git.sesse.net Git - fjl/blob - unstuff.c

   1 #include <string.h>
   2 #include <assert.h>
   3 #include <mmintrin.h>
   4 #include <xmmintrin.h>
   5 #include <smmintrin.h>
   6
   7 #include "unstuff.h"
   8
   9 #define MARKER_CHAR 0xff
  10 #define STUFF_MARKER 0x00
  11
  12 int unstuff_reference(uint8_t* dst, const uint8_t* src, size_t len)
  13 {
  14         size_t bytes_written = 0;
  15
  16         for (unsigned i = 0; i < len; ++i, ++dst, ++src, ++bytes_written) {
  17                 *dst = *src;
  18                 if (*src == MARKER_CHAR) {
  19                         if (i == len - 1 || src[1] != STUFF_MARKER) {
  20                                 return -1;
  21                         }
  22
  23                         // Skip the stuff byte.
  24                         ++src, ++i;
  25                 }
  26         }
  27
  28         assert(bytes_written <= len);
  29         return bytes_written;
  30 }
  31
  32 int unstuff_fast(uint8_t* dst, const uint8_t* src, size_t len)
  33 {
  34         const uint8_t* sptr = src;
  35         uint8_t* dptr = dst;
  36
  37         while (len > 0) {
  38                 // Find the first marker byte in the rest of the stream.
  39                 const uint8_t* ptr = memchr(sptr, MARKER_CHAR, len);
  40                 if (ptr == NULL) {
  41                         // No marker bytes left.
  42                         memcpy(dptr, sptr, len);
  43                         dptr += len;
  44                         break;
  45                 }
  46
  47                 const size_t len_to_copy = ptr - sptr + 1;
  48                 memcpy(dptr, sptr, len_to_copy);
  49
  50                 sptr += len_to_copy;
  51                 dptr += len_to_copy;
  52                 len -= len_to_copy;
  53
  54                 if (len == 0) {
  55                         // Partial marker.
  56                         return -1;
  57                 } else {
  58                         if (*sptr != STUFF_MARKER) {
  59                                 return -1;
  60                         }
  61                         ++sptr;
  62                         --len;
  63                 }
  64         }
  65
  66         return dptr - dst;
  67 }
  68
  69 int unstuff_sse41(uint8_t* dst, const uint8_t* src, size_t len)
  70 {
  71         __m128i marker_search = _mm_set1_epi8(MARKER_CHAR);
  72
  73         const uint8_t* sptr = src;
  74         uint8_t* dptr = dst;
  75         while (len >= 16) {
  76                 __m128i data = _mm_lddqu_si128((const __m128i*)sptr);
  77
  78                 // The store here is safe (if there's stuff bytes, the data
  79                 // will simply get overwritten in the slow path); fire it off
  80                 // here so it can run in parallel with the compare.
  81                 _mm_storeu_si128((__m128i*)dptr, data);
  82
  83                 __m128i eq_mask = _mm_cmpeq_epi8(data, marker_search);
  84                 if (_mm_test_all_zeros(eq_mask, eq_mask)) {
  85                         // Fast path; no stuff byte found.
  86                         sptr += 16;
  87                         dptr += 16;
  88                         len -= 16;
  89                         continue;
  90                 }
  91
  92                 // We found a stuff byte. If it was the last byte, we just
  93                 // defer that to the next chunk. Apart from that, we just keep
  94                 // going one by one byte. We could perhaps speed this up with
  95                 // the data from eq_mask(), but we're not doing that yet.
  96                 size_t len_this_chunk = (sptr[15] == 0xff ? 15 : 16);
  97                 for (unsigned j = 0; j < len_this_chunk; ++j, ++dptr, ++sptr) {
  98                         *dptr = *sptr;
  99
 100                         if (*sptr == MARKER_CHAR) {
 101                                 assert(j != 15);
 102                                 if (sptr[1] != STUFF_MARKER) {
 103                                         return -1;
 104                                 }
 105
 106                                 // Skip the stuff byte.
 107                                 ++sptr, ++j;
 108                         }
 109                 }
 110                 len -= len_this_chunk;
 111         }
 112
 113         // Do the final bytes via the reference path.
 114         int ret = unstuff_reference(dptr, sptr, len);
 115         if (ret == -1) {
 116                 return -1;
 117         } else {
 118                 return (dptr - dst) + ret;
 119         }
 120 }