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deflate_compress: produce same results on all CPUs

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Eric Biggers 2017-05-29 17:38:35 -07:00
parent e42013f92e
commit aadf6d8198
1 changed files with 14 additions and 5 deletions
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19
lib/deflate_compress.c
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@ -491,10 +491,19 @@ struct deflate_output_bitstream {
u8 *end; u8 *end;
}; };
#define MIN_OUTPUT_SIZE (UNALIGNED_ACCESS_IS_FAST ? sizeof(bitbuf_t) : 1) /*
* OUTPUT_END_PADDING is the size, in bytes, of the extra space that must be
* present following os->end, in order to not overrun the buffer when generating
* output. When UNALIGNED_ACCESS_IS_FAST, we need at least sizeof(bitbuf_t)
* bytes for put_unaligned_leword(). Otherwise we need only 1 byte. However,
* to make the compression algorithm produce the same result on all CPU
* architectures (which is sometimes desirable), we have to unconditionally use
* the maximum for any CPU, which is sizeof(bitbuf_t) == 8.
*/
#define OUTPUT_END_PADDING 8
/* Initialize the output bitstream. 'size' is assumed to be at least /* Initialize the output bitstream. 'size' is assumed to be at least
* MIN_OUTPUT_SIZE. */ * OUTPUT_END_PADDING. */
static void static void
deflate_init_output(struct deflate_output_bitstream *os, deflate_init_output(struct deflate_output_bitstream *os,
void *buffer, size_t size) void *buffer, size_t size)
@ -503,7 +512,7 @@ deflate_init_output(struct deflate_output_bitstream *os,
os->bitcount = 0; os->bitcount = 0;
os->begin = buffer; os->begin = buffer;
os->next = os->begin; os->next = os->begin;
os->end = os->begin + size - MIN_OUTPUT_SIZE; os->end = os->begin + size - OUTPUT_END_PADDING;
} }
/* Add some bits to the bitbuffer variable of the output bitstream. The caller /* Add some bits to the bitbuffer variable of the output bitstream. The caller
@ -2774,7 +2783,7 @@ libdeflate_deflate_compress(struct libdeflate_compressor *c,
const void *in, size_t in_nbytes, const void *in, size_t in_nbytes,
void *out, size_t out_nbytes_avail) void *out, size_t out_nbytes_avail)
{ {
if (unlikely(out_nbytes_avail < MIN_OUTPUT_SIZE)) if (unlikely(out_nbytes_avail < OUTPUT_END_PADDING))
return 0; return 0;
/* For extremely small inputs just use a single uncompressed block. */ /* For extremely small inputs just use a single uncompressed block. */
@ -2813,5 +2822,5 @@ libdeflate_deflate_compress_bound(struct libdeflate_compressor *c,
* and alignment to a byte boundary; 2 for LEN; and 2 for NLEN. * and alignment to a byte boundary; 2 for LEN; and 2 for NLEN.
*/ */
size_t max_num_blocks = MAX(DIV_ROUND_UP(in_nbytes, MIN_BLOCK_LENGTH), 1); size_t max_num_blocks = MAX(DIV_ROUND_UP(in_nbytes, MIN_BLOCK_LENGTH), 1);
return (5 * max_num_blocks) + in_nbytes + 1 + MIN_OUTPUT_SIZE; return (5 * max_num_blocks) + in_nbytes + 1 + OUTPUT_END_PADDING;
} }