From c0ba090335207d82765c652fb9f637231499ac38 Mon Sep 17 00:00:00 2001
From: James Zern <jzern@google.com>
Date: Wed, 20 Feb 2013 21:22:04 +0000
Subject: [PATCH] backward_references: avoid signed integer overflow
signed integer overflow behavior is undefined, split PrefixEncode() to
two branches to avoid this.
Change-Id: I6e2761d0d77f0aaceafdc4e07232e089c22beb64
---
src/enc/backward_references.h | 31 ++++++++++++++++++-------------
1 file changed, 18 insertions(+), 13 deletions(-)
diff --git a/src/enc/backward_references.h b/src/enc/backward_references.h
index 7ad5ade6..8cb1a7a6 100644
--- a/src/enc/backward_references.h
+++ b/src/enc/backward_references.h
@@ -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;
+ }
}
// -----------------------------------------------------------------------------