On Mon, Nov 28, 2016 at 7:41 PM, Jeff Law <l...@redhat.com> wrote:
> On 11/28/2016 06:10 AM, Paolo Bonzini wrote:
>>
>>
>>
>> On 27/11/2016 00:28, Marc Glisse wrote:
>>>
>>> On Sat, 26 Nov 2016, Paolo Bonzini wrote:
>>>
>>>> --- match.pd (revision 242742)
>>>> +++ match.pd (working copy)
>>>> @@ -2554,6 +2554,19 @@
>>>> (cmp (bit_and@2 @0 integer_pow2p@1) @1)
>>>> (icmp @2 { build_zero_cst (TREE_TYPE (@0)); })))
>>>>
>>>> +/* If we have (A & C) != 0 ? D : 0 where C and D are powers of 2,
>>>> + convert this into a shift of (A & C). */
>>>> +(simplify
>>>> + (cond
>>>> + (ne (bit_and@2 @0 integer_pow2p@1) integer_zerop)
>>>> + integer_pow2p@3 integer_zerop)
>>>> + (with {
>>>> + int shift = wi::exact_log2 (@3) - wi::exact_log2 (@1);
>>>> + }
>>>> + (if (shift > 0)
>>>> + (lshift (convert @2) { build_int_cst (integer_type_node, shift); })
>>>> + (convert (rshift @2 { build_int_cst (integer_type_node, -shift);
>>>> })))))
>>>
>>>
>>> What happens if @1 is the sign bit, in a signed type? Do we get an
>>> arithmetic shift right?
>>
>>
>> It shouldn't happen because the canonical form of a sign bit test is A <
>> 0 (that's the pattern immediately after). However I can add an "if" if
>> preferred, or change the pattern to do the AND after the shift.
>
> But are we absolutely sure it'll be in canonical form every time?
No, of course not (though it would be a bug). If the pattern generates wrong
code when the non-canonical form is met that would be bad, if it merely
does not optimize (or optimize non-optimally) then that's not too bad.
> Is there
> a pattern in in match.pd which turns an (x & SIGN_BIT) tests into canonical
> form?
>
>
>>
>> (bit_and
>> (if (shift > 0)
>> (lshift (convert @0) { build_int_cst (integer_type_node, shift); })
>> (convert (rshift @0 { build_int_cst (integer_type_node, -shift); })))
>> @3)
>>
>> What do you think?
>
> Shouldn't be necessary if we're working on unsigned types. May be necessary
> on signed types unless we're 100% sure sign bit tests will be canonicalized.
Instead of the bit_and better put a if() condition around this.
Richard.
> jeff
