string.h

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#ifndef X86_BITS_STRING_H
#define X86_BITS_STRING_H

/*
 * Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );

/** @file
 *
 * Optimised string operations
 *
 */

extern void * __memcpy ( void *dest, const void *src, size_t len );
extern void * __memcpy_reverse ( void *dest, const void *src, size_t len );

/**
 * Copy memory area (where length is a compile-time constant)
 *
 * @v dest              Destination address
 * @v src               Source address
 * @v len               Length
 * @ret dest            Destination address
 */
static inline __attribute__ (( always_inline )) void *
__constant_memcpy ( void *dest, const void *src, size_t len ) {
        union {
                uint32_t u32[2];
                uint16_t u16[4];
                uint8_t  u8[8];
        } __attribute__ (( __may_alias__ )) *dest_u = dest;
        const union {
                uint32_t u32[2];
                uint16_t u16[4];
                uint8_t  u8[8];
        } __attribute__ (( __may_alias__ )) *src_u = src;
        const void *esi;
        void *edi;

        switch ( len ) {
        case 0 : /* 0 bytes */
                return dest;
        /*
         * Single-register moves; these are always better than a
         * string operation.  We can clobber an arbitrary two
         * registers (data, source, dest can re-use source register)
         * instead of being restricted to esi and edi.  There's also a
         * much greater potential for optimising with nearby code.
         *
         */
        case 1 : /* 4 bytes */
                dest_u->u8[0]  = src_u->u8[0];
                return dest;
        case 2 : /* 6 bytes */
                dest_u->u16[0] = src_u->u16[0];
                return dest;
        case 4 : /* 4 bytes */
                dest_u->u32[0] = src_u->u32[0];
                return dest;
        /*
         * Double-register moves; these are probably still a win.
         *
         */
        case 3 : /* 12 bytes */
                dest_u->u16[0] = src_u->u16[0];
                dest_u->u8[2]  = src_u->u8[2];
                return dest;
        case 5 : /* 10 bytes */
                dest_u->u32[0] = src_u->u32[0];
                dest_u->u8[4]  = src_u->u8[4];
                return dest;
        case 6 : /* 12 bytes */
                dest_u->u32[0] = src_u->u32[0];
                dest_u->u16[2] = src_u->u16[2];
                return dest;
        case 8 : /* 10 bytes */
                dest_u->u32[0] = src_u->u32[0];
                dest_u->u32[1] = src_u->u32[1];
                return dest;
        }

        /* Even if we have to load up esi and edi ready for a string
         * operation, we can sometimes save space by using multiple
         * single-byte "movs" operations instead of loading up ecx and
         * using "rep movsb".
         *
         * "load ecx, rep movsb" is 7 bytes, plus an average of 1 byte
         * to allow for saving/restoring ecx 50% of the time.
         *
         * "movsl" and "movsb" are 1 byte each, "movsw" is two bytes.
         * (In 16-bit mode, "movsl" is 2 bytes and "movsw" is 1 byte,
         * but "movsl" moves twice as much data, so it balances out).
         *
         * The cutoff point therefore occurs around 26 bytes; the byte
         * requirements for each method are:
         *
         * len             16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
         * #bytes (ecx)     8  8  8  8  8  8  8  8  8  8  8  8  8  8  8  8
         * #bytes (no ecx)  4  5  6  7  5  6  7  8  6  7  8  9  7  8  9 10
         */

        esi = src;
        edi = dest;
        
        if ( len >= 26 )
                return __memcpy ( dest, src, len );
        
