Entry assumptions:
state->mode == LEN
- strm->avail_in >= 6
- strm->avail_out >= 258
+ strm->avail_in >= INFLATE_FAST_MIN_HAVE
+ strm->avail_out >= INFLATE_FAST_MIN_LEFT
start >= strm->avail_out
state->bits < 8
Therefore if strm->avail_in >= 6, then there is enough input to avoid
checking for available input while decoding.
+ - On some architectures, it can be significantly faster (e.g. up to 1.2x
+ faster on x86_64) to load from strm->next_in 64 bits, or 8 bytes, at a
+ time, so INFLATE_FAST_MIN_HAVE == 8.
+
- The maximum bytes that a single length/distance pair can output is 258
bytes, which is the maximum length that can be coded. inflate_fast()
requires strm->avail_out >= 258 for each loop to avoid checking for
unsigned whave; /* valid bytes in the window */
unsigned wnext; /* window write index */
unsigned char *window; /* allocated sliding window, if wsize != 0 */
- uint32_t hold; /* local strm->hold */
+
+ /* hold is a local copy of strm->hold. By default, hold satisfies the same
+ invariants that strm->hold does, namely that (hold >> bits) == 0. This
+ invariant is kept by loading bits into hold one byte at a time, like:
+
+ hold |= next_byte_of_input << bits; in++; bits += 8;
+
+ If we need to ensure that bits >= 15 then this code snippet is simply
+ repeated. Over one iteration of the outermost do/while loop, this
+ happens up to six times (48 bits of input), as described in the NOTES
+ above.
+
+ However, on some little endian architectures, it can be significantly
+ faster to load 64 bits once instead of 8 bits six times:
+
+ if (bits <= 16) {
+ hold |= next_8_bytes_of_input << bits; in += 6; bits += 48;
+ }
+
+ Unlike the simpler one byte load, shifting the next_8_bytes_of_input
+ by bits will overflow and lose those high bits, up to 2 bytes' worth.
+ The conservative estimate is therefore that we have read only 6 bytes
+ (48 bits). Again, as per the NOTES above, 48 bits is sufficient for the
+ rest of the iteration, and we will not need to load another 8 bytes.
+
+ Inside this function, we no longer satisfy (hold >> bits) == 0, but
+ this is not problematic, even if that overflow does not land on an 8 bit
+ byte boundary. Those excess bits will eventually shift down lower as the
+ Huffman decoder consumes input, and when new input bits need to be loaded
+ into the bits variable, the same input bits will be or'ed over those
+ existing bits. A bitwise or is idempotent: (a | b | b) equals (a | b).
+ Note that we therefore write that load operation as "hold |= etc" and not
+ "hold += etc".
+
+ Outside that loop, at the end of the function, hold is bitwise and'ed
+ with (1<<bits)-1 to drop those excess bits so that, on function exit, we
+ keep the invariant that (state->hold >> state->bits) == 0.
+ */
+ uint64_t hold; /* local strm->hold */
unsigned bits; /* local strm->bits */
code const *lcode; /* local strm->lencode */
code const *dcode; /* local strm->distcode */
/* copy state to local variables */
state = (struct inflate_state *)strm->state;
in = strm->next_in;
- last = in + (strm->avail_in - 5);
+ last = in + (strm->avail_in - (INFLATE_FAST_MIN_HAVE - 1));
out = strm->next_out;
beg = out - (start - strm->avail_out);
- end = out + (strm->avail_out - 257);
+ end = out + (strm->avail_out - (INFLATE_FAST_MIN_LEFT - 1));
#ifdef INFFAST_CHUNKSIZE
safe = out + (strm->avail_out - INFFAST_CHUNKSIZE);
input data or output space */
do {
if (bits < 15) {
- hold += load_short(in, bits);
- in += 2;
- bits += 16;
+ hold |= load_64_bits(in, bits);
+ in += 6;
+ bits += 48;
}
here = lcode[hold & lmask];
dolen:
op &= 15; /* number of extra bits */
if (op) {
if (bits < op) {
- hold += (uint32_t)(*in++) << bits;
- bits += 8;
+ hold |= load_64_bits(in, bits);
+ in += 6;
+ bits += 48;
}
len += BITS(op);
DROPBITS(op);
}
Tracevv((stderr, "inflate: length %u\n", len));
if (bits < 15) {
- hold += load_short(in, bits);
- in += 2;
- bits += 16;
+ hold |= load_64_bits(in, bits);
+ in += 6;
+ bits += 48;
}
here = dcode[hold & dmask];
dodist:
dist = here.val;
op &= 15; /* number of extra bits */
if (bits < op) {
- hold += (uint32_t)(*in++) << bits;
- bits += 8;
- if (bits < op) {
- hold += (uint32_t)(*in++) << bits;
- bits += 8;
- }
+ hold |= load_64_bits(in, bits);
+ in += 6;
+ bits += 48;
}
dist += BITS(op);
#ifdef INFLATE_STRICT
/* update state and return */
strm->next_in = in;
strm->next_out = out;
- strm->avail_in = (unsigned)(in < last ? 5 + (last - in) : 5 - (in - last));
- strm->avail_out = (unsigned)(out < end ? 257 + (end - out) : 257 - (out - end));
+ strm->avail_in =
+ (unsigned)(in < last ? (INFLATE_FAST_MIN_HAVE - 1) + (last - in)
+ : (INFLATE_FAST_MIN_HAVE - 1) - (in - last));
+ strm->avail_out =
+ (unsigned)(out < end ? (INFLATE_FAST_MIN_LEFT - 1) + (end - out)
+ : (INFLATE_FAST_MIN_LEFT - 1) - (out - end));
state->hold = hold;
state->bits = bits;
return;
projects / thirdparty / zlib-ng.git / commitdiff
