memset_32.c

/*
 * 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);