nodemcu-firmware/app/lua/lrotable.c

/* Read-only tables for Lua */
#define LUAC_CROSS_FILE

#include "lua.h"
#include C_HEADER_STRING
#include "lrotable.h"
#include "lauxlib.h"
#include "lstring.h"
#include "lobject.h"
#include "lapi.h"

#ifdef _MSC_VER
#define ALIGNED_STRING (__declspec( align( 4 ) ) char*)
#else
#define ALIGNED_STRING (__attribute__((aligned(4))) char *)
#endif

#define LA_LINES 32
#define LA_SLOTS 4
//#define COLLECT_STATS

/*
 * All keyed ROtable access passes through luaR_findentry().  ROTables
 * are simply a list of <key><TValue value> pairs.  The existing algo
 * did a linear scan of this vector of pairs looking for a match.
 *
 * A N×M lookaside cache has been added, with a simple hash on the key's
 * TString addr and the ROTable addr to identify one of N lines.  Each
 * line has M slots which are scanned. This is all done in RAM and is
 * perhaps 20x faster than the corresponding random Flash accesses which
 * will cause flash faults.
 *
 * If a match is found and the table addresses match, then this entry is
 * probed first. In practice the hit-rate here is over 99% so the code
 * rarely fails back to doing the linear scan in ROM.
 *
 * Note that this hash does a couple of prime multiples and a modulus 2^X
 * with is all evaluated in H/W, and adequately randomizes the lookup.
 */
#define HASH(a,b) (unsigned)((((519*(size_t)(a)))>>4) + ((b) ? (b)->tsv.hash: 0))

typedef struct {
  unsigned hash;
  unsigned addr:24;
  unsigned ndx:8;
} cache_line_t;

static cache_line_t cache [LA_LINES][LA_SLOTS];

#ifdef COLLECT_STATS
unsigned cache_stats[3];
#define COUNT(i) cache_stats[i]++
#else
#define COUNT(i)
#endif

static int lookup_cache(unsigned hash, ROTable *rotable) {
  int i = (hash>>2) & (LA_LINES-1), j;

  for (j = 0; j<LA_SLOTS; j++) {
    cache_line_t cl = cache[i][j];
    if (cl.hash == hash && ((size_t)rotable & 0xffffffu) == cl.addr) {
      COUNT(0);
      return cl.ndx;
    }
  }
  COUNT(1);
  return -1;
}

static void update_cache(unsigned hash, ROTable *rotable, unsigned ndx) {
  int i = (hash)>>2 & (LA_LINES-1), j;
#ifndef _MSC_VER
  cache_line_t cl = {hash, (size_t) rotable, ndx};
#else
  cache_line_t cl;             // MSC doesn't allow non-scalar initialisers, which
  cl.hash = hash;              // is a pity because xtensa gcc generates optimum   
  cl.addr = (size_t) rotable;  // code using them.
  cl.ndx  = ndx;
#endif

  COUNT(2);
  if (ndx>0xffu)
    return;
  for (j = LA_SLOTS-1; j>0; j--)
    cache[i][j] = cache[i][j-1];
  cache[i][0] = cl;
}
/*
 * Find a string key entry in a rotable and return it.  Note that this internally
 * uses a null key to denote a metatable search.
 */
const TValue* luaR_findentry(ROTable *rotable, TString *key, unsigned *ppos) {
  const luaR_entry *pentry = rotable;
  const char *strkey = key ? getstr(key) : ALIGNED_STRING "__metatable" ;
  unsigned   hash    = HASH(rotable, key);

  unsigned    i      = 0;
  int         j      = lookup_cache(hash, rotable);
  unsigned    l      = key ? key->tsv.len : sizeof("__metatable")-1;

  if (pentry) {
    if (j >= 0 && !c_strcmp(pentry[j].key, strkey)) {
      if (ppos)
        *ppos = j;
//dbg_printf("%3d hit  %p %s\n", (hash>>2) & (LA_LINES-1), rotable, strkey);
      return &pentry[j].value;
    }
    /*
     * The invariants for 1st word comparison are deferred to here since they
     * aren't needed if there is a cache hit. Note that the termination null
     * is included so a "on\0" has a mask of 0xFFFFFF and "a\0" has 0xFFFF.
     */
    unsigned name4, mask4 = l > 2 ? (~0u) : (~0u)>>((3-l)*8);
    c_memcpy(&name4, strkey, sizeof(name4));

    for(;pentry->key != NULL; i++, pentry++) {
      if (((*(unsigned *)pentry->key ^ name4) & mask4) == 0 &&
          !c_strcmp(pentry->key, strkey)) {
//dbg_printf("%p %s hit after %d probes \n", rotable, strkey, (int)(rotable-pentry));
        if (ppos)
          *ppos = i;
        update_cache(hash, rotable, pentry - rotable);
//dbg_printf("%3d %3d  %p %s\n", (hash>>2) & (LA_LINES-1), (int)(pentry-rotable), rotable, strkey);
       return &pentry->value;
      }
    }
  }
//dbg_printf("%p %s miss after %d probes \n", rotable, strkey, (int)(rotable-pentry));
  return luaO_nilobject;
}

/* Find the metatable of a given table */
void* luaR_getmeta(ROTable *rotable) {
  const TValue *res = luaR_findentry(rotable, NULL, NULL);
  return res && ttisrotable(res) ? rvalue(res) : NULL;
}

static void luaR_next_helper(lua_State *L, ROTable *pentries, int pos,
                             TValue *key, TValue *val) {
  if (pentries[pos].key) {
    /* Found an entry */
    setsvalue(L, key, luaS_new(L, pentries[pos].key));
    setobj2s(L, val, &pentries[pos].value);
  } else {
    setnilvalue(key);
    setnilvalue(val);
  }
}


/* next (used for iteration) */
void luaR_next(lua_State *L, ROTable *rotable, TValue *key, TValue *val) {
  unsigned keypos;

  /* Special case: if key is nil, return the first element of the rotable */
  if (ttisnil(key))
    luaR_next_helper(L, rotable, 0, key, val);
  else if (ttisstring(key)) {
    /* Find the previous key again */
    if (ttisstring(key)) {
      luaR_findentry(rotable, rawtsvalue(key), &keypos);
    }
    /* Advance to next key */
    keypos ++;
    luaR_next_helper(L, rotable, keypos, key, val);
  }
}