        if ( len >= 6*4 )
                __asm__ __volatile__ ( "movsl" : "=&D" ( edi ), "=&S" ( esi )
                                       : "0" ( edi ), "1" ( esi ) : "memory" );
        if ( len >= 5*4 )
                __asm__ __volatile__ ( "movsl" : "=&D" ( edi ), "=&S" ( esi )
                                       : "0" ( edi ), "1" ( esi ) : "memory" );
        if ( len >= 4*4 )
                __asm__ __volatile__ ( "movsl" : "=&D" ( edi ), "=&S" ( esi )
                                       : "0" ( edi ), "1" ( esi ) : "memory" );
        if ( len >= 3*4 )
                __asm__ __volatile__ ( "movsl" : "=&D" ( edi ), "=&S" ( esi )
                                       : "0" ( edi ), "1" ( esi ) : "memory" );
        if ( len >= 2*4 )
                __asm__ __volatile__ ( "movsl" : "=&D" ( edi ), "=&S" ( esi )
                                       : "0" ( edi ), "1" ( esi ) : "memory" );
        if ( len >= 1*4 )
                __asm__ __volatile__ ( "movsl" : "=&D" ( edi ), "=&S" ( esi )
                                       : "0" ( edi ), "1" ( esi ) : "memory" );
        if ( ( len % 4 ) >= 2 )
                __asm__ __volatile__ ( "movsw" : "=&D" ( edi ), "=&S" ( esi )
                                       : "0" ( edi ), "1" ( esi ) : "memory" );
        if ( ( len % 2 ) >= 1 )
                __asm__ __volatile__ ( "movsb" : "=&D" ( edi ), "=&S" ( esi )
                                       : "0" ( edi ), "1" ( esi ) : "memory" );

        return dest;
}

/**
 * Copy memory area
 *
 * @v dest              Destination address
 * @v src               Source address
 * @v len               Length
 * @ret dest            Destination address
 */
static inline __attribute__ (( always_inline )) void *
memcpy ( void *dest, const void *src, size_t len ) {
        if ( __builtin_constant_p ( len ) ) {
                return __constant_memcpy ( dest, src, len );
        } else {
                return __memcpy ( dest, src, len );
        }
}

extern void * __memmove ( void *dest, const void *src, size_t len );

/**
 * Copy (possibly overlapping) memory area
 *
 * @v dest              Destination address
 * @v src               Source address
 * @v len               Length
 * @ret dest            Destination address
 */
static inline __attribute__ (( always_inline )) void *
memmove ( void *dest, const void *src, size_t len ) {
        ssize_t offset = ( dest - src );

        if ( __builtin_constant_p ( offset ) ) {
                if ( offset <= 0 ) {
                        return memcpy ( dest, src, len );
                } else {
                        return __memcpy_reverse ( dest, src, len );
                }
        } else {
                return __memmove ( dest, src, len );
        }
}

/**
 * Fill memory region
 *
 * @v dest              Destination address
 * @v fill              Fill pattern
 * @v len               Length
 * @ret dest            Destination address
 */
static inline __attribute__ (( always_inline )) void *
__memset ( void *dest, int fill, size_t len ) {
        void *discard_D;
        size_t discard_c;

        __asm__ __volatile__ ( "rep stosb"
                               : "=&D" ( discard_D ), "=&c" ( discard_c )
                               : "0" ( dest ), "1" ( len ), "a" ( fill )
                               : "memory" );
        return dest;
}

/**
 * Fill memory region with zero (where length is a compile-time constant)
 *
 * @v dest              Destination address
 * @v len               Length
 * @ret dest            Destination address
 */
static inline __attribute__ (( always_inline )) void *
__constant_memset_zero ( void *dest, size_t len ) {
        union {
                uint32_t u32[2];
                uint16_t u16[4];
                uint8_t  u8[8];
        } __attribute__ (( __may_alias__ )) *dest_u = dest;
        void *edi;
        uint32_t eax;

        switch ( len ) {
        case 0 : /* 0 bytes */
                return dest;

