math: amd64 versions of fdim, fmax, fmin

Uses the SSE2 max, min instructions. Also shorter sqrt_amd64.s

R=rsc
CC=golang-dev
https://golang.org/cl/1216042

src/pkg/math/Makefile

@@ -7,6 +7,7 @@ include ../../Make.$(GOARCH)
 TARG=math
 
 OFILES_amd64=\
+	fdim_amd64.$O\
 	sqrt_amd64.$O\
 
 OFILES_386=\

src/pkg/math/fdim_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.
+
+// func Fdim(x, y float64) float64
+TEXT ·Fdim(SB),7,$0
+	MOVSD x+0(FP), X0
+	SUBSD y+8(FP), X0
+	MOVSD $(0.0), X1
+	MAXSD X1, X0
+	MOVSD X0, r+16(FP)
+	RET
+
+// func Fmax(x, y float64) float64
+TEXT ·Fmax(SB),7,$0
+	MOVSD x+0(FP), X0
+	MAXSD y+8(FP), X0
+	MOVSD X0, r+16(FP)
+	RET
+
+// func Fmin(x, y float64) float64
+TEXT ·Fmin(SB),7,$0
+	MOVSD x+0(FP), X0
+	MINSD y+8(FP), X0
+	MOVSD X0, r+16(FP)
+	RET

src/pkg/math/fdim_decl.go

+// 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.
+
+package math
+
+func Fdim(x, y float64) float64
+func Fmax(x, y float64) float64
+func Fmin(x, y float64) float64

src/pkg/math/sqrt_amd64.s

@@ -4,7 +4,6 @@
 
 // func Sqrt(x float64) float64
 TEXT ·Sqrt(SB),7,$0
-	MOVSD x+0(FP), X0
-	SQRTSD X0, X0
+	SQRTSD x+0(FP), X0
 	MOVSD X0, r+8(FP)
 	RET