From: "H.J. Lu" <hjl.tools@gmail.com>
To: Bernd Schmidt <bernds@codesourcery.com>
Cc: Richard Henderson <rth@redhat.com>,
GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: Add __builtin_clrsb, similar to clz/ctz
Date: Thu, 23 Jun 2011 06:23:00 -0000 [thread overview]
Message-ID: <BANLkTinkF6ggUfFyfzkoqKYVT49cZgjy0A@mail.gmail.com> (raw)
In-Reply-To: <4DFFA1AE.7070405@codesourcery.com>
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.
next prev parent reply other threads:[~2011-06-23 5:40 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-16 13:06 Bernd Schmidt
2011-06-16 13:10 ` Georg-Johann Lay
2011-06-16 13:56 ` Laurent Desnogues
2011-06-16 13:59 ` Bernd Schmidt
2011-06-16 17:03 ` Richard Henderson
2011-06-20 20:32 ` Bernd Schmidt
2011-06-20 20:48 ` Richard Henderson
2011-06-21 16:39 ` [PATCH] Fix __bultin_clrsb* (PR middle-end/49489) Jakub Jelinek
2011-06-21 16:46 ` Bernd Schmidt
2011-06-23 6:23 ` H.J. Lu [this message]
2011-07-12 3:50 ` Add __builtin_clrsb, similar to clz/ctz Hans-Peter Nilsson
2011-08-23 10:07 ` Jakub Jelinek
2011-08-23 10:08 ` Bernd Schmidt
2011-08-23 10:19 ` Richard Guenther
2011-08-23 10:25 ` Jakub Jelinek
2011-08-23 10:34 ` Bernd Schmidt
2011-08-23 13:43 ` [PATCH] For FFS/CLZ/CTZ/CLRSB/POPCOUNT/PARITY/BSWAP require operand mode equal to operation mode (or VOIDmode) (PR middle-end/50161) Jakub Jelinek
2011-08-23 14:54 ` Georg-Johann Lay
2011-08-23 15:58 ` Jakub Jelinek
2011-08-23 16:16 ` Bernd Schmidt
2011-08-23 10:45 ` Add __builtin_clrsb, similar to clz/ctz Richard Sandiford
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