Re: [patch, aarch64] additional bics patterns

[Richard Earnshaw <rearnsha at arm dot com>]

On 13/11/14 17:05, Ramana Radhakrishnan wrote:
> On Thu, Nov 13, 2014 at 4:55 PM, Sandra Loosemore
> <sandra@codesourcery.com> wrote:
>> This patch to the AArch64 back end adds a couple of additional bics patterns
>> to match code of the form
>>
>>   if ((x & y) == x) ...;
>>
>> This is testing whether the bits set in x are a subset of the bits set in y;
>> or, that no bits in x are set that are not set in y.  So, it is equivalent
>> to
>>
>>   if ((x & ~y) == 0) ...;
>>
>> Presently this generates code like
>>   and     x21, x21, x20
>>   cmp     x21, x20
>>   b.eq    c0 <main+0xc0>
>>
>> and this patch allows it to be written more concisely as:
>>   bics     x21, x20, x21
>>   b.eq     c0 <main+0xc0>
>>
>> Since the bics instruction sets the condition codes itself, no explicit
>> comparison is required and the result of the bics computation can be
>> discarded.
>>
>> Regression-tested on aarch64-linux-gnu.  OK to commit?
> 
> Is this not a duplicate of
> https://gcc.gnu.org/ml/gcc-patches/2014-11/msg00943.html ?
> 
> regards
> Ramana
> 
>>
>> -Sandra
>>
>>
>> 2014-11-12  Sandra Loosemore  <sandra@codesourcery.com>
>>             Chris Jones <chrisj@nvidia.com>
>>
>>         gcc/
>>         * config/aarch64/aarch64.md (*and<mode>3_compare_op1): New.
>>         (*and<mode>3_compare_op2): New.
>>
>>         gcc/testsuite/
>>         * gcc.target/aarch64/bics_3.c: New.
>>
>>
> 
I don't think so.  However, I think it is something that should be
caught in generic simplification code

ie map  ((a & b) == b) ==> ((~a & b) == 0), etc

Bit-clear operations are not that uncommon.  Furthermore, A may be a
constant.

R.