Re: [Qemu-devel] [PATCH v3 5/6] bitops: add BITNR macro

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 11/11/2013 10:57 PM, Andreas Färber wrote:
> Am 11.11.2013 08:44, schrieb Alexey Kardashevskiy:
>> This adds a macro to calculate the highest bit set.
> 
> Isn't that already available as ffs / clz GCC builtin with wrapper in
> qemu/bitops.h? What's the difference to your macro? CC'ing Paolo.


I looked further and did not find any use of ffs/clz so I wonder what did
you mean about bitops.h and what did I miss? I am confused.

So I would suggest the following instead (if I really needed this BITNR but
I really do not :) )

===
    bitops: add BITNR macro

    This adds a macro to calculate the highest single bit set. If more than
    one bit is set, returns -1.

    Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
index 304c90c..24361b2 100644
--- a/include/qemu/bitops.h
+++ b/include/qemu/bitops.h
@@ -23,6 +23,8 @@
 #define BIT_WORD(nr)           ((nr) / BITS_PER_LONG)
 #define BITS_TO_LONGS(nr)      DIV_ROUND_UP(nr, BITS_PER_BYTE * sizeof(long))

+#define BITNR(m)   (__builtin_popcount(m) == 1)?(__builtin_ffs(m) - 1) : -1
+
 /**
  * set_bit - Set a bit in memory
  * @nr: the bit to set
===



> 
> Andreas
> 
>> If used on constant
>> values, no code will be generated.
>>
>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>> ---
>>  include/qemu/bitops.h | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
>> index 304c90c..98ba42a 100644
>> --- a/include/qemu/bitops.h
>> +++ b/include/qemu/bitops.h
>> @@ -23,6 +23,18 @@
>>  #define BIT_WORD(nr)		((nr) / BITS_PER_LONG)
>>  #define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_BYTE * sizeof(long))
>>  
>> +#define __BITNR(m, n)  ((m) == ((m) & (1<<(n)))) ? (n) :
>> +#define BITNR(m) \
>> +    __BITNR((m), 31) __BITNR((m), 30) __BITNR((m), 29) __BITNR((m), 28) \
>> +    __BITNR((m), 27) __BITNR((m), 26) __BITNR((m), 25) __BITNR((m), 24) \
>> +    __BITNR((m), 23) __BITNR((m), 22) __BITNR((m), 21) __BITNR((m), 20) \
>> +    __BITNR((m), 19) __BITNR((m), 18) __BITNR((m), 17) __BITNR((m), 16) \
>> +    __BITNR((m), 15) __BITNR((m), 14) __BITNR((m), 13) __BITNR((m), 12) \
>> +    __BITNR((m), 11) __BITNR((m), 10) __BITNR((m),  9) __BITNR((m),  8) \
>> +    __BITNR((m),  7) __BITNR((m),  6) __BITNR((m),  5) __BITNR((m),  4) \
>> +    __BITNR((m),  3) __BITNR((m),  2) __BITNR((m),  1) __BITNR((m),  0) \
>> +    -1
>> +
>>  /**
>>   * set_bit - Set a bit in memory
>>   * @nr: the bit to set
>>
> 
> 


-- 
Alexey