[FFmpeg-devel] [PATCH v2] compat: Fix the fallback definition of stdc_trailing_zeros

[Martin Storsjö]

While shifting "value" to left, we would iterate through all bits
of an unsigned long long, while we only expect to count through
"size * CHAR_BIT" bits; instead shift bits to the right and just
count the trailing zeros.

This fixes fate with MSVC.
---
Fixed the UB by shifting to the right instead of to the left.
---
 compat/stdbit/stdbit.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/compat/stdbit/stdbit.h b/compat/stdbit/stdbit.h
index b434fc2357..53419cf9f9 100644
--- a/compat/stdbit/stdbit.h
+++ b/compat/stdbit/stdbit.h
@@ -178,11 +178,14 @@ static inline unsigned int stdc_trailing_zeros_uc(unsigned char value)
 static inline unsigned int __stdc_trailing_zeros(unsigned long long value,
                                                  unsigned int size)
 {
-    unsigned int zeros = size * CHAR_BIT;
+    unsigned int zeros = 0;
 
-    while (value != 0) {
-        value <<= 1;
-        zeros--;
+    if (!value)
+        return size * CHAR_BIT;
+
+    while ((value & 1) == 0) {
+        value >>= 1;
+        zeros++;
     }
 
     return zeros;
-- 
2.39.5 (Apple Git-154)