cmd/compile: lazily create true and false Values in shortcircuit

It is mildly wasteful to always create values
that must sometimes then be dead code eliminated.
Given that it is very easy to avoid, do so.

Noticed when examining a package with thousands
of generated wrappers, each of which uses
only a handful of Values to compile.

Change-Id: If02eb4aa786dfa20f7aa43e8d729dad8b3db2786
Reviewed-on: https://go-review.googlesource.com/41502
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

diff --git a/src/cmd/compile/internal/ssa/shortcircuit.go b/src/cmd/compile/internal/ssa/shortcircuit.go
index 54e186980d23c050b534f21fc19cef6109b107c9..506be4e7a0752c3d142ca1b967033131389c67ea 100644 (file)
--- a/src/cmd/compile/internal/ssa/shortcircuit.go
+++ b/src/cmd/compile/internal/ssa/shortcircuit.go
@@ -17,8 +17,7 @@ func shortcircuit(f *Func) {
        //    x = phi(a, ...)
        //
        // We can replace the "a" in the phi with the constant true.
-       ct := f.ConstBool(f.Entry.Pos, f.Config.Types.Bool, true)
-       cf := f.ConstBool(f.Entry.Pos, f.Config.Types.Bool, false)
+       var ct, cf *Value
        for _, b := range f.Blocks {
                for _, v := range b.Values {
                        if v.Op != OpPhi {
@@ -37,8 +36,14 @@ func shortcircuit(f *Func) {
                                        continue
                                }
                                if e.i == 0 {
+                                       if ct == nil {
+                                               ct = f.ConstBool(f.Entry.Pos, f.Config.Types.Bool, true)
+                                       }
                                        v.SetArg(i, ct)
                                } else {
+                                       if cf == nil {
+                                               cf = f.ConstBool(f.Entry.Pos, f.Config.Types.Bool, false)
+                                       }
                                        v.SetArg(i, cf)
                                }
                        }