cmd/compile: int64(uint64 >> x) >= 0 if x > 0

This rewrite rule triggers only once, in math/big.quotToFloat64,
as part of converting a uint64 to a float64.

Nevertheless, it is cheap; let's add it.

Change-Id: I3ed4a197a559110fec1bc04b3a8abb4c7fcc2c89
Reviewed-on: https://go-review.googlesource.com/c/go/+/167500
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -425,6 +425,8 @@
 (Geq32 (And32 _ (Const32 [c])) (Const32 [0])) && int32(c) >= 0 -> (ConstBool [1])
 (Geq64 (And64 _ (Const64 [c])) (Const64 [0])) && int64(c) >= 0 -> (ConstBool [1])
 
+(Geq64 (Rsh64Ux64 _ (Const64 [c])) (Const64 [0])) && 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

@@ -10714,6 +10714,35 @@ func rewriteValuegeneric_OpGeq64_0(v *Value) bool {
 		v.AuxInt = 1
 		return true
 	}
+	// match: (Geq64 (Rsh64Ux64 _ (Const64 [c])) (Const64 [0]))
+	// cond: c > 0
+	// result: (ConstBool [1])
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpRsh64Ux64 {
+			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 !(c > 0) {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpGeq64F_0(v *Value) bool {