cmd/compile: simplify less with non-negative number and constant 0 or 1

The most common cases:
len(s) > 0
len(s) < 1

and they can be simplified to:
len(s) != 0
len(s) == 0

Fixes #48054

Change-Id: I16e5b0cffcfab62a4acc2a09977a6cd3543dd000
Reviewed-on: https://go-review.googlesource.com/c/go/+/346050
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

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

@@ -500,6 +500,9 @@
 (Leq32 (Const32 [0]) (Rsh32Ux64 _ (Const64 [c]))) && c > 0 => (ConstBool [true])
 (Leq64 (Const64 [0]) (Rsh64Ux64 _ (Const64 [c]))) && c > 0 => (ConstBool [true])
 
+(Less(64|32|16|8) (Const(64|32|16|8) <t> [0]) x) && isNonNegative(x) => (Neq(64|32|16|8) (Const(64|32|16|8) <t> [0]) x)
+(Less(64|32|16|8) x (Const(64|32|16|8) <t> [1])) && isNonNegative(x) => (Eq(64|32|16|8) (Const(64|32|16|8) <t> [0]) x)
+
 // constant floating point comparisons
 (Eq32F   (Const32F [c]) (Const32F [d])) => (ConstBool [c == d])
 (Eq64F   (Const64F [c]) (Const64F [d])) => (ConstBool [c == d])

