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

Philip Reames via llvm-dev llvm-dev at lists.llvm.org
Sat Jul 15 12:26:10 PDT 2017

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.

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