backward_references: avoid signed integer overflow

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

View File

@ -35,7 +35,8 @@ extern "C" {
#if defined(__GNUC__) && \ #if defined(__GNUC__) && \
((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4) ((__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || __GNUC__ >= 4)
static WEBP_INLINE int BitsLog2Floor(uint32_t n) { 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)) #elif defined(_MSC_VER) && (defined(_M_X64) || defined(_M_IX86))
#include <intrin.h> #include <intrin.h>
@ -43,15 +44,18 @@ static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
static WEBP_INLINE int BitsLog2Floor(uint32_t n) { static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
unsigned long first_set_bit; 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 #else
// Returns (int)floor(log2(n)). n must be > 0.
static WEBP_INLINE int BitsLog2Floor(uint32_t n) { static WEBP_INLINE int BitsLog2Floor(uint32_t n) {
int log = 0; int log = 0;
uint32_t value = n; uint32_t value = n;
int i; int i;
if (value == 0) return -1; assert(n != 0);
for (i = 4; i >= 0; --i) { for (i = 4; i >= 0; --i) {
const int shift = (1 << i); const int shift = (1 << i);
const uint32_t x = value >> shift; 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, static WEBP_INLINE void PrefixEncode(int distance, int* const code,
int* const extra_bits_count, int* const extra_bits_count,
int* const extra_bits_value) { int* const extra_bits_value) {
// Collect the two most significant bits where the highest bit is 1. if (distance > 2) { // Collect the two most significant bits.
const int highest_bit = BitsLog2Floor(--distance); const int highest_bit = BitsLog2Floor(--distance);
// & 0x3f is to make behavior well defined when highest_bit const int second_highest_bit = (distance >> (highest_bit - 1)) & 1;
// does not exist or is the least significant bit. *extra_bits_count = highest_bit - 1;
const int second_highest_bit = *extra_bits_value = distance & ((1 << *extra_bits_count) - 1);
(distance >> ((highest_bit - 1) & 0x3f)) & 1; *code = 2 * highest_bit + second_highest_bit;
*extra_bits_count = (highest_bit > 0) ? (highest_bit - 1) : 0; } else {
*extra_bits_value = distance & ((1 << *extra_bits_count) - 1); *extra_bits_count = 0;
*code = (highest_bit > 0) ? (2 * highest_bit + second_highest_bit) *extra_bits_value = 0;
: (highest_bit == 0) ? 1 : 0; *code = (distance == 2) ? 1 : 0;
}
} }
// ----------------------------------------------------------------------------- // -----------------------------------------------------------------------------