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

@@ -10083,6 +10083,7 @@ func rewriteValuegeneric_OpLeq8U(v *Value) bool {
 func rewriteValuegeneric_OpLess16(v *Value) bool {
 	v_1 := v.Args[1]
 	v_0 := v.Args[0]
+	b := v.Block
 	// match: (Less16 (Const16 [c]) (Const16 [d]))
 	// result: (ConstBool [c < d])
 	for {
@@ -10098,6 +10099,45 @@ func rewriteValuegeneric_OpLess16(v *Value) bool {
 		v.AuxInt = boolToAuxInt(c < d)
 		return true
 	}
+	// match: (Less16 (Const16 <t> [0]) x)
+	// cond: isNonNegative(x)
+	// result: (Neq16 (Const16 <t> [0]) x)
+	for {
+		if v_0.Op != OpConst16 {
+			break
+		}
+		t := v_0.Type
+		if auxIntToInt16(v_0.AuxInt) != 0 {
+			break
+		}
+		x := v_1
+		if !(isNonNegative(x)) {
+			break
+		}
+		v.reset(OpNeq16)
+		v0 := b.NewValue0(v.Pos, OpConst16, t)
+		v0.AuxInt = int16ToAuxInt(0)
+		v.AddArg2(v0, x)
+		return true
+	}
+	// match: (Less16 x (Const16 <t> [1]))
+	// cond: isNonNegative(x)
+	// result: (Eq16 (Const16 <t> [0]) x)
+	for {
+		x := v_0
+		if v_1.Op != OpConst16 {
+			break
+		}
+		t := v_1.Type
+		if auxIntToInt16(v_1.AuxInt) != 1 || !(isNonNegative(x)) {
+			break
+		}
+		v.reset(OpEq16)
+		v0 := b.NewValue0(v.Pos, OpConst16, t)
+		v0.AuxInt = int16ToAuxInt(0)
+		v.AddArg2(v0, x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpLess16U(v *Value) bool {
@@ -10133,6 +10173,7 @@ func rewriteValuegeneric_OpLess16U(v *Value) bool {
 func rewriteValuegeneric_OpLess32(v *Value) bool {
 	v_1 := v.Args[1]
 	v_0 := v.Args[0]
+	b := v.Block
 	// match: (Less32 (Const32 [c]) (Const32 [d]))
 	// result: (ConstBool [c < d])
 	for {
@@ -10148,6 +10189,45 @@ func rewriteValuegeneric_OpLess32(v *Value) bool {
 		v.AuxInt = boolToAuxInt(c < d)
 		return true
 	}
+	// match: (Less32 (Const32 <t> [0]) x)
+	// cond: isNonNegative(x)
+	// result: (Neq32 (Const32 <t> [0]) x)
+	for {
+		if v_0.Op != OpConst32 {
+			break
+		}
+		t := v_0.Type
+		if auxIntToInt32(v_0.AuxInt) != 0 {
+			break
+		}
+		x := v_1
+		if !(isNonNegative(x)) {
+			break
+		}
+		v.reset(OpNeq32)
+		v0 := b.NewValue0(v.Pos, OpConst32, t)
+		v0.AuxInt = int32ToAuxInt(0)
+		v.AddArg2(v0, x)
+		return true
+	}
+	// match: (Less32 x (Const32 <t> [1]))
+	// cond: isNonNegative(x)
+	// result: (Eq32 (Const32 <t> [0]) x)
+	for {
+		x := v_0
+		if v_1.Op != OpConst32 {
+			break
+		}
+		t := v_1.Type
+		if auxIntToInt32(v_1.AuxInt) != 1 || !(isNonNegative(x)) {
+			break
+		}
+		v.reset(OpEq32)
+		v0 := b.NewValue0(v.Pos, OpConst32, t)
+		v0.AuxInt = int32ToAuxInt(0)
+		v.AddArg2(v0, x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpLess32F(v *Value) bool {
@@ -10203,6 +10283,7 @@ func rewriteValuegeneric_OpLess32U(v *Value) bool {
 func rewriteValuegeneric_OpLess64(v *Value) bool {
 	v_1 := v.Args[1]
 	v_0 := v.Args[0]
+	b := v.Block
 	// match: (Less64 (Const64 [c]) (Const64 [d]))
 	// result: (ConstBool [c < d])
 	for {
@@ -10218,6 +10299,45 @@ func rewriteValuegeneric_OpLess64(v *Value) bool {
 		v.AuxInt = boolToAuxInt(c < d)
 		return true
 	}
+	// match: (Less64 (Const64 <t> [0]) x)
+	// cond: isNonNegative(x)
+	// result: (Neq64 (Const64 <t> [0]) x)
+	for {
+		if v_0.Op != OpConst64 {
+			break
+		}
+		t := v_0.Type
+		if auxIntToInt64(v_0.AuxInt) != 0 {
+			break
+		}
+		x := v_1
+		if !(isNonNegative(x)) {
+			break
+		}
+		v.reset(OpNeq64)
+		v0 := b.NewValue0(v.Pos, OpConst64, t)
+		v0.AuxInt = int64ToAuxInt(0)
+		v.AddArg2(v0, x)
+		return true
+	}
+	// match: (Less64 x (Const64 <t> [1]))
+	// cond: isNonNegative(x)
+	// result: (Eq64 (Const64 <t> [0]) x)
+	for {
+		x := v_0
+		if v_1.Op != OpConst64 {
+			break
+		}
+		t := v_1.Type
+		if auxIntToInt64(v_1.AuxInt) != 1 || !(isNonNegative(x)) {
+			break
+		}
+		v.reset(OpEq64)
+		v0 := b.NewValue0(v.Pos, OpConst64, t)
+		v0.AuxInt = int64ToAuxInt(0)
+		v.AddArg2(v0, x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpLess64F(v *Value) bool {
@@ -10273,6 +10393,7 @@ func rewriteValuegeneric_OpLess64U(v *Value) bool {
 func rewriteValuegeneric_OpLess8(v *Value) bool {
 	v_1 := v.Args[1]
 	v_0 := v.Args[0]
+	b := v.Block
 	// match: (Less8 (Const8 [c]) (Const8 [d]))
 	// result: (ConstBool [c < d])
 	for {
@@ -10288,6 +10409,45 @@ func rewriteValuegeneric_OpLess8(v *Value) bool {
 		v.AuxInt = boolToAuxInt(c < d)
 		return true
 	}
+	// match: (Less8 (Const8 <t> [0]) x)
+	// cond: isNonNegative(x)
+	// result: (Neq8 (Const8 <t> [0]) x)
+	for {
+		if v_0.Op != OpConst8 {
+			break
+		}
+		t := v_0.Type
+		if auxIntToInt8(v_0.AuxInt) != 0 {
+			break
+		}
+		x := v_1
+		if !(isNonNegative(x)) {
+			break
+		}
+		v.reset(OpNeq8)
+		v0 := b.NewValue0(v.Pos, OpConst8, t)
+		v0.AuxInt = int8ToAuxInt(0)
+		v.AddArg2(v0, x)
+		return true
+	}
+	// match: (Less8 x (Const8 <t> [1]))
+	// cond: isNonNegative(x)
+	// result: (Eq8 (Const8 <t> [0]) x)
+	for {
+		x := v_0
+		if v_1.Op != OpConst8 {
+			break
+		}
+		t := v_1.Type
+		if auxIntToInt8(v_1.AuxInt) != 1 || !(isNonNegative(x)) {
+			break
+		}
+		v.reset(OpEq8)
+		v0 := b.NewValue0(v.Pos, OpConst8, t)
+		v0.AuxInt = int8ToAuxInt(0)
+		v.AddArg2(v0, x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpLess8U(v *Value) bool {

test/codegen/issue48054.go

@@ -0,0 +1,31 @@
+// asmcheck
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func a(n string) bool {
+	// arm64:"CBZ"
+	if len(n) > 0 {
+		return true
+	}
+	return false
+}
+
+func a2(n []int) bool {
+	// arm64:"CBZ"
+	if len(n) > 0 {
+		return true
+	}
+	return false
+}
+
+func a3(n []int) bool {
+	// amd64:"TESTQ"
+	if len(n) < 1 {
+		return true
+	}
+	return false
+}