/* Copyright (c) 2017 LiteSpeed Technologies Inc.  See LICENSE. */
/*
 * lsquic_malo.c -- malo allocator implementation.
 *
 * The malo allocator is a pool of objects of fixed size.  It tries to
 * allocate and deallocate objects as fast as possible.  To do so, it
 * does the following:
 *
 *  1. Allocations occur 4 KB at a time.
 *  2. No division or multiplication operations are performed.
 *
 * (In recent testing, malo was about 2.7 times faster than malloc for
 * 64-byte objects.)
 *
 * Besides speed, two other important characteristics distinguish it
 * from other pool allocators:
 *
 *  1. To free (put) an object, one does not need a pointer to the malo
 *     object.  This makes this allocator easy to use.
 *  2. A built-in iterator is provided to iterate over all allocated
 *     objects (with ability to safely release objects while iterator
 *     is active).  This may be useful in some circumstances.
 *
 * To gain all these advantages, there are trade-offs:
 *
 *  1. There are two memory penalties:
 *      a. Per object overhead.  To avoid division and multiplication,
 *         the object sizes is rounded up to the nearest power or two,
 *         starting with 64 bytes (minumum) and up to 2 KB (maximum).
 *         Thus, a 104-byte object will have a 24-byte overhead; a
 *         130-byte object will have 126-byte overhead.  This is
 *         something to keep in mind.
 *      b. Per page overhead.  Page links occupy some bytes in the
 *         page.  To keep things fast, at least one slot per page is
 *         always occupied, independent of object size.  Thus, for a
 *         1 KB object size, 25% of the page is used for the page
 *         header.
 *  2. 4 KB pages are not freed until the malo allocator is destroyed.
 *     This is something to keep in mind.
 *
 * P.S. In Russian, "malo" (мало) means "little" or "few".  Thus, the
 *      malo allocator aims to perform its job in as few CPU cycles as
 *      possible.
 */

#include <assert.h>
#include <errno.h>
#include <stdint.h>
#include <stdlib.h>
#include <sys/queue.h>
#ifdef WIN32
#include <vc_compat.h>
#endif

#include "fiu-local.h"
#include "lsquic_malo.h"

/* 64 slots in a 4KB page means that the smallest object is 64 bytes.
 * The largest object is 2KB.
 */
#define MALO_MIN_NBITS 6
#define MALO_MAX_NBITS 11

/* A "free page" is a page with free slots available.
 */

static unsigned find_free_slot (uint64_t slots);
static unsigned size_in_bits (size_t sz);

struct malo_page {
    SLIST_ENTRY(malo_page)  next_page;
    LIST_ENTRY(malo_page)   next_free_page;
    struct malo            *malo;
    uint64_t                slots,
                            full_slot_mask;
    unsigned                nbits;
    unsigned                initial_slot;
};

typedef char malo_header_fits_in_one_slot
    [(sizeof(struct malo_page) > (1 << MALO_MAX_NBITS)) ? -1 : 1];

struct malo {
    struct malo_page        page_header;
    SLIST_HEAD(, malo_page) all_pages;
    LIST_HEAD(, malo_page)  free_pages;
    struct {
        struct malo_page   *cur_page;
        unsigned            next_slot;
    }                       iter;
};

struct malo *
lsquic_malo_create (size_t obj_size)
{
    unsigned nbits = size_in_bits(obj_size);
    if (nbits < MALO_MIN_NBITS)
        nbits = MALO_MIN_NBITS;
    else if (nbits > MALO_MAX_NBITS)
    {
        errno = EOVERFLOW;
        return NULL;
    }

    struct malo *malo;
    if (0 != posix_memalign((void **) &malo, 0x1000, 0x1000))
        return NULL;

    SLIST_INIT(&malo->all_pages);
    LIST_INIT(&malo->free_pages);
    malo->iter.cur_page = &malo->page_header;
    malo->iter.next_slot = 0;

    int n_slots =   sizeof(*malo) / (1 << nbits)
                + ((sizeof(*malo) % (1 << nbits)) > 0);

    struct malo_page *const page = &malo->page_header;
    SLIST_INSERT_HEAD(&malo->all_pages, page, next_page);
    LIST_INSERT_HEAD(&malo->free_pages, page, next_free_page);
    page->malo = malo;
    if (nbits == MALO_MIN_NBITS)
        page->full_slot_mask = ~0ULL;
    else
        page->full_slot_mask = (1ULL << (1 << (12 - nbits))) - 1;
    page->slots = (1ULL << n_slots) - 1;
    page->nbits = nbits;
    page->initial_slot = n_slots;

    return malo;
}


static struct malo_page *
allocate_page (struct malo *malo)
{
    struct malo_page *page;
    if (0 != posix_memalign((void **) &page, 0x1000, 0x1000))
        return NULL;
    SLIST_INSERT_HEAD(&malo->all_pages, page, next_page);
    LIST_INSERT_HEAD(&malo->free_pages, page, next_free_page);
    page->slots = 1;
    page->full_slot_mask = malo->page_header.full_slot_mask;
    page->nbits = malo->page_header.nbits;
    page->malo = malo;
    page->initial_slot = 1;
    return page;
}


