On 10/29/2012 10:44 AM, Måns Rullgård wrote:
> Justin Ruggles <justin.rugg...@gmail.com> writes:
>
>> +#if HAVE_FAST_CLZ && AV_GCC_VERSION_AT_LEAST(3,4)
>> +#ifndef ff_ctz
>> +#define ff_ctz(v) (__builtin_ctz(v))
>
> Note that __builtin_ctz is undefined for an input of zero. Also, no
> need for the parens.
>
>> +#endif
>> +#endif
>> +
>> +#ifndef ff_ctz
>> +#define ff_ctz ff_ctz_c
>> +static av_always_inline av_const int ff_ctz_c(int v)
>> +{
>> + int c;
>> +
>> + if (v & 0x1)
>> + return 0;
>> +
>> + c = 1;
>> + if (!(v & 0xffff)) {
>> + v >>= 16;
>> + c += 16;
>> + }
>> + if (!(v & 0xff)) {
>> + v >>= 8;
>> + c += 8;
>> + }
>> + if (!(v & 0xf)) {
>> + v >>= 4;
>> + c += 4;
>> + }
>> + if (!(v & 0x3)) {
>> + v >>= 2;
>> + c += 2;
>> + }
>> + c -= v & 0x1;
>> +
>> + return c;
>> +}
>> +#endif
>
> Is it intentional for ff_ctz(0) to return 31?
Yes.
See the part of the patch that has:
+/**
+ * Trailing zero bit count.
+ *
+ * @param v input value. If v is 0, the result is undefined.
+ * @return the number of trailing 0-bits
+ */
+int av_ctz(int v);
So it doesn't matter what it returns for 0.
-Justin
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