[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>
diff --git a/src/cmd/compile/internal/ssa/shortcircuit.go b/src/cmd/compile/internal/ssa/shortcircuit.go
new file mode 100644
index 0000000..d22a61a
--- /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")
+}
