math/big: initial vector arithmetic in riscv64 assembly

Provide an assembly implementation of mulWW - for now all others run the
Go code.

Change-Id: Icb594c31048255f131bdea8d64f56784fc9db4d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/220919
Reviewed-by: Robert Griesemer <[email protected]>

Author: 4a6f656c

src/math/big/arith_decl.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !math_big_pure_go,!riscv64
+// +build !math_big_pure_go
 
 package big
 

src/math/big/arith_decl_pure.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build math_big_pure_go riscv64
+// +build math_big_pure_go
 
 package big
 

src/math/big/arith_riscv64.s

@@ -0,0 +1,51 @@
+// 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.
+
+// +build !math_big_pure_go,riscv64
+
+#include "textflag.h"
+
+// This file provides fast assembly versions for the elementary
+// arithmetic operations on vectors implemented in arith.go.
+
+// func mulWW(x, y Word) (z1, z0 Word)
+TEXT ·mulWW(SB),NOSPLIT,$0
+	MOV	x+0(FP), X5
+	MOV	y+8(FP), X6
+	MULHU	X5, X6, X7
+	MUL	X5, X6, X8
+	MOV	X7, z1+16(FP)
+	MOV	X8, z0+24(FP)
+	RET
+
+// func divWW(x1, x0, y Word) (q, r Word)
+TEXT ·divWW(SB),NOSPLIT,$0
+	JMP ·divWW_g(SB)		// riscv64 has no multiword division
+
+TEXT ·addVV(SB),NOSPLIT,$0
+	JMP ·addVV_g(SB)
+
+TEXT ·subVV(SB),NOSPLIT,$0
+	JMP ·subVV_g(SB)
+
+TEXT ·addVW(SB),NOSPLIT,$0
+	JMP ·addVW_g(SB)
+
+TEXT ·subVW(SB),NOSPLIT,$0
+	JMP ·subVW_g(SB)
+
+TEXT ·shlVU(SB),NOSPLIT,$0
+	JMP ·shlVU_g(SB)
+
+TEXT ·shrVU(SB),NOSPLIT,$0
+	JMP ·shrVU_g(SB)
+
+TEXT ·mulAddVWW(SB),NOSPLIT,$0
+	JMP ·mulAddVWW_g(SB)
+
+TEXT ·addMulVVW(SB),NOSPLIT,$0
+	JMP ·addMulVVW_g(SB)
+
+TEXT ·divWVW(SB),NOSPLIT,$0
+	JMP ·divWVW_g(SB)