math: replace assembly implementations of Abs with pure Go version

The compiler can do a fine job, and can also inline it.

From Jeremy Jackins's observation and rsc's recommendation in thread:

"Pure Go math.Abs outperforms assembly version"
https://groups.google.com/forum/#!topic/golang-dev/nP5mWvwAXZo

Updates #13095

Change-Id: I3066f8eaa327bb403173b29791cc8661d7c0532c
Reviewed-on: https://go-review.googlesource.com/16444Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/abs.go

@@ -9,9 +9,10 @@ package math
 // Special cases are:
 //	Abs(±Inf) = +Inf
 //	Abs(NaN) = NaN
-func Abs(x float64) float64
-
-func abs(x float64) float64 {
+func Abs(x float64) float64 {
+	// TODO: once golang.org/issue/13905 is fixed, change this to:
+	// return Float64frombits(Float64bits(x) &^ (1 << 63))
+	// But for now, this generates better code and can also be inlined:
 	switch {
 	case x < 0:
 		return -x

src/math/abs_386.s

-// Copyright 2010 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"
-
-// func Abs(x float64) float64
-TEXT ·Abs(SB),NOSPLIT,$0
-	FMOVD   x+0(FP), F0  // F0=x
-	FABS                 // F0=|x|
-	FMOVDP  F0, ret+8(FP)
-	RET

src/math/abs_amd64.s

-// Copyright 2010 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"
-
-// func Abs(x float64) float64
-TEXT ·Abs(SB),NOSPLIT,$0
-	MOVQ   $(1<<63), BX
-	MOVQ   BX, X0 // movsd $(-0.0), x0
-	MOVSD  x+0(FP), X1
-	ANDNPD X1, X0
-	MOVSD  X0, ret+8(FP)
-	RET

src/math/abs_amd64p32.s

-// Copyright 2013 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 "abs_amd64.s"

src/math/abs_arm.s

-// Copyright 2011 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"
-
-TEXT ·Abs(SB),NOSPLIT,$0
-	MOVW	x_lo+0(FP), R0
-	MOVW	x_hi+4(FP), R1
-	AND 	$((1<<31)-1), R1
-	MOVW	R0, ret_lo+8(FP)
-	MOVW	R1, ret_hi+12(FP)
-	RET

src/math/abs_arm64.s

-// Copyright 2011 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"
-
-TEXT ·Abs(SB),NOSPLIT,$0-16
-	FMOVD	x+0(FP), F3
-	FABSD	F3, F3
-	FMOVD	F3, ret+8(FP)
-	RET

src/math/abs_ppc64x.s

-// Copyright 2011 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.
-
-// +build ppc64 ppc64le
-
-#include "textflag.h"
-
-TEXT ·Abs(SB),NOSPLIT,$0-16
-	MOVD	x+0(FP), R3
-	MOVD 	$((1<<63)-1), R4
-	AND	R4, R3
-	MOVD	R3, ret+8(FP)
-	RET