/*
 * Copyright (c) 2023 The FreeBSD Foundation
 *
 * This software was developed by Robert Clausecker <fuz@FreeBSD.org>
 * under sponsorship from the FreeBSD Foundation.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ''AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE
 */

#include <machine/asm.h>

#include "amd64_archlevel.h"

#define ALIGN_TEXT      .p2align 4, 0x90

        .weak stpncpy
        .set stpncpy, __stpncpy
ARCHFUNCS(__stpncpy)
        ARCHFUNC(__stpncpy, scalar)
        ARCHFUNC(__stpncpy, baseline)
ENDARCHFUNCS(__stpncpy)

ARCHENTRY(__stpncpy, scalar)
        push    %rbp            # establish stack frame
        mov     %rsp, %rbp

        push    %rdx
        push    %rdi
        push    %rsi
        push    %rax            # dummy push for alignment

        mov     %rsi, %rdi
        xor     %esi, %esi
        call    CNAME(__memchr) # memchr(src, '\0', len)
        pop     %rcx            # dummy pop
        pop     %rsi
        mov     -16(%rbp), %rdi

        test    %rax, %rax      # NUL found?
        jz      .Lfullcopy

        mov     %rax, %rdx
        sub     %rsi, %rdx      # copy until the NUL byte
        add     %rdx, -16(%rbp) # advance destination by string length
        sub     %rdx, -8(%rbp)  # and shorten buffer size by string length
        call    CNAME(memcpy)

        pop     %rdi
        pop     %rdx
        xor     %esi, %esi
        pop     %rbp
        jmp     CNAME(memset)   # clear remaining buffer

.Lfullcopy:
        mov     -8(%rbp), %rdx
        call    CNAME(memcpy)   # copy whole string
        add     -8(%rbp), %rax  # point to dest[n]
        leave
        ret
ARCHEND(__stpncpy, scalar)

        /*
         * this mask allows us to generate masks of 16-n 0xff bytes
         * followed by n 0x00 bytes by loading from .Lmask+n.
         */
        .section        .rodata
.Lmask: .quad           0xffffffffffffffff
        .quad           0xffffffffffffffff
        .quad           0x0000000000000000
        .quad           0x0000000000000000

/* stpncpy(char *restrict rdi, const char *rsi, size_t rdx) */
ARCHENTRY(__stpncpy, baseline)
#define bounce          (-3*16-8)               /* location of on-stack bounce buffer */
        test            %rdx, %rdx              # no bytes to copy?
        jz              .L0

        mov             %esi, %ecx
        and             $~0xf, %rsi             # align source to 16 bytes
        movdqa          (%rsi), %xmm0           # load head
        and             $0xf, %ecx              # offset from alignment
        mov             $-1, %r9d
        lea             -33(%rcx), %rax         # set up overflow-proof comparison rdx+rcx<=32
        shl             %cl, %r9d               # mask of bytes belonging to the string
        sub             %rcx, %rdi              # adjust RDI to correspond to RSI
        pxor            %xmm1, %xmm1
        movdqa          %xmm0, bounce(%rsp)     # stash copy of head on the stack
        pcmpeqb         %xmm1, %xmm0
        pmovmskb        %xmm0, %r8d

        lea             (%rdx, %rcx, 1), %r10   # buffer length from alignment boundary
        add             %rdx, %rax              # less than 2 chunks (32 bytes) to play with?
        jnc             .Lrunt                  # if yes, use special runt processing

        movdqu          %xmm1, -16(%rdi, %r10, 1) # clear final bytes of destination
        and             %r9d, %r8d              # end of string within head?
        jnz             .Lheadnul

        movdqu          (%rsi, %rcx, 1), %xmm2  # load head from source buffer
        movdqu          %xmm2, (%rdi, %rcx, 1)  # an deposit

        add             $16, %rsi
        add             $16, %rdi
        sub             $32, %r10

        /* main loop unrolled twice */
        ALIGN_TEXT
0:      movdqa          (%rsi), %xmm0
        pxor            %xmm1, %xmm1
        pcmpeqb         %xmm0, %xmm1            # NUL byte encountered?
        pmovmskb        %xmm1, %r8d
        test            %r8d, %r8d
        jnz             3f

        movdqu          %xmm0, (%rdi)
        cmp             $16, %r10               # more than a full chunk left?
        jbe             1f

        movdqa          16(%rsi), %xmm0
        add             $32, %rdi               # advance pointers to next chunk
        add             $32, %rsi
        pxor            %xmm1, %xmm1
        pcmpeqb         %xmm0, %xmm1            # NUL byte encountered?
        pmovmskb        %xmm1, %r8d
        test            %r8d, %r8d
        jnz             2f

        movdqu          %xmm0, -16(%rdi)
        sub             $32, %r10               # more than another full chunk left?
        ja              0b

        sub             $16, %rdi               # undo second advancement
        sub             $16, %rsi
        add             $16, %r10d              # restore number of remaining bytes

        /* 1--16 bytes left but string has not ended yet */
1:      pxor            %xmm1, %xmm1
        pcmpeqb         16(%rsi), %xmm1         # NUL byte in source tail?
        pmovmskb        %xmm1, %r8d
        bts             %r10d, %r8d             # treat end of buffer as NUL
        tzcnt           %r8d, %r8d              # where is the NUL byte?
        movdqu          (%rsi, %r8, 1), %xmm0   # load source tail before NUL
        lea             16(%rdi, %r8, 1), %rax  # point return value to NUL byte
                                                # or end of buffer
        movdqu          %xmm0, (%rdi, %r8, 1)   # store tail into the buffer
        ret

