On Nov 21, 2013, at 2:48 PM, Ben Pfaff <b...@nicira.com> wrote:
> On Thu, Nov 21, 2013 at 02:25:30PM -0800, Jarno Rajahalme wrote:
>> Count leading zeroes using builtins if available.
>>
>> Signed-off-by: Jarno Rajahalme <jrajaha...@nicira.com>
>
> Could we rename raw_clz() to raw_clz64()? clz is a function that
> doesn't make sense without a defined length.
Should clz() be clz32() to then as well?
>
>> +static inline int
>> +raw_clz(uint64_t n)
>> +{
>> + /* With GCC 4.7 on 32-bit x86, if a 32-bit integer is passed as 'n',
>> using
>> + * a plain __builtin_ctzll() here always generates an out-of-line
>> function
>> + * call. The test below helps it to emit a single 'bsf' instruction. */
>> + return (__builtin_constant_p(n <= UINT32_MAX) && n <= UINT32_MAX
>> + ? __builtin_clz(n) + 32
>> + : __builtin_clzll(n));
>
> In my tests the above trick isn't necessary for clz. Just writing
> __builtin_clzll() generates good code. (But the comment would need an
> update anyway: s/ctz/clz/ and s/bsf/bsr/.)
OK. Are you saying that clz32() could call raw_clz64() without penalty, or
should I add a raw_clz32() as well?
Jarno
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