cmd/compile: reduce bits.Div64(0, lo, y) to 64 bit division

With this change, these two functions generate identical code:

func f(x uint64) (uint64, uint64) {
	return bits.Div64(0, x, 5)
}

func g(x uint64) (uint64, uint64) {
	return x / 5, x % 5
}

Updates #31582

Change-Id: Ia96c2e67f8af5dd985823afee5f155608c04a4b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/173197Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -146,6 +146,8 @@
 (Div64u (Const64 [c])  (Const64 [d])) && d != 0 -> (Const64 [int64(uint64(c)/uint64(d))])
 (Div32F (Const32F [c]) (Const32F [d])) -> (Const32F [auxFrom32F(auxTo32F(c) / auxTo32F(d))])
 (Div64F (Const64F [c]) (Const64F [d])) -> (Const64F [auxFrom64F(auxTo64F(c) / auxTo64F(d))])
+(Select0 (Div128u (Const64 [0]) lo y)) -> (Div64u lo y)
+(Select1 (Div128u (Const64 [0]) lo y)) -> (Mod64u lo y)
 
 (Not (ConstBool [c])) -> (ConstBool [1-c])
 

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

@@ -389,6 +389,10 @@ func rewriteValuegeneric(v *Value) bool {
 		return rewriteValuegeneric_OpRsh8x64_0(v)
 	case OpRsh8x8:
 		return rewriteValuegeneric_OpRsh8x8_0(v)
+	case OpSelect0:
+		return rewriteValuegeneric_OpSelect0_0(v)
+	case OpSelect1:
+		return rewriteValuegeneric_OpSelect1_0(v)
 	case OpSignExt16to32:
 		return rewriteValuegeneric_OpSignExt16to32_0(v)
 	case OpSignExt16to64:
@@ -26587,6 +26591,56 @@ func rewriteValuegeneric_OpRsh8x8_0(v *Value) bool {
 	}
 	return false
 }
+func rewriteValuegeneric_OpSelect0_0(v *Value) bool {
+	// match: (Select0 (Div128u (Const64 [0]) lo y))
+	// cond:
+	// result: (Div64u lo y)
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpDiv128u {
+			break
+		}
+		y := v_0.Args[2]
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpConst64 {
+			break
+		}
+		if v_0_0.AuxInt != 0 {
+			break
+		}
+		lo := v_0.Args[1]
+		v.reset(OpDiv64u)
+		v.AddArg(lo)
+		v.AddArg(y)
+		return true
+	}
+	return false
+}
+func rewriteValuegeneric_OpSelect1_0(v *Value) bool {
+	// match: (Select1 (Div128u (Const64 [0]) lo y))
+	// cond:
+	// result: (Mod64u lo y)
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpDiv128u {
+			break
+		}
+		y := v_0.Args[2]
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpConst64 {
+			break
+		}
+		if v_0_0.AuxInt != 0 {
+			break
+		}
+		lo := v_0.Args[1]
+		v.reset(OpMod64u)
+		v.AddArg(lo)
+		v.AddArg(y)
+		return true
+	}
+	return false
+}
 func rewriteValuegeneric_OpSignExt16to32_0(v *Value) bool {
 	// match: (SignExt16to32 (Const16 [c]))
 	// cond:

test/codegen/mathbits.go

@@ -538,3 +538,8 @@ func Div64(hi, lo, x uint64) (q, r uint64) {
 	// amd64:"DIVQ"
 	return bits.Div64(hi, lo, x)
 }
+
+func Div64degenerate(x uint64) (q, r uint64) {
+	// amd64:-"DIVQ"
+	return bits.Div64(0, x, 5)
+}