memset_32.c

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/*
 * Copyright 2010 Tilera Corporation. All Rights Reserved.
 *
 *   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, version 2.
 *
 *   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, GOOD TITLE or
 *   NON INFRINGEMENT.  See the GNU General Public License for
 *   more details.
 */

#include <arch/chip.h>

#include <linux/types.h>
#include <linux/string.h>
#include <linux/module.h>

#undef memset

void *memset(void *s, int c, size_t n)
{
    uint32_t *out32;
    int n32;
    uint32_t v16, v32;
    uint8_t *out8 = s;
#if !CHIP_HAS_WH64()
    int ahead32;
#else
    int to_align32;
#endif

    /* Experimentation shows that a trivial tight loop is a win up until
     * around a size of 20, where writing a word at a time starts to win.
     */
#define BYTE_CUTOFF 20

#if BYTE_CUTOFF < 3
    /* This must be at least at least this big, or some code later
     * on doesn't work.
     */
#error "BYTE_CUTOFF is too small"
#endif

    if (n < BYTE_CUTOFF) {
        /* Strangely, this turns out to be the tightest way to
         * write this loop.
         */
        if (n != 0) {
            do {
                /* Strangely, combining these into one line
                 * performs worse.
                 */
                *out8 = c;
                out8++;
            } while (--n != 0);
        }

        return s;
    }

#if !CHIP_HAS_WH64()
    /* Use a spare issue slot to start prefetching the first cache
     * line early. This instruction is free as the store can be buried
     * in otherwise idle issue slots doing ALU ops.
     */
    __insn_prefetch(out8);

    /* We prefetch the end so that a short memset that spans two cache
     * lines gets some prefetching benefit. Again we believe this is free
     * to issue.
     */
    __insn_prefetch(&out8[n - 1]);
#endif /* !CHIP_HAS_WH64() */


    /* Align 'out8'. We know n >= 3 so this won't write past the end. */
    while (((uintptr_t) out8 & 3) != 0) {
        *out8++ = c;
        --n;
    }

    /* Align 'n'. */
    while (n & 3)
        out8[--n] = c;

    out32 = (uint32_t *) out8;
    n32 = n >> 2;

    /* Tile input byte out to 32 bits. */
    v16 = __insn_intlb(c, c);
    v32 = __insn_intlh(v16, v16);

    /* This must be at least 8 or the following loop doesn't work. */
#define CACHE_LINE_SIZE_IN_WORDS (CHIP_L2_LINE_SIZE() / 4)

#if !CHIP_HAS_WH64()

    ahead32 = CACHE_LINE_SIZE_IN_WORDS;

    /* We already prefetched the first and last cache lines, so
     * we only need to do more prefetching if we are storing
     * to more than two cache lines.
     */
    if (n32 > CACHE_LINE_SIZE_IN_WORDS * 2) {
        int i;

        /* Prefetch the next several cache lines.
         * This is the setup code for the software-pipelined
         * loop below.
         */
#define MAX_PREFETCH 5
        ahead32 = n32 & -CACHE_LINE_SIZE_IN_WORDS;
        if (ahead32 > MAX_PREFETCH * CACHE_LINE_SIZE_IN_WORDS)
            ahead32 = MAX_PREFETCH * CACHE_LINE_SIZE_IN_WORDS;

        for (i = CACHE_LINE_SIZE_IN_WORDS;
             i < ahead32; i += CACHE_LINE_SIZE_IN_WORDS)
            __insn_prefetch(&out32[i]);
    }

    if (n32 > ahead32) {
        while (1) {
            int j;

            /* Prefetch by reading one word several cache lines
             * ahead.  Since loads are non-blocking this will
             * cause the full cache line to be read while we are
             * finishing earlier cache lines.  Using a store
             * here causes microarchitectural performance
             * problems where a victimizing store miss goes to
             * the head of the retry FIFO and locks the pipe for
             * a few cycles.  So a few subsequent stores in this
             * loop go into the retry FIFO, and then later
             * stores see other stores to the same cache line
             * are already in the retry FIFO and themselves go
             * into the retry FIFO, filling it up and grinding
             * to a halt waiting for the original miss to be
             * satisfied.
             */
            __insn_prefetch(&out32[ahead32]);

#if CACHE_LINE_SIZE_IN_WORDS % 4 != 0
#error "Unhandled CACHE_LINE_SIZE_IN_WORDS"
#endif

            n32 -= CACHE_LINE_SIZE_IN_WORDS;

            /* Save icache space by only partially unrolling
             * this loop.
             */
            for (j = CACHE_LINE_SIZE_IN_WORDS / 4; j > 0; j--) {
                *out32++ = v32;
                *out32++ = v32;
                *out32++ = v32;
                *out32++ = v32;
            }

            /* To save compiled code size, reuse this loop even
             * when we run out of prefetching to do by dropping
             * ahead32 down.
             */
            if (n32 <= ahead32) {
                /* Not even a full cache line left,
                 * so stop now.
                 */
                if (n32 < CACHE_LINE_SIZE_IN_WORDS)
                    break;

                /* Choose a small enough value that we don't
                 * prefetch past the end.  There's no sense
                 * in touching cache lines we don't have to.
                 */
                ahead32 = CACHE_LINE_SIZE_IN_WORDS - 1;
            }
        }
    }

#else /* CHIP_HAS_WH64() */

    /* Determine how many words we need to emit before the 'out32'
     * pointer becomes aligned modulo the cache line size.
     */
    to_align32 =
        (-((uintptr_t)out32 >> 2)) & (CACHE_LINE_SIZE_IN_WORDS - 1);

    /* Only bother aligning and using wh64 if there is at least
     * one full cache line to process.  This check also prevents
     * overrunning the end of the buffer with alignment words.
     */
    if (to_align32 <= n32 - CACHE_LINE_SIZE_IN_WORDS) {
        int lines_left;

        /* Align out32 mod the cache line size so we can use wh64. */
        n32 -= to_align32;
        for (; to_align32 != 0; to_align32--) {
            *out32 = v32;
            out32++;
        }

        /* Use unsigned divide to turn this into a right shift. */
        lines_left = (unsigned)n32 / CACHE_LINE_SIZE_IN_WORDS;

        do {
            /* Only wh64 a few lines at a time, so we don't
             * exceed the maximum number of victim lines.
             */
            int x = ((lines_left < CHIP_MAX_OUTSTANDING_VICTIMS())
                  ? lines_left
                  : CHIP_MAX_OUTSTANDING_VICTIMS());
            uint32_t *wh = out32;
            int i = x;
            int j;

            lines_left -= x;

            do {
                __insn_wh64(wh);
                wh += CACHE_LINE_SIZE_IN_WORDS;
            } while (--i);

            for (j = x * (CACHE_LINE_SIZE_IN_WORDS / 4);
                 j != 0; j--) {
                *out32++ = v32;
                *out32++ = v32;
                *out32++ = v32;
                *out32++ = v32;
            }
        } while (lines_left != 0);

        /* We processed all full lines above, so only this many
         * words remain to be processed.
         */
        n32 &= CACHE_LINE_SIZE_IN_WORDS - 1;
    }

#endif /* CHIP_HAS_WH64() */

    /* Now handle any leftover values. */
    if (n32 != 0) {
        do {
            *out32 = v32;
            out32++;
        } while (--n32 != 0);
    }

    return s;
}
EXPORT_SYMBOL(memset);