, ...)
  If x goto t else u

Note that if we take the edge p->b, then we are guaranteed
to take the edge b->t also.  So in this situation, we might
as well go directly from p to t.

Change-Id: I6974f1e6367119a2ddf2014f9741fdb490edcc12
Reviewed-on: https://go-review.googlesource.com/18910
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 144 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/shortcircuit.go b/src/cmd/compile/internal/ssa/shortcircuit.go
new file mode 100644
index 0000000000..d22a61a0af
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/shortcircuit.go
@@ -0,0 +1,144 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ssa
+
+// Shortcircuit finds situations where branch directions
+// are always correlated and rewrites the CFG to take
+// advantage of that fact.
+// This optimization is useful for compiling && and || expressions.
+func shortcircuit(f *Func) {
+	// Step 1: Replace a phi arg with a constant if that arg
+	// is the control value of a preceding If block.
+	// b1:
+	//    If a goto b2 else b3
+	// b2: <- b1 ...
+	//    x = phi(a, ...)
+	//
+	// We can replace the "a" in the phi with the constant true.
+	ct := f.ConstBool(f.Entry.Line, f.Config.fe.TypeBool(), true)
+	cf := f.ConstBool(f.Entry.Line, f.Config.fe.TypeBool(), false)
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			if v.Op != OpPhi {
+				continue
+			}
+			if !v.Type.IsBoolean() {
+				continue
+			}
+			for i, a := range v.Args {
+				p := b.Preds[i]
+				if p.Kind != BlockIf {
+					continue
+				}
+				if p.Control != a {
+					continue
+				}
+				if p.Succs[0] == b {
+					v.Args[i] = ct
+				} else {
+					v.Args[i] = cf
+				}
+			}
+		}
+	}
+
+	// Step 2: Compute which values are live across blocks.
+	live := make([]bool, f.NumValues())
+	for _, b := range f.Blocks {
+		for _, v := range b.Values {
+			for _, a := range v.Args {
+				if a.Block != v.Block {
+					live[a.ID] = true
+				}
+			}
+		}
+		if b.Control != nil && b.Control.Block != b {
+			live[b.Control.ID] = true
+		}
+	}
+
+	// Step 3: Redirect control flow around known branches.
+	// p:
+	//   ... goto b ...
+	// b: <- p ...
+	//   v = phi(true, ...)
+	//   if v goto t else u
+	// We can redirect p to go directly to t instead of b.
+	// (If v is not live after b).
+	for _, b := range f.Blocks {
+		if b.Kind != BlockIf {
+			continue
+		}
+		if len(b.Values) != 1 {
+			continue
+		}
+		v := b.Values[0]
+		if v.Op != OpPhi {
+			continue
+		}
+		if b.Control != v {
+			continue
+		}
+		if live[v.ID] {
+			continue
+		}
+		for i := 0; i < len(v.Args); i++ {
+			a := v.Args[i]
+			if a.Op != OpConstBool {
+				continue
+			}
+
+			// The predecessor we come in from.
+			p := b.Preds[i]
+			// The successor we always go to when coming in
+			// from that predecessor.
+			t := b.Succs[1-a.AuxInt]
+
+			// Change the edge p->b to p->t.
+			for j, x := range p.Succs {
+				if x == b {
+					p.Succs[j] = t
+					break
+				}
+			}
+
+			// Fix up t to have one more predecessor.
+			j := predIdx(t, b)
+			t.Preds = append(t.Preds, p)
+			for _, w := range t.Values {
+				if w.Op != OpPhi {
+					continue
+				}
+				w.Args = append(w.Args, w.Args[j])
+			}
+
+			// 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] = v.Args[n]
+			v.Args[n] = nil
+			v.Args = v.Args[:n]
+			if n == 1 {
+				v.Op = OpCopy
+				// No longer a phi, stop optimizing here.
+				break
+			}
+			i--
+		}
+	}
+}
+
+// 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")
+}