[llvm-dev] failing to optimize boolean ops on cmps

Philip Reames via llvm-dev llvm-dev at lists.llvm.org
Sat Jul 15 13:20:46 PDT 2017

On 07/15/2017 01:09 PM, Daniel Berlin wrote:
> Which is, of course, precisely why I didn't suggest that, it was Hal's 
> straw man.
Ah, sorry for the misread then.  Skimming the thread, that's exactly 
what I thought you *were* arguing for.  Glad to know we're in agreement.
> On Sat, Jul 15, 2017, 12:26 PM Philip Reames 
> <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>     On 07/14/2017 11:46 AM, Hal Finkel via llvm-dev wrote:
>>     On 07/14/2017 01:38 PM, Daniel Berlin wrote:
>>>         Not sure about this last part. It is really going to require
>>>         work by us to rewrite things. :-) In the mean time, I think
>>>         we should go ahead with this.
>>>     FWIW: My problem is, when put in this framework, we will
>>>     repeatedly make this same decision, this same way, again and
>>>     again, and never actually get even started on fixing it :)
>>>     IE "it's just another small patch!"
>>     You're correct. However, as I'm sure you're aware, developer time
>>     is not directly transferable in a way that makes any other
>>     decision optimal. It's not like blocking all improvements to
>>     InstCombine will cause a movement to appear to rewrite
>>     InstCombine. It will only cause a shrink in the pool of
>>     developers with recent experience working on InstCombine (and a
>>     lot of out-of-tree patches). Frankly, we don't even have a
>>     concrete plan for how we'd do this, or even a target design, and
>>     that's the first item on the critical path to a rewrite. We
>>     should start an RFC and iterate on this until we have a concrete
>>     plan and migration plan.
>     Just want to chime in here in support of what Hal said.  I
>     strongly believe we should not block incremental progress unless
>     we have a plan in place for a better design.  In particular, part
>     of being able to come up with a better design is having multiple
>     people with recent knowledge of the system which needs replaced
>     and stopping evolution of the old system by definition inhibits
>     having such a group of people.
>     Philip

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