cmd/compile: don't generate panicshift for masked int shifts

We know that a & 31 is non-negative for all a, signed or not.
We can avoid checking that and needing to write out an
unreachable call to panicshift.

Change-Id: I32f32fb2c950d2b2b35ac5c0e99b7b2dbd47f917
Reviewed-on: https://go-review.googlesource.com/c/go/+/167499
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -420,6 +420,11 @@
 (Less(64|32|16|8)    (Const(64|32|16|8) [c]) (Const(64|32|16|8) [d])) -> (ConstBool [b2i(c < d)])
 (Leq(64|32|16|8)     (Const(64|32|16|8) [c]) (Const(64|32|16|8) [d])) -> (ConstBool [b2i(c <= d)])
 
+(Geq8  (And8  _ (Const8  [c])) (Const8  [0])) && int8(c)  >= 0 -> (ConstBool [1])
+(Geq16 (And16 _ (Const16 [c])) (Const16 [0])) && int16(c) >= 0 -> (ConstBool [1])
+(Geq32 (And32 _ (Const32 [c])) (Const32 [0])) && int32(c) >= 0 -> (ConstBool [1])
+(Geq64 (And64 _ (Const64 [c])) (Const64 [0])) && int64(c) >= 0 -> (ConstBool [1])
+
 (Greater64U (Const64 [c]) (Const64 [d])) -> (ConstBool [b2i(uint64(c) > uint64(d))])
 (Greater32U (Const32 [c]) (Const32 [d])) -> (ConstBool [b2i(uint32(c) > uint32(d))])
 (Greater16U (Const16 [c]) (Const16 [d])) -> (ConstBool [b2i(uint16(c) > uint16(d))])