#define FAIL_NOMEM do { errno = ENOMEM; return NULL; } while (0)

/* Get a new object. */
void *
lsquic_malo_get (struct malo *malo)
{
    fiu_do_on("malo/get", FAIL_NOMEM);
    struct malo_page *page = LIST_FIRST(&malo->free_pages);
    if (!page)
    {
        page = allocate_page(malo);
        if (!page)
            return NULL;
    }
    unsigned slot = find_free_slot(page->slots);
    page->slots |= (1ULL << slot);
    if (page->full_slot_mask == page->slots)
        LIST_REMOVE(page, next_free_page);
    return (char *) page + (slot << page->nbits);
}


/* Return obj to the pool */
void
lsquic_malo_put (void *obj)
{
    uintptr_t page_addr = (uintptr_t) obj & ~((1 << 12) - 1);
    struct malo_page *page = (void *) page_addr;
    unsigned slot = ((uintptr_t) obj - page_addr) >> page->nbits;
    if (page->full_slot_mask == page->slots)
        LIST_INSERT_HEAD(&page->malo->free_pages, page, next_free_page);
    page->slots &= ~(1ULL << slot);
}


void
lsquic_malo_destroy (struct malo *malo)
{
    struct malo_page *page, *next;
    page = SLIST_FIRST(&malo->all_pages);
    while (page != &malo->page_header)
    {
        next = SLIST_NEXT(page, next_page);
#ifndef WIN32
        free(page);
#else
        _aligned_free(page);
#endif
        page = next;
    }
#ifndef WIN32
    free(page);
#else
        _aligned_free(page);
#endif
}


/* The iterator is built-in.  Usage:
 * void *obj;
 * for (obj = lsquic_malo_first(malo); obj; lsquic_malo_next(malo))
 *     do_stuff(obj);
 */
void *
lsquic_malo_first (struct malo *malo)
{
    malo->iter.cur_page = SLIST_FIRST(&malo->all_pages);
    malo->iter.next_slot = malo->iter.cur_page->initial_slot;
    return lsquic_malo_next(malo);
}


void *
lsquic_malo_next (struct malo *malo)
{
    struct malo_page *page;
    unsigned max_slot, slot;

    page = malo->iter.cur_page;
    if (page)
    {
        max_slot = 1 << (12 - page->nbits);     /* Same for all pages */
        slot = malo->iter.next_slot;
        while (1)
        {
            for (; slot < max_slot; ++slot)
            {
                if (page->slots & (1ULL << slot))
                {
                    malo->iter.cur_page  = page;
                    malo->iter.next_slot = slot + 1;
                    return (char *) page + (slot << page->nbits);
                }
            }
            page = SLIST_NEXT(page, next_page);
            if (page)
                slot = page->initial_slot;
            else
            {
                malo->iter.cur_page = NULL;     /* Stop iterator */
                return NULL;
            }
        }
    }

    return NULL;
}


static unsigned
size_in_bits (size_t sz)
{
#if __GNUC__
    unsigned clz = __builtin_clz(sz - 1);
    return 32 - clz;
#else
    unsigned clz;
    size_t y;

    --sz;
    clz = 32;
    y = sz >> 16;   if (y) { clz -= 16; sz = y; }
    y = sz >>  8;   if (y) { clz -=  8; sz = y; }
    y = sz >>  4;   if (y) { clz -=  4; sz = y; }
    y = sz >>  2;   if (y) { clz -=  2; sz = y; }
    y = sz >>  1;   if (y) return 32 - clz + 1;
    return 32 - clz + sz;
#endif
}


static unsigned
find_free_slot (uint64_t slots)
{
#if __GNUC__
    return __builtin_ffsll(~slots) - 1;
#else
    unsigned n;

    slots =~ slots;
    n = 0;

    if (0 == (slots & ((1ULL << 32) - 1))) { n += 32; slots >>= 32; }
    if (0 == (slots & ((1ULL << 16) - 1))) { n += 16; slots >>= 16; }
    if (0 == (slots & ((1ULL <<  8) - 1))) { n +=  8; slots >>=  8; }
    if (0 == (slots & ((1ULL <<  4) - 1))) { n +=  4; slots >>=  4; }
    if (0 == (slots & ((1ULL <<  2) - 1))) { n +=  2; slots >>=  2; }
    if (0 == (slots & ((1ULL <<  1) - 1))) { n +=  1; slots >>=  1; }
    return n;
#endif
}


size_t
lsquic_malo_mem_used (const struct malo *malo)
{
    const struct malo_page *page;
    size_t size;

    size = 0;
    SLIST_FOREACH(page, &malo->all_pages, next_page)
        size += sizeof(*page);

    return size;
}