// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: BSD-3-Clause

#include "textflag.h"

// The vector link-regression kernel for riscv64: the RVV load/store and
// arithmetic families computing a checksum end to end.  bytetotal
// strip-mines a byte buffer with VSETVLI, loads each chunk with VLE8V,
// widens it to E64 lanes with VSEXTVF8 and reduces the chunk with
// VREDSUMVS, so the result never depends on the hardware VLEN.  checksum
// passes the slice to bytetotal through the ABI0 stack slots and mixes the
// reduction with the seed through the intra-file mix call.  mix multiplies
// by the odd golden-ratio constant and XORs the seed, the same arithmetic
// the Go side of the regression mirrors bit for bit.

// func bytetotal(buf []byte) int64
TEXT ·bytetotal(SB), NOSPLIT, $0-32
	MOV       buf+0(FP), X10
	MOV       buf_len+8(FP), X12
	MOV       $0, X13
	BEQZ      X12, empty

loop:
	VSETVLI   X12, E64, M1, TA, MA, X11
	VLE8V     (X10), V16
	VSEXTVF8  V16, V9
	VMVVI     $0, V10
	VREDSUMVS V10, V9, V11
	VMVXS     V11, X14
	ADD       X14, X13, X13
	ADD       X11, X10, X10
	SUB       X11, X12, X12
	BNE       X12, X0, loop

empty:
	MOV       X13, ret+24(FP)
	RET

// func mix(x, seed int64) int64
TEXT ·mix(SB), NOSPLIT, $0-24
	MOV x+0(FP), X10
	MOV seed+8(FP), X11
	MOV $0x0101010101010101, X12
	MUL X12, X10, X10
	XOR X11, X10, X10
	MOV X10, ret+16(FP)
	RET

// func checksum(buf []byte, seed int64) int64
TEXT ·checksum(SB), NOSPLIT, $40-40
	MOV  buf+0(FP), X10
	MOV  buf+8(FP), X11
	MOV  buf+16(FP), X12
	MOV  X10, 8(SP)
	MOV  X11, 16(SP)
	MOV  X12, 24(SP)
	CALL ·bytetotal(SB)
	MOV  32(SP), X10
	MOV  seed+24(FP), X11
	MOV  X10, 8(SP)
	MOV  X11, 16(SP)
	CALL ·mix(SB)
	MOV  24(SP), X10
	MOV  X10, ret+32(FP)
	RET