cmd/compile: trunc(and(x,K)) rewrite to trunc(x) for some K

uint8(s.b & 0xff) ought to produce same code as uint8(s.b)
but it did not.  RLH found this one looking for moles to
whack in the GC code.

Change-Id: I883d68ec7a5746d652712be84a274a11256b3b33
Reviewed-on: https://go-review.googlesource.com/20141Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -355,6 +355,13 @@
 (Neg32 (Sub32 x y)) -> (Sub32 y x)
 (Neg64 (Sub64 x y)) -> (Sub64 y x)
 
+(Trunc64to8 (And64 (Const64 [y]) x)) && y&0xFF == 0xFF -> (Trunc64to8 x)
+(Trunc64to16 (And64 (Const64 [y]) x)) && y&0xFFFF == 0xFFFF -> (Trunc64to16 x)
+(Trunc64to32 (And64 (Const64 [y]) x)) && y&0xFFFFFFFF == 0xFFFFFFFF -> (Trunc64to32 x)
+(Trunc32to8 (And32 (Const32 [y]) x)) && y&0xFF == 0xFF -> (Trunc32to8 x)
+(Trunc32to16 (And32 (Const32 [y]) x)) && y&0xFFFF == 0xFFFF -> (Trunc32to16 x)
+(Trunc16to8 (And16 (Const16 [y]) x)) && y&0xFF == 0xFF -> (Trunc16to8 x)
+
 // Rewrite AND of consts as shifts if possible, slightly faster for 32/64 bit operands
 // leading zeros can be shifted left, then right
 (And64 <t> (Const64 [y]) x) && nlz(y) + nto(y) == 64 -> (Rsh64Ux64 (Lsh64x64 <t> x (Const64 <t> [nlz(y)])) (Const64 <t> [nlz(y)]))

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

@@ -7551,6 +7551,25 @@ func rewriteValuegeneric_OpTrunc16to8(v *Value, config *Config) bool {
 		v.AuxInt = int64(int8(c))
 		return true
 	}
+	// match: (Trunc16to8 (And16 (Const16 [y]) x))
+	// cond: y&0xFF == 0xFF
+	// result: (Trunc16to8 x)
+	for {
+		if v.Args[0].Op != OpAnd16 {
+			break
+		}
+		if v.Args[0].Args[0].Op != OpConst16 {
+			break
+		}
+		y := v.Args[0].Args[0].AuxInt
+		x := v.Args[0].Args[1]
+		if !(y&0xFF == 0xFF) {
+			break
+		}
+		v.reset(OpTrunc16to8)
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpTrunc32to16(v *Value, config *Config) bool {
@@ -7568,6 +7587,25 @@ func rewriteValuegeneric_OpTrunc32to16(v *Value, config *Config) bool {
 		v.AuxInt = int64(int16(c))
 		return true
 	}
+	// match: (Trunc32to16 (And32 (Const32 [y]) x))
+	// cond: y&0xFFFF == 0xFFFF
+	// result: (Trunc32to16 x)
+	for {
+		if v.Args[0].Op != OpAnd32 {
+			break
+		}
+		if v.Args[0].Args[0].Op != OpConst32 {
+			break
+		}
+		y := v.Args[0].Args[0].AuxInt
+		x := v.Args[0].Args[1]
+		if !(y&0xFFFF == 0xFFFF) {
+			break
+		}
+		v.reset(OpTrunc32to16)
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpTrunc32to8(v *Value, config *Config) bool {
@@ -7585,6 +7623,25 @@ func rewriteValuegeneric_OpTrunc32to8(v *Value, config *Config) bool {
 		v.AuxInt = int64(int8(c))
 		return true
 	}
+	// match: (Trunc32to8 (And32 (Const32 [y]) x))
+	// cond: y&0xFF == 0xFF
+	// result: (Trunc32to8 x)
+	for {
+		if v.Args[0].Op != OpAnd32 {
+			break
+		}
+		if v.Args[0].Args[0].Op != OpConst32 {
+			break
+		}
+		y := v.Args[0].Args[0].AuxInt
+		x := v.Args[0].Args[1]
+		if !(y&0xFF == 0xFF) {
+			break
+		}
+		v.reset(OpTrunc32to8)
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpTrunc64to16(v *Value, config *Config) bool {
@@ -7602,6 +7659,25 @@ func rewriteValuegeneric_OpTrunc64to16(v *Value, config *Config) bool {
 		v.AuxInt = int64(int16(c))
 		return true
 	}
+	// match: (Trunc64to16 (And64 (Const64 [y]) x))
+	// cond: y&0xFFFF == 0xFFFF
+	// result: (Trunc64to16 x)
+	for {
+		if v.Args[0].Op != OpAnd64 {
+			break
+		}
+		if v.Args[0].Args[0].Op != OpConst64 {
+			break
+		}
+		y := v.Args[0].Args[0].AuxInt
+		x := v.Args[0].Args[1]
+		if !(y&0xFFFF == 0xFFFF) {
+			break
+		}
+		v.reset(OpTrunc64to16)
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpTrunc64to32(v *Value, config *Config) bool {
@@ -7619,6 +7695,25 @@ func rewriteValuegeneric_OpTrunc64to32(v *Value, config *Config) bool {
 		v.AuxInt = int64(int32(c))
 		return true
 	}
+	// match: (Trunc64to32 (And64 (Const64 [y]) x))
+	// cond: y&0xFFFFFFFF == 0xFFFFFFFF
+	// result: (Trunc64to32 x)
+	for {
+		if v.Args[0].Op != OpAnd64 {
+			break
+		}
+		if v.Args[0].Args[0].Op != OpConst64 {
+			break
+		}
+		y := v.Args[0].Args[0].AuxInt
+		x := v.Args[0].Args[1]
+		if !(y&0xFFFFFFFF == 0xFFFFFFFF) {
+			break
+		}
+		v.reset(OpTrunc64to32)
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpTrunc64to8(v *Value, config *Config) bool {
@@ -7636,6 +7731,25 @@ func rewriteValuegeneric_OpTrunc64to8(v *Value, config *Config) bool {
 		v.AuxInt = int64(int8(c))
 		return true
 	}
+	// match: (Trunc64to8 (And64 (Const64 [y]) x))
+	// cond: y&0xFF == 0xFF
+	// result: (Trunc64to8 x)
+	for {
+		if v.Args[0].Op != OpAnd64 {
+			break
+		}
+		if v.Args[0].Args[0].Op != OpConst64 {
+			break
+		}
+		y := v.Args[0].Args[0].AuxInt
+		x := v.Args[0].Args[1]
+		if !(y&0xFF == 0xFF) {
+			break
+		}
+		v.reset(OpTrunc64to8)
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpXor16(v *Value, config *Config) bool {