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

@@ -10430,6 +10430,64 @@ func rewriteValuegeneric_OpGeq16_0(v *Value) bool {
 		v.AuxInt = b2i(c >= d)
 		return true
 	}
+	// match: (Geq16 (And16 _ (Const16 [c])) (Const16 [0]))
+	// cond: int16(c) >= 0
+	// result: (ConstBool [1])
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpAnd16 {
+			break
+		}
+		_ = v_0.Args[1]
+		v_0_1 := v_0.Args[1]
+		if v_0_1.Op != OpConst16 {
+			break
+		}
+		c := v_0_1.AuxInt
+		v_1 := v.Args[1]
+		if v_1.Op != OpConst16 {
+			break
+		}
+		if v_1.AuxInt != 0 {
+			break
+		}
+		if !(int16(c) >= 0) {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
+	// match: (Geq16 (And16 (Const16 [c]) _) (Const16 [0]))
+	// cond: int16(c) >= 0
+	// result: (ConstBool [1])
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpAnd16 {
+			break
+		}
+		_ = v_0.Args[1]
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpConst16 {
+			break
+		}
+		c := v_0_0.AuxInt
+		v_1 := v.Args[1]
+		if v_1.Op != OpConst16 {
+			break
+		}
+		if v_1.AuxInt != 0 {
+			break
+		}
+		if !(int16(c) >= 0) {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpGeq16U_0(v *Value) bool {
@@ -10474,6 +10532,64 @@ func rewriteValuegeneric_OpGeq32_0(v *Value) bool {
 		v.AuxInt = b2i(c >= d)
 		return true
 	}
+	// match: (Geq32 (And32 _ (Const32 [c])) (Const32 [0]))
+	// cond: int32(c) >= 0
+	// result: (ConstBool [1])
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpAnd32 {
+			break
+		}
+		_ = v_0.Args[1]
+		v_0_1 := v_0.Args[1]
+		if v_0_1.Op != OpConst32 {
+			break
+		}
+		c := v_0_1.AuxInt
+		v_1 := v.Args[1]
+		if v_1.Op != OpConst32 {
+			break
+		}
+		if v_1.AuxInt != 0 {
+			break
+		}
+		if !(int32(c) >= 0) {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
+	// match: (Geq32 (And32 (Const32 [c]) _) (Const32 [0]))
+	// cond: int32(c) >= 0
+	// result: (ConstBool [1])
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpAnd32 {
+			break
+		}
+		_ = v_0.Args[1]
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpConst32 {
+			break
+		}
+		c := v_0_0.AuxInt
+		v_1 := v.Args[1]
+		if v_1.Op != OpConst32 {
+			break
+		}
+		if v_1.AuxInt != 0 {
+			break
+		}
+		if !(int32(c) >= 0) {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpGeq32F_0(v *Value) bool {
@@ -10540,6 +10656,64 @@ func rewriteValuegeneric_OpGeq64_0(v *Value) bool {
 		v.AuxInt = b2i(c >= d)
 		return true
 	}
+	// match: (Geq64 (And64 _ (Const64 [c])) (Const64 [0]))
+	// cond: int64(c) >= 0
+	// result: (ConstBool [1])
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpAnd64 {
+			break
+		}
+		_ = v_0.Args[1]
+		v_0_1 := v_0.Args[1]
+		if v_0_1.Op != OpConst64 {
+			break
+		}
+		c := v_0_1.AuxInt
+		v_1 := v.Args[1]
+		if v_1.Op != OpConst64 {
+			break
+		}
+		if v_1.AuxInt != 0 {
+			break
+		}
+		if !(int64(c) >= 0) {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
+	// match: (Geq64 (And64 (Const64 [c]) _) (Const64 [0]))
+	// cond: int64(c) >= 0
+	// result: (ConstBool [1])
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpAnd64 {
+			break
+		}
+		_ = v_0.Args[1]
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpConst64 {
+			break
+		}
+		c := v_0_0.AuxInt
+		v_1 := v.Args[1]
+		if v_1.Op != OpConst64 {
+			break
+		}
+		if v_1.AuxInt != 0 {
+			break
+		}
+		if !(int64(c) >= 0) {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpGeq64F_0(v *Value) bool {
@@ -10606,6 +10780,64 @@ func rewriteValuegeneric_OpGeq8_0(v *Value) bool {
 		v.AuxInt = b2i(c >= d)
 		return true
 	}
+	// match: (Geq8 (And8 _ (Const8 [c])) (Const8 [0]))
+	// cond: int8(c) >= 0
+	// result: (ConstBool [1])
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpAnd8 {
+			break
+		}
+		_ = v_0.Args[1]
+		v_0_1 := v_0.Args[1]
+		if v_0_1.Op != OpConst8 {
+			break
+		}
+		c := v_0_1.AuxInt
+		v_1 := v.Args[1]
+		if v_1.Op != OpConst8 {
+			break
+		}
+		if v_1.AuxInt != 0 {
+			break
+		}
+		if !(int8(c) >= 0) {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
+	// match: (Geq8 (And8 (Const8 [c]) _) (Const8 [0]))
+	// cond: int8(c) >= 0
+	// result: (ConstBool [1])
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpAnd8 {
+			break
+		}
+		_ = v_0.Args[1]
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpConst8 {
+			break
+		}
+		c := v_0_0.AuxInt
+		v_1 := v.Args[1]
+		if v_1.Op != OpConst8 {
+			break
+		}
+		if v_1.AuxInt != 0 {
+			break
+		}
+		if !(int8(c) >= 0) {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpGeq8U_0(v *Value) bool {

test/codegen/shift.go

@@ -70,6 +70,34 @@ func rshMask64x32Ext(v int64, s int32) int64 {
 	return v >> uint(s&63)
 }
 
+// --------------- //
+//  signed shifts  //
+// --------------- //
+
+// We do want to generate a test + panicshift for these cases.
+func lshSigned(v8 int8, v16 int16, v32 int32, v64 int64, x int) {
+	// amd64:"TESTB"
+	_ = x << v8
+	// amd64:"TESTW"
+	_ = x << v16
+	// amd64:"TESTL"
+	_ = x << v32
+	// amd64:"TESTQ"
+	_ = x << v64
+}
+
+// We want to avoid generating a test + panicshift for these cases.
+func lshSignedMasked(v8 int8, v16 int16, v32 int32, v64 int64, x int) {
+	// amd64:-"TESTB"
+	_ = x << (v8 & 7)
+	// amd64:-"TESTW"
+	_ = x << (v16 & 15)
+	// amd64:-"TESTL"
+	_ = x << (v32 & 31)
+	// amd64:-"TESTQ"
+	_ = x << (v64 & 63)
+}
+
 // ------------------ //
 //   bounded shifts   //
 // ------------------ //