| Message ID | 20240924085338.89777-1-martin@martin.st |
|---|---|
| State | New |
| Headers | show |
| Series | [FFmpeg-devel] compat: Fix the fallback definition of stdc_trailing_zeros | expand |
| Context | Check | Description |
|---|---|---|
| andriy/make_x86 | success | Make finished |
| andriy/make_fate_x86 | success | Make fate finished |
Quoting Martin Storsjö (2024-09-24 10:53:38) > 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. > > This fixes fate with MSVC. > --- > compat/stdbit/stdbit.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/compat/stdbit/stdbit.h b/compat/stdbit/stdbit.h > index b434fc2357..3197a24938 100644 > --- a/compat/stdbit/stdbit.h > +++ b/compat/stdbit/stdbit.h > @@ -179,9 +179,10 @@ static inline unsigned int __stdc_trailing_zeros(unsigned long long value, > unsigned int size) > { > unsigned int zeros = size * CHAR_BIT; > + unsigned long long mask = (1ULL << (size * CHAR_BIT)) - 1; This is UB for size == sizeof(unsigned long long). > > while (value != 0) { > - value <<= 1; > + value = (value << 1) & mask; Otherwise looks ok.
diff --git a/compat/stdbit/stdbit.h b/compat/stdbit/stdbit.h index b434fc2357..3197a24938 100644 --- a/compat/stdbit/stdbit.h +++ b/compat/stdbit/stdbit.h @@ -179,9 +179,10 @@ static inline unsigned int __stdc_trailing_zeros(unsigned long long value, unsigned int size) { unsigned int zeros = size * CHAR_BIT; + unsigned long long mask = (1ULL << (size * CHAR_BIT)) - 1; while (value != 0) { - value <<= 1; + value = (value << 1) & mask; zeros--; }