1 files changed, 86 insertions, 0 deletions
diff --git a/src/cmd/compile/internal/ssa/phiopt.go b/src/cmd/compile/internal/ssa/phiopt.go
new file mode 100644
index 0000000000..fb17727242
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/phiopt.go
@@ -0,0 +1,86 @@
+package ssa
+
+// phiopt eliminates boolean Phis based on the previous if.
+//
+// Main use case is to transform:
+//   x := false
+//   if b {
+//     x = true
+//   }
+// into x = b.
+//
+// In SSA code this appears as
+//
+// b0
+//   If b -> b1 b2
+// b1
+//   Plain -> b2
+// b2
+//   x = (OpPhi (ConstBool [true]) (ConstBool [false]))
+//
+// In this case we can replace x with a copy of b.
+func phiopt(f *Func) {
+	for _, b := range f.Blocks {
+		if len(b.Preds) != 2 || len(b.Values) == 0 {
+			continue
+		}
+
+		pb0, b0 := b, b.Preds[0]
+		for b0.Kind != BlockIf && len(b0.Preds) == 1 {
+			pb0, b0 = b0, b0.Preds[0]
+		}
+		if b0.Kind != BlockIf {
+			continue
+		}
+		pb1, b1 := b, b.Preds[1]
+		for b1.Kind != BlockIf && len(b1.Preds) == 1 {
+			pb1, b1 = b1, b1.Preds[0]
+		}
+		if b1 != b0 {
+			continue
+		}
+		// b0 is the if block giving the boolean value.
+
+		var reverse bool
+		if b0.Succs[0] == pb0 && b0.Succs[1] == pb1 {
+			reverse = false
+		} else if b0.Succs[0] == pb1 && b0.Succs[1] == pb0 {
+			reverse = true
+		} else {
+			b.Fatalf("invalid predecessors\n")
+		}
+
+		for _, v := range b.Values {
+			if v.Op != OpPhi || !v.Type.IsBoolean() || v.Args[0].Op != OpConstBool || v.Args[1].Op != OpConstBool {
+				continue
+			}
+
+			ok, isCopy := false, false
+			if v.Args[0].AuxInt == 1 && v.Args[1].AuxInt == 0 {
+				ok, isCopy = true, !reverse
+			} else if v.Args[0].AuxInt == 0 && v.Args[1].AuxInt == 1 {
+				ok, isCopy = true, reverse
+			}
+
+			// (Phi (ConstBool [x]) (ConstBool [x])) is already handled by opt / phielim.
+
+			if ok && isCopy {
+				if f.pass.debug > 0 {
+					f.Config.Warnl(int(b.Line), "converted OpPhi to OpCopy")
+				}
+				v.reset(OpCopy)
+				v.AddArg(b0.Control)
+				continue
+			}
+			if ok && !isCopy {
+				if f.pass.debug > 0 {
+					f.Config.Warnl(int(b.Line), "converted OpPhi to OpNot")
+				}
+				v.reset(OpNot)
+				v.AddArg(b0.Control)
+				continue
+			}
+		}
+	}
+
+}