math: Add ARM64 implementation of frexp

The implementation of Frexp on  ARM64 has been implemented in this PR.

The following benchmark was run on Apple Silicon M1 chips.
BenchmarkArchFrexp-8    1.581 ns/op  0 B/op  0 allocs/op
BenchmarkFrexpGO-8   	2.079 ns/op  0 B/op  0 allocs/op

The time spent on each iteration drop 23.95%.

Author: howjmay

src/math/all_test.go

@@ -2541,7 +2541,7 @@ func TestFrexp(t *testing.T) {
 		}
 	}
 	for i := 0; i < len(vffrexpBC); i++ {
-		if f, j := Frexp(vffrexpBC[i]); !alike(frexpBC[i].f, f) || frexpBC[i].i != j {
+		if f, j := Frexp(vffrexpBC[i]); !veryclose(frexpBC[i].f, f) || frexpBC[i].i != j {
 			t.Errorf("Frexp(%g) = %g, %d, want %g, %d", vffrexpBC[i], f, j, frexpBC[i].f, frexpBC[i].i)
 		}
 	}
@@ -3467,6 +3467,16 @@ func BenchmarkFrexp(b *testing.B) {
 	GlobalI = y
 }
 
+func BenchmarkFrexpGO(b *testing.B) {
+	x := 0.0
+	y := 0
+	for i := 0; i < b.N; i++ {
+		x, y = FrexpGO(8)
+	}
+	GlobalF = x
+	GlobalI = y
+}
+
 func BenchmarkGamma(b *testing.B) {
 	x := 0.0
 	for i := 0; i < b.N; i++ {

src/math/arith_s390x.go

@@ -135,12 +135,6 @@ func archPow(x, y float64) float64
 func powTrampolineSetup(x, y float64) float64
 func powAsm(x, y float64) float64
 
-const haveArchFrexp = false
-
-func archFrexp(x float64) (float64, int) {
-	panic("not implemented")
-}
-
 const haveArchLdexp = false
 
 func archLdexp(frac float64, exp int) float64 {

src/math/export_test.go

@@ -8,6 +8,7 @@ package math
 var ExpGo = exp
 var Exp2Go = exp2
 var HypotGo = hypot
+var FrexpGo = frexp
 var SqrtGo = sqrt
 var TrigReduce = trigReduce
 

src/math/frexp_arm64.s

@@ -0,0 +1,57 @@
+// 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.
+
+#include "textflag.h"
+
+#define PosInf 0x7FF0000000000000
+#define Float64SmallestNormal 2.2250738585072014e-308
+
+// func archFrexp(x float64) (frac float64, exp int)
+TEXT ·archFrexp(SB),NOSPLIT,$0-24
+	FMOVD	x+0(FP), F0
+	MOVD	ZR, R1	// R1 is the ret exp
+	FABSD	F0, F3 // F3 is the absolute of 'x'
+	FCMPD	$(0.0), F0
+	BEQ	isInfOrNaNOrZero
+
+	MOVD	$PosInf, R3
+	FMOVD	R3, F30 // F30 is PosInf
+	FCMPD	F30, F3
+	BGE	isInfOrNaNOrZero // isInf
+	FCMPED	F0, F0
+	BNE	isInfOrNaNOrZero // isNaN
+
+	FMOVD	F3, R0
+	UBFX $52, R0, $11, R0
+
+	FMOVD	$Float64SmallestNormal, F29
+	FCMPD	F29, F3
+	BGE	pass
+
+	// if abs(x) < SmallestNormal ,then run the following special case
+	FMOVD	$(4503599627370496.0), F2 // 4503599627370496.0 is (1<<52)
+	FMULD	F2, F0, F0
+	SUB	$(52), R1, R1 // set R1 to -52
+	FMOVD	F0, R0
+	// tmp = int((x>>shift)&mask)
+	UBFX $52, R0, $11, R0
+	
+pass:
+	// tmp = tmp - bias + 1
+	SUB	$0x3FE, R0, R3
+	// exp = exp + tmp
+	ADD R3, R1, R1
+
+	FMOVD	F0, R0
+	AND	$0x800FFFFFFFFFFFFF, R0, R0
+	ORR	$0x3FE0000000000000, R0, R0
+	FMOVD	R0, F0
+  	FMOVD	F0, ret+8(FP)
+	MOVD	R1, ret+16(FP)
+	RET
+
+isInfOrNaNOrZero:
+	FMOVD	F0, ret+8(FP)
+	MOVD	R1, ret+16(FP)
+	RET

src/math/frexp_asm.go

@@ -0,0 +1,12 @@
+// 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.
+
+//go:build arm64
+// +build arm64
+
+package math
+
+const haveArchFrexp = true
+
+func archFrexp(x float64) (frac float64, exp int)

src/math/frexp_noasm.go

@@ -0,0 +1,14 @@
+// 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.
+
+//go:build !arm64 && !s390x
+// +build !arm64,!s390x
+
+package math
+
+const haveArchFrexp = false
+
+func archFrexp(x float64) (frac float64, exp int)
+	panic("not implemented")
+}

src/math/stubs.go

@@ -88,12 +88,6 @@ func archExpm1(x float64) float64 {
 	panic("not implemented")
 }
 
-const haveArchFrexp = false
-
-func archFrexp(x float64) (float64, int) {
-	panic("not implemented")
-}
-
 const haveArchLdexp = false
 
 func archLdexp(frac float64, exp int) float64 {