[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, 50 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/shortcircuit_test.go b/src/cmd/compile/internal/ssa/shortcircuit_test.go
new file mode 100644
index 0000000000..d518dfbabf
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/shortcircuit_test.go
@@ -0,0 +1,50 @@
+// 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
+
+import "testing"
+
+func TestShortCircuit(t *testing.T) {
+	c := testConfig(t)
+
+	fun := Fun(c, "entry",
+		Bloc("entry",
+			Valu("mem", OpInitMem, TypeMem, 0, ".mem"),
+			Valu("arg1", OpArg, TypeInt64, 0, nil),
+			Valu("arg2", OpArg, TypeInt64, 0, nil),
+			Valu("arg3", OpArg, TypeInt64, 0, nil),
+			Goto("b1")),
+		Bloc("b1",
+			Valu("cmp1", OpLess64, TypeBool, 0, nil, "arg1", "arg2"),
+			If("cmp1", "b2", "b3")),
+		Bloc("b2",
+			Valu("cmp2", OpLess64, TypeBool, 0, nil, "arg2", "arg3"),
+			Goto("b3")),
+		Bloc("b3",
+			Valu("phi2", OpPhi, TypeBool, 0, nil, "cmp1", "cmp2"),
+			If("phi2", "b4", "b5")),
+		Bloc("b4",
+			Valu("cmp3", OpLess64, TypeBool, 0, nil, "arg3", "arg1"),
+			Goto("b5")),
+		Bloc("b5",
+			Valu("phi3", OpPhi, TypeBool, 0, nil, "phi2", "cmp3"),
+			If("phi3", "b6", "b7")),
+		Bloc("b6",
+			Exit("mem")),
+		Bloc("b7",
+			Exit("mem")))
+
+	CheckFunc(fun.f)
+	shortcircuit(fun.f)
+	CheckFunc(fun.f)
+
+	for _, b := range fun.f.Blocks {
+		for _, v := range b.Values {
+			if v.Op == OpPhi {
+				t.Errorf("phi %s remains", v)
+			}
+		}
+	}
+}