[dev.ssa] cmd/compile/internal/ssa: handle rewrite of Phis.

* Phis can have variable number of arguments, but rulegen assumed that
each operation has fixed number of arguments.
* Rewriting Phis is necessary to handle the following case:

func f1_ssa(a bool, x int) int {
        v := 0
        if a {
                v = -1
        } else {
                v = -1
        }
        return x|v
}

Change-Id: Iff6bd411b854f3d1d6d3ce21934bf566757094f2
Reviewed-on: https://go-review.googlesource.com/19412Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/testdata/arith_ssa.go

@@ -358,6 +358,26 @@ func testSubConst() {
 	}
 }
 
+//go:noinline
+func orPhi_ssa(a bool, x int) int {
+	v := 0
+	if a {
+		v = -1
+	} else {
+		v = -1
+	}
+	return x | v
+}
+
+func testOrPhi() {
+	if want, got := -1, orPhi_ssa(true, 4); got != want {
+		println("orPhi_ssa(true, 4)=", got, " want ", want)
+	}
+	if want, got := -1, orPhi_ssa(false, 0); got != want {
+		println("orPhi_ssa(false, 0)=", got, " want ", want)
+	}
+}
+
 var failed = false
 
 func main() {

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -263,6 +263,12 @@
 (Sub8 (Add8 x y) x) -> y
 (Sub8 (Add8 x y) y) -> x
 
+// basic phi simplifications
+(Phi (Const8 [c]) (Const8 [d])) && int8(c) == int8(d) -> (Const8 [c])
+(Phi (Const16 [c]) (Const16 [d])) && int16(c) == int16(d) -> (Const16 [c])
+(Phi (Const32 [c]) (Const32 [d])) && int32(c) == int32(d) -> (Const32 [c])
+(Phi (Const64 [c]) (Const64 [c])) -> (Const64 [c])
+
 // user nil checks
 (NeqPtr p (ConstNil)) -> (IsNonNil p)
 (NeqPtr (ConstNil) p) -> (IsNonNil p)

src/cmd/compile/internal/ssa/gen/genericOps.go

@@ -240,8 +240,8 @@ var genericOps = []opData{
 	{name: "Sqrt"}, // sqrt(arg0), float64 only
 
 	// Data movement
-	{name: "Phi"},  // select an argument based on which predecessor block we came from
-	{name: "Copy"}, // output = arg0
+	{name: "Phi", variableLength: true}, // select an argument based on which predecessor block we came from
+	{name: "Copy"},                      // output = arg0
 	// Convert converts between pointers and integers.
 	// We have a special op for this so as to not confuse GC
 	// (particularly stack maps).  It takes a memory arg so it

src/cmd/compile/internal/ssa/gen/main.go

@@ -32,6 +32,7 @@ type opData struct {
 	typ               string // default result type
 	aux               string
 	rematerializeable bool
+	variableLength    bool // if true the operation has a variable number of arguments
 }
 
 type blockData struct {

src/cmd/compile/internal/ssa/gen/rulegen.go

@@ -395,6 +395,23 @@ func genMatch0(w io.Writer, arch arch, match, v string, m map[string]string, top
 			argnum++
 		}
 	}
+
+	variableLength := false
+	for _, op := range genericOps {
+		if op.name == s[0] {
+			variableLength = op.variableLength
+			break
+		}
+	}
+	for _, op := range arch.ops {
+		if op.name == s[0] {
+			variableLength = op.variableLength
+			break
+		}
+	}
+	if variableLength {
+		fmt.Fprintf(w, "if len(%s.Args) != %d {\nbreak\n}\n", v, argnum)
+	}
 }
 
 func genResult(w io.Writer, arch arch, result string) {

src/cmd/compile/internal/ssa/rewritegeneric.go

@@ -213,6 +213,8 @@ func rewriteValuegeneric(v *Value, config *Config) bool {
 		return rewriteValuegeneric_OpOr64(v, config)
 	case OpOr8:
 		return rewriteValuegeneric_OpOr8(v, config)
+	case OpPhi:
+		return rewriteValuegeneric_OpPhi(v, config)
 	case OpPtrIndex:
 		return rewriteValuegeneric_OpPtrIndex(v, config)
 	case OpRsh16Ux16:
@@ -3965,6 +3967,98 @@ func rewriteValuegeneric_OpOr8(v *Value, config *Config) bool {
 	}
 	return false
 }
+func rewriteValuegeneric_OpPhi(v *Value, config *Config) bool {
+	b := v.Block
+	_ = b
+	// match: (Phi (Const8 [c]) (Const8 [d]))
+	// cond: int8(c) == int8(d)
+	// result: (Const8 [c])
+	for {
+		if v.Args[0].Op != OpConst8 {
+			break
+		}
+		c := v.Args[0].AuxInt
+		if v.Args[1].Op != OpConst8 {
+			break
+		}
+		d := v.Args[1].AuxInt
+		if len(v.Args) != 2 {
+			break
+		}
+		if !(int8(c) == int8(d)) {
+			break
+		}
+		v.reset(OpConst8)
+		v.AuxInt = c
+		return true
+	}
+	// match: (Phi (Const16 [c]) (Const16 [d]))
+	// cond: int16(c) == int16(d)
+	// result: (Const16 [c])
+	for {
+		if v.Args[0].Op != OpConst16 {
+			break
+		}
+		c := v.Args[0].AuxInt
+		if v.Args[1].Op != OpConst16 {
+			break
+		}
+		d := v.Args[1].AuxInt
+		if len(v.Args) != 2 {
+			break
+		}
+		if !(int16(c) == int16(d)) {
+			break
+		}
+		v.reset(OpConst16)
+		v.AuxInt = c
+		return true
+	}
+	// match: (Phi (Const32 [c]) (Const32 [d]))
+	// cond: int32(c) == int32(d)
+	// result: (Const32 [c])
+	for {
+		if v.Args[0].Op != OpConst32 {
+			break
+		}
+		c := v.Args[0].AuxInt
+		if v.Args[1].Op != OpConst32 {
+			break
+		}
+		d := v.Args[1].AuxInt
+		if len(v.Args) != 2 {
+			break
+		}
+		if !(int32(c) == int32(d)) {
+			break
+		}
+		v.reset(OpConst32)
+		v.AuxInt = c
+		return true
+	}
+	// match: (Phi (Const64 [c]) (Const64 [c]))
+	// cond:
+	// result: (Const64 [c])
+	for {
+		if v.Args[0].Op != OpConst64 {
+			break
+		}
+		c := v.Args[0].AuxInt
+		if v.Args[1].Op != OpConst64 {
+			break
+		}
+		if v.Args[1].AuxInt != v.Args[0].AuxInt {
+			break
+		}
+		if len(v.Args) != 2 {
+			break
+		}
+		v.reset(OpConst64)
+		v.AuxInt = c
+		return true
+	}
+	return false
+}
 func rewriteValuegeneric_OpPtrIndex(v *Value, config *Config) bool {
 	b := v.Block
 	_ = b