internal/chacha8rand: provide vector implementation for riscv64

Provide a vector implementation of chacha8rand for riscv64,
which improves performance.

goos: linux
goarch: riscv64
pkg: internal/chacha8rand
cpu: Spacemit(R) X60
      │ /root/chacha8.rand.old.log │     /root/chacha8.rand.new.log      │
      │           sec/op           │   sec/op     vs base                │
Block                  1.640µ ± 0%   1.294µ ± 0%  -21.10% (p=0.000 n=10)

      │ /root/chacha8.rand.old.log │      /root/chacha8.rand.new.log      │
      │            B/s             │     B/s       vs base                │
Block                 148.9Mi ± 0%   188.6Mi ± 0%  +26.72% (p=0.000 n=10)

Change-Id: I1e04c5c44e5ce0c78814a6a48c5ab65e4d758937
Reviewed-on: https://go-review.googlesource.com/c/go/+/710035
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: mengzhuo

src/internal/chacha8rand/chacha8.go

@@ -16,6 +16,7 @@ import (
 // Offsets into internal/cpu records for use in assembly.
 const (
 	offsetLOONG64HasLSX = unsafe.Offsetof(cpu.Loong64.HasLSX)
+	offsetRISCV64HasV   = unsafe.Offsetof(cpu.RISCV64.HasV)
 )
 
 const (

src/internal/chacha8rand/chacha8_riscv64.s

@@ -0,0 +1,113 @@
+// Copyright 2025 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 "asm_riscv64.h"
+#include "go_asm.h"
+#include "textflag.h"
+
+// TODO(mzh): use Zvkb if possible
+
+#define QR(A, B, C, D) \
+	VADDVV	A, B, A \
+	VXORVV	D, A, D \
+	VSLLVI	$16, D, V28 \
+	VSRLVI	$16, D, D \
+	VXORVV	V28, D, D \
+	VADDVV	D, C, C  \
+	VXORVV	C, B, B \
+	VSLLVI	$12, B, V29 \
+	VSRLVI	$20, B, B \
+	VXORVV	V29, B, B \
+	VADDVV	B, A, A  \
+	VXORVV	A, D, D \
+	VSLLVI	$8, D, V30 \
+	VSRLVI	$24, D, D \
+	VXORVV	V30, D, D \
+	VADDVV	D, C, C  \
+	VXORVV	C, B, B \
+	VSLLVI	$7, B, V31 \
+	VSRLVI	$25, B, B \
+	VXORVV	V31, B, B
+
+// block runs four ChaCha8 block transformations using four elements in each V register.
+// func block(seed *[8]uint32, blocks *[16][4]uint32, counter uint32)
+TEXT ·block<ABIInternal>(SB), NOSPLIT, $0
+	// seed in X10
+	// blocks in X11
+	// counter in X12
+
+#ifndef hasV
+	MOVB	internal∕cpu·RISCV64+const_offsetRISCV64HasV(SB), X13
+	BNEZ	X13, vector_chacha8
+	JMP	·block_generic<ABIInternal>(SB)
+#endif
+
+vector_chacha8:
+	// At least VLEN >= 128
+	VSETIVLI	$4, E32, M1, TA, MA, X0
+	// Load initial constants into top row.
+	MOV $·chachaConst(SB), X14
+	VLSSEG4E32V	(X14), X0, V0 // V0, V1, V2, V3 = const row
+	VLSSEG8E32V	(X10), X0, V4 // V4 ... V11, seed
+	VIDV	V12
+	VADDVX	X12, V12, V12		// counter
+
+	// Clear all nonces.
+	VXORVV	V13, V13, V13
+	VXORVV	V14, V14, V14
+	VXORVV	V15, V15, V15
+
+	// Copy initial state.
+	VMV4RV V4, V20
+	VMV4RV V8, V24
+
+	MOV	$4, X15
+	PCALIGN	$16
+loop:
+	QR(V0, V4, V8, V12)
+	QR(V1, V5, V9, V13)
+	QR(V2, V6, V10, V14)
+	QR(V3, V7, V11, V15)
+
+	QR(V0, V5, V10, V15)
+	QR(V1, V6, V11, V12)
+	QR(V2, V7, V8, V13)
+	QR(V3, V4, V9, V14)
+
+	SUB	$1, X15
+	BNEZ	X15, loop
+
+	VADDVV	V20, V4, V4
+	VADDVV	V21, V5, V5
+	VADDVV	V22, V6, V6
+	VADDVV	V23, V7, V7
+	VADDVV	V24, V8, V8
+	VADDVV	V25, V9, V9
+	VADDVV	V26, V10, V10
+	VADDVV	V27, V11, V11
+
+	VSE32V	V0, (X11); ADD $16, X11;
+	VSE32V	V1, (X11); ADD $16, X11;
+	VSE32V	V2, (X11); ADD $16, X11;
+	VSE32V	V3, (X11); ADD $16, X11;
+	VSE32V	V4, (X11); ADD $16, X11;
+	VSE32V	V5, (X11); ADD $16, X11;
+	VSE32V	V6, (X11); ADD $16, X11;
+	VSE32V	V7, (X11); ADD $16, X11;
+	VSE32V	V8, (X11); ADD $16, X11;
+	VSE32V	V9, (X11); ADD $16, X11;
+	VSE32V	V10, (X11); ADD $16, X11;
+	VSE32V	V11, (X11); ADD $16, X11;
+	VSE32V	V12, (X11); ADD $16, X11;
+	VSE32V	V13, (X11); ADD $16, X11;
+	VSE32V	V14, (X11); ADD $16, X11;
+	VSE32V	V15, (X11); ADD $16, X11;
+
+	RET
+
+GLOBL	·chachaConst(SB), NOPTR|RODATA, $32
+DATA	·chachaConst+0x00(SB)/4, $0x61707865
+DATA	·chachaConst+0x04(SB)/4, $0x3320646e
+DATA	·chachaConst+0x08(SB)/4, $0x79622d32
+DATA	·chachaConst+0x0c(SB)/4, $0x6b206574

src/internal/chacha8rand/chacha8_stub.s

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !amd64 && !arm64 && !loong64
+//go:build !amd64 && !arm64 && !loong64 && !riscv64
 
 #include "textflag.h"