Use BitCtz for FastSLog2Slow_C
@@ -329,6 +329,15 @@ const uint8_t kPrefixEncodeExtraBitsValue[PREFIX_LOOKUP_IDX_MAX] = {
static float FastSLog2Slow_C(uint32_t v) {
assert(v >= LOG_LOOKUP_IDX_MAX);
if (v < APPROX_LOG_WITH_CORRECTION_MAX) {
+#if !defined(WEBP_HAVE_SLOW_CLZ_CTZ)
+ // use clz if available
+ const int log_cnt = BitsLog2Floor(v) - 7;
+ const uint32_t y = 1 << log_cnt;
+ int correction = 0;
+ const float v_f = (float)v;
+ const uint32_t orig_v = v;
+ v >>= log_cnt;
+#else
int log_cnt = 0;
uint32_t y = 1;
int correction = 0;
@@ -339,6 +348,7 @@ static float FastSLog2Slow_C(uint32_t v) {
v = v >> 1;
y = y << 1;
} while (v >= LOG_LOOKUP_IDX_MAX);
+#endif
// vf = (2^log_cnt) * Xf; where y = 2^log_cnt and Xf < 256
// Xf = floor(Xf) * (1 + (v % y) / v)
// log2(Xf) = log2(floor(Xf)) + log2(1 + (v % y) / v)
@@ -355,6 +365,14 @@ static float FastSLog2Slow_C(uint32_t v) {
static float FastLog2Slow_C(uint32_t v) {
assert(v >= LOG_LOOKUP_IDX_MAX);
if (v < APPROX_LOG_WITH_CORRECTION_MAX) {
+#if !defined(WEBP_HAVE_SLOW_CLZ_CTZ)
+ // use clz if available
+ const int log_cnt = BitsLog2Floor(v) - 7;
+ const uint32_t y = 1 << log_cnt;
+ const uint32_t orig_v = v;
+ double log_2;
+ v >>= log_cnt;
+#else
int log_cnt = 0;
uint32_t y = 1;
const uint32_t orig_v = v;
@@ -364,6 +382,7 @@ static float FastLog2Slow_C(uint32_t v) {
v = v >> 1;
y = y << 1;
} while (v >= LOG_LOOKUP_IDX_MAX);
+#endif
log_2 = kLog2Table[v] + log_cnt;
if (orig_v >= APPROX_LOG_MAX) {
// Since the division is still expensive, add this correction factor only
