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backward_references: avoid signed integer overflow

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signed integer overflow behavior is undefined, split PrefixEncode() to
two branches to avoid this.

Change-Id: I6e2761d0d77f0aaceafdc4e07232e089c22beb64
This commit is contained in:
James Zern 2013-02-20 21:22:04 +00:00
parent 943386db4b
commit c0ba090335
1 changed files with 18 additions and 13 deletions
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31
src/enc/backward_references.h
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@ -35,7 +35,8 @@ extern "C" {
#if defined(__GNUC__) && \
((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)
static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
return n == 0 ? -1 : 31 ^ __builtin_clz(n);
assert(n != 0);
return 31 ^ __builtin_clz(n);
}
#elif defined(_MSC_VER) && (defined(_M_X64) || defined(_M_IX86))
#include <intrin.h>
@ -43,15 +44,18 @@ static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
unsigned long first_set_bit;
return _BitScanReverse(&first_set_bit, n) ? first_set_bit : -1;
assert(n != 0);
_BitScanReverse(&first_set_bit, n);
return first_set_bit;
}
#else
// Returns (int)floor(log2(n)). n must be > 0.
static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
int log = 0;
uint32_t value = n;
int i;
if (value == 0) return -1;
assert(n != 0);
for (i = 4; i >= 0; --i) {
const int shift = (1 << i);
const uint32_t x = value >> shift;
@ -78,16 +82,17 @@ static WEBP_INLINE int VP8LBitsLog2Ceiling(uint32_t n) {
static WEBP_INLINE void PrefixEncode(int distance, int* const code,
int* const extra_bits_count,
int* const extra_bits_value) {
// Collect the two most significant bits where the highest bit is 1.
const int highest_bit = BitsLog2Floor(--distance);
// & 0x3f is to make behavior well defined when highest_bit
// does not exist or is the least significant bit.
const int second_highest_bit =
(distance >> ((highest_bit - 1) & 0x3f)) & 1;
*extra_bits_count = (highest_bit > 0) ? (highest_bit - 1) : 0;
*extra_bits_value = distance & ((1 << *extra_bits_count) - 1);
*code = (highest_bit > 0) ? (2 * highest_bit + second_highest_bit)
: (highest_bit == 0) ? 1 : 0;
if (distance > 2) { // Collect the two most significant bits.
const int highest_bit = BitsLog2Floor(--distance);
const int second_highest_bit = (distance >> (highest_bit - 1)) & 1;
*extra_bits_count = highest_bit - 1;
*extra_bits_value = distance & ((1 << *extra_bits_count) - 1);
*code = 2 * highest_bit + second_highest_bit;
} else {
*extra_bits_count = 0;
*extra_bits_value = 0;
*code = (distance == 2) ? 1 : 0;
}
}
// -----------------------------------------------------------------------------