Add __builtin_clrsb, similar to clz/ctz

H.J. Lu hjl.tools@gmail.com
Thu Jun 23 06:23:00 GMT 2011


On Mon, Jun 20, 2011 at 12:38 PM, Bernd Schmidt <bernds@codesourcery.com> wrote:
> On 06/16/2011 06:25 PM, Richard Henderson wrote:
>> On 06/16/2011 05:44 AM, Bernd Schmidt wrote:
>>> +@deftypefn {Built-in Function} int __builtin_clrsb (unsigned int x)
>>> +Returns the number of leading redundant sign bits in @var{x}, starting
>>> +at the most significant bit position.
>>> +@end deftypefn
>>
>> Do we want a signed argument, since we're talking about signs?
>
> Err, yes. It's signed everywhere else (builtins.def etc.).
>
>> It would seem that unlike clz, this function is not undefined for zero.
>> What about INT_MIN?  Do all cpus handle those edge cases the same way?
>
> -1 and zero should both produce the same value, 31 (for a 32 bit
> integer). I don't see why INT_MIN should be special - the return value
> is zero. This is true for C6X and Blackfin; ARM documentation suggests
> it's also true for their VCLS instruction. I've not found proper
> picochip documentation but some other documents that suggest it's also
> implemented this way.
>
>> Do you get smaller code in general from
>>
>>   if (x < 0)
>>     x = ~x;
>>   if (x == 0)
>>     return W_TYPE_SIZE - 1;
>>   count_leading_zeros(ret, x);
>>   return ret - 1;
>
> Probably.
>
>>> -(define_insn "signbitssi2"
>>> +(define_insn "clrsbsi2"
>>>    [(set (match_operand:HI 0 "register_operand" "=d")
>>>      (if_then_else:HI
>>>       (lt (match_operand:SI 1 "register_operand" "d") (const_int 0))
>>
>> No use of the new rtx code?
>
> D'oh. Blackfin has a (clrsb:HI (operand:SI)) instruction, so adding this
> showed a problem with some of the existing simplify_const_unop cases:
> for ffs/clz/ctz/clrsb/parity/popcount, we should look at the mode of the
> operand, rather than the mode of the operation. This limits what we can
> do in that function, since op_mode is sometimes VOIDmode - we really
> should add builtin folders for these at some point.
>
> New patch below. Retested on i686 and bfin.

This caused:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49512


-- 
H.J.



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