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Re: [PATCH 0/3][POPCOUNT]


On July 6, 2018 12:03:11 AM GMT+02:00, Jeff Law <law@redhat.com> wrote:
>On 06/24/2018 08:41 PM, Kugan Vivekanandarajah wrote:
>> Hi Jeff,
>> 
>> Thanks for the comments.
>> 
>> On 23 June 2018 at 02:06, Jeff Law <law@redhat.com> wrote:
>>> On 06/22/2018 03:11 AM, Kugan Vivekanandarajah wrote:
>>>> When we set niter with maybe_zero, currently final_value_relacement
>>>> will not happen due to expression_expensive_p not handling. Patch 1
>>>> adds this.
>>>>
>>>> With that we have the following optimized gimple.
>>>>
>>>>   <bb 2> [local count: 118111601]:
>>>>   if (b_4(D) != 0)
>>>>     goto <bb 3>; [89.00%]
>>>>   else
>>>>     goto <bb 4>; [11.00%]
>>>>
>>>>   <bb 3> [local count: 105119324]:
>>>>   _2 = (unsigned long) b_4(D);
>>>>   _9 = __builtin_popcountl (_2);
>>>>   c_3 = b_4(D) != 0 ? _9 : 1;
>>>>
>>>>   <bb 4> [local count: 118111601]:
>>>>   # c_12 = PHI <c_3(3), 0(2)>
>>>>
>>>> I assume that 1 in  b_4(D) != 0 ? _9 : 1; is OK (?) because when
>the
>>>> latch execute zero times for b_4 == 0 means that the body will
>execute
>>>> ones.
>>> ISTM that DOM ought to have simplified the conditional, unless
>there's
>>> some other way to get to bb3.  We know that b_4 is nonzero and thus
>c_3
>>> must have the value _9.
>> As of now, dom is not optimizing it. With the attached hack, it can
>be made to.
>What's strange is I'm not getting the c_3 = (b_4 != 0) ... in any of
>the
>dumps I'm looking at.  Instead it's c_3 = _9, which is what I would
>expect since we know that b_4 != 0
>
>
>My tests have been on x86_64 and aarch64 linux targets.  I've tried
>with
>patch#1 installed as well as with patch #1 and patch #2 together.
>
>What target, what flags and what patches do I need to see this?

I believe it has been fixed in niters analysis to avoid the condition if it is known to be true. 

Richard. 

>Jeff


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