        /* Single-register moves.  Almost certainly better than a
         * string operation.  We can avoid clobbering any registers,
         * we can reuse a zero that happens to already be in a
         * register, and we can optimise away the code entirely if the
         * memset() is used to clear a region which then gets
         * immediately overwritten.
         */
        case 1 : /* 3 bytes */
                dest_u->u8[0] = 0;
                return dest;
        case 2: /* 5 bytes */
                dest_u->u16[0] = 0;
                return dest;
        case 4: /* 6 bytes */
                dest_u->u32[0] = 0;
                return dest;

        /* Double-register moves.  Very probably better than a string
         * operation.
         */
        case 3 : /* 9 bytes */
                dest_u->u16[0] = 0;
                dest_u->u8[2]  = 0;
                return dest;
        case 5 : /* 10 bytes */
                dest_u->u32[0] = 0;
                dest_u->u8[4]  = 0;
                return dest;
        case 6 : /* 12 bytes */
                dest_u->u32[0] = 0;
                dest_u->u16[2] = 0;
                return dest;
        case 8 : /* 13 bytes */
                dest_u->u32[0] = 0;
                dest_u->u32[1] = 0;
                return dest;
        }

        /* As with memcpy(), we can potentially save space by using
         * multiple single-byte "stos" instructions instead of loading
         * up ecx and using "rep stosb".
         *
         * "load ecx, rep movsb" is 7 bytes, plus an average of 1 byte
         * to allow for saving/restoring ecx 50% of the time.
         *
         * "stosl" and "stosb" are 1 byte each, "stosw" is two bytes.
         *
         * The calculations are therefore the same as for memcpy(),
         * giving a cutoff point of around 26 bytes.
         */

        edi = dest;
        eax = 0;

        if ( len >= 26 )
                return __memset ( dest, 0, len );

        if ( len >= 6*4 )
                __asm__ __volatile__ ( "stosl" : "=&D" ( edi ), "=&a" ( eax )
                                       : "0" ( edi ), "1" ( eax ) : "memory" );
        if ( len >= 5*4 )
                __asm__ __volatile__ ( "stosl" : "=&D" ( edi ), "=&a" ( eax )
                                       : "0" ( edi ), "1" ( eax ) : "memory" );
        if ( len >= 4*4 )
                __asm__ __volatile__ ( "stosl" : "=&D" ( edi ), "=&a" ( eax )
                                       : "0" ( edi ), "1" ( eax ) : "memory" );
        if ( len >= 3*4 )
                __asm__ __volatile__ ( "stosl" : "=&D" ( edi ), "=&a" ( eax )
                                       : "0" ( edi ), "1" ( eax ) : "memory" );
        if ( len >= 2*4 )
                __asm__ __volatile__ ( "stosl" : "=&D" ( edi ), "=&a" ( eax )
                                       : "0" ( edi ), "1" ( eax ) : "memory" );
        if ( len >= 1*4 )
                __asm__ __volatile__ ( "stosl" : "=&D" ( edi ), "=&a" ( eax )
                                       : "0" ( edi ), "1" ( eax ) : "memory" );
        if ( ( len % 4 ) >= 2 )
                __asm__ __volatile__ ( "stosw" : "=&D" ( edi ), "=&a" ( eax )
                                       : "0" ( edi ), "1" ( eax ) : "memory" );
        if ( ( len % 2 ) >= 1 )
                __asm__ __volatile__ ( "stosb" : "=&D" ( edi ), "=&a" ( eax )
                                       : "0" ( edi ), "1" ( eax ) : "memory" );

        return dest;
}

/**
 * Fill memory region
 *
 * @v dest              Destination address
 * @v fill              Fill pattern
 * @v len               Length
 * @ret dest            Destination address
 */
static inline __attribute__ (( always_inline )) void *
memset ( void *dest, int fill, size_t len ) {

        if ( __builtin_constant_p ( fill ) && ( fill == 0 ) &&
             __builtin_constant_p ( len ) ) {
                return __constant_memset_zero ( dest, len );
        } else {
                return __memset ( dest, fill, len );
        }
}

#endif /* X86_BITS_STRING_H */