math: implement Min/Max in riscv64 assembly

Change-Id: If34422859d47bc8f44974a00c6b7908e7655ff41
Reviewed-on: https://go-review.googlesource.com/c/go/+/223561
Reviewed-by: Cherry Zhang <[email protected]>

Author: 4a6f656c

src/math/dim_riscv64.s

@@ -0,0 +1,70 @@
+// Copyright 2020 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"
+
+// Values returned from an FCLASS instruction.
+#define	NegInf	0x001
+#define	PosInf	0x080
+#define	NaN	0x200
+
+// func Max(x, y float64) float64
+TEXT ·Max(SB),NOSPLIT,$0
+	MOVD	x+0(FP), F0
+	MOVD	y+8(FP), F1
+	FCLASSD	F0, X5
+	FCLASSD	F1, X6
+
+	// +Inf special cases
+	MOV	$PosInf, X7
+	BEQ	X7, X5, isMaxX
+	BEQ	X7, X6, isMaxY
+
+	// NaN special cases
+	MOV	$NaN, X7
+	BEQ	X7, X5, isMaxX
+	BEQ	X7, X6, isMaxY
+
+	// normal case
+	FMAXD	F0, F1, F0
+	MOVD	F0, ret+16(FP)
+	RET
+
+isMaxX: // return x
+	MOVD	F0, ret+16(FP)
+	RET
+
+isMaxY: // return y
+	MOVD	F1, ret+16(FP)
+	RET
+
+// func Min(x, y float64) float64
+TEXT ·Min(SB),NOSPLIT,$0
+	MOVD	x+0(FP), F0
+	MOVD	y+8(FP), F1
+	FCLASSD	F0, X5
+	FCLASSD	F1, X6
+
+	// -Inf special cases
+	MOV	$NegInf, X7
+	BEQ	X7, X5, isMinX
+	BEQ	X7, X6, isMinY
+
+	// NaN special cases
+	MOV	$NaN, X7
+	BEQ	X7, X5, isMinX
+	BEQ	X7, X6, isMinY
+
+	// normal case
+	FMIND	F0, F1, F0
+	MOVD	F0, ret+16(FP)
+	RET
+
+isMinX: // return x
+	MOVD	F0, ret+16(FP)
+	RET
+
+isMinY: // return y
+	MOVD	F1, ret+16(FP)
+	RET

src/math/stubs_riscv64.s

@@ -25,12 +25,6 @@ TEXT ·Atan(SB),NOSPLIT,$0
 TEXT ·Atanh(SB),NOSPLIT,$0
 	JMP ·atanh(SB)
 
-TEXT ·Min(SB),NOSPLIT,$0
-	JMP ·min(SB)
-
-TEXT ·Max(SB),NOSPLIT,$0
-	JMP ·max(SB)
-
 TEXT ·Erf(SB),NOSPLIT,$0
 	JMP ·erf(SB)