2:      sub             $16, %rdi               # undo second advancement
        sub             $16, %rsi
        sub             $16, %r10

        /* string has ended and buffer has not */
3:      tzcnt           %r8d, %r8d              # where did the string end?
        lea             .Lmask+16(%rip), %rcx
        lea             (%rdi, %r8, 1), %rax    # where the NUL byte will be
        neg             %r8
        movdqu          (%rcx, %r8, 1), %xmm1   # mask with FF where the string is,
                                                # 00 where it is not
        pand            %xmm1, %xmm0            # mask out bytes after the string
        movdqu          %xmm0, (%rdi)           # store masked current chunk
        pxor            %xmm1, %xmm1
        sub             $16, %r10               # another full chunk left?
        jbe             1f

        /* clear remaining destination buffer (tail has been cleared earlier) */
        ALIGN_TEXT
0:      movdqu          %xmm1, 16(%rdi)
        cmp             $16, %r10
        jbe             1f

        movdqu          %xmm1, 32(%rdi)
        add             $32, %rdi
        sub             $32, %r10
        ja              0b

1:      ret

        /* at least two chunks to play with and NUL while processing head */
.Lheadnul:
        movdqu          bounce(%rsp, %rcx, 1), %xmm0 # load start of source from stack
        tzcnt           %r8d, %r8d              # find location of NUL byte
        movdqu          %xmm0, (%rdi, %rcx, 1)  # deposit head in the destination
        movdqu          %xmm1, (%rdi, %r8, 1)   # clear out following bytes
        movdqu          %xmm1, 16(%rdi)         # clear out second chunk
        lea             (%rdi, %r8, 1), %rax    # make RAX point to the NUL byte

        add             $32, %rdi               # advance past first two chunks
        sub             $32+16, %r10            # advance past first three chunks
        jbe             1f                      # did we pass the end of the buffer?

        /* clear remaining destination buffer (tail has been cleared earlier) */
        ALIGN_TEXT
0:      movdqu          %xmm1, (%rdi)           # clear out buffer chunk
        cmp             $16, %r10
        jbe             1f

        movdqu          %xmm1, 16(%rdi)
        add             $32, %rdi
        sub             $32, %r10
        ja              0b

1:      ret

        /* 1--32 bytes to copy, bounce through the stack */
.Lrunt: movdqa          %xmm1, bounce+16(%rsp)  # clear out rest of on-stack copy
        and             %r9d, %r8d              # mask out head before string
        bts             %r10, %r8               # treat end of buffer as end of string
        test            $0x1ffff, %r8d          # end of string within first chunk or right after?
        jnz             0f                      # if yes, do not inspect second buffer

        movdqa          16(%rsi), %xmm0         # load second chunk of input
        movdqa          %xmm0, bounce+16(%rsp)  # stash copy on stack
        pcmpeqb         %xmm1, %xmm0            # NUL in second chunk?
        pmovmskb        %xmm0, %r9d
        shl             $16, %r9d
        or              %r9, %r8                # merge found NUL bytes into NUL mask

        /* end of string after one buffer */
0:      tzcnt           %r8, %r8                # location of last char in string
        movdqu          %xmm1, bounce(%rsp, %r8, 1) # clear bytes behind string
        lea             bounce(%rsp, %rcx, 1), %rsi # start of string copy on stack
        lea             (%rdi, %r8, 1), %rax    # return pointer to NUL byte

        cmp             $16, %edx               # at least 16 bytes to transfer?
        jae             .L1631

        mov             (%rsi), %r8             # load string head
        cmp             $8, %edx                # at least 8 bytes to transfer?
        jae             .L0815

        cmp             $4, %edx                # at least 4 bytes to transfer?
        jae             .L0407

        movzwl          -2(%rsi, %rdx, 1), %esi # load last two bytes of string
        mov             %r8b, (%rdi, %rcx, 1)   # store first byte

        cmp             $2, %edx                # at least 2 bytes to transfer?
        jb              .L1

        mov             %si, -2(%rdi, %r10, 1)  # store last two bytes of string
.L1:    ret

.L1631: movdqu          (%rsi), %xmm0           # load first 16 bytes of string
        movdqu          -16(%rsi, %rdx, 1), %xmm1 # load last 16 bytes of string
        movdqu          %xmm0, (%rdi, %rcx, 1)
        movdqu          %xmm1, -16(%rdi, %r10, 1)
        ret

.L0815: mov             -8(%rsi, %rdx, 1), %rdx # load last 8 bytes of string
        mov             %r8, (%rdi, %rcx, 1)
        mov             %rdx, -8(%rdi, %r10, 1)
        ret

.L0407: mov             -4(%rsi, %rdx, 1), %edx # load last four bytes of string
        mov             %r8d, (%rdi, %rcx, 1)
        mov             %edx, -4(%rdi, %r10, 1)
        ret

        /* length 0 buffer: just return dest */
.L0:    mov             %rdi, %rax
        ret
ARCHEND(__stpncpy, baseline)

        .section .note.GNU-stack,"",%progbits