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

Daniel Berlin via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 14 12:04:32 PDT 2017

On Fri, Jul 14, 2017 at 11:46 AM, Hal Finkel <hfinkel at anl.gov> 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.
Of course not, but this  sets it up as an all or nothing thing, and it's
Though you probably will need a stop loss point in the future.

> 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).
This is not the only effect it will have, and i don't believe you think
that either (since this argument is globally applicable to the degree that
it would mean we should just accept *every patch* rather than push people
to better design stuff).
It would also, for example, push people towards putting transformations

> 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.
 Sure, and frankly, you will never get one until either someone decides to
be super-altruistic, or you start pushing on people a little more than "not
at all".

We should start an RFC and iterate on this until we have a concrete plan
> and migration plan.

I don't believe that will actually achieve anything at this point (really,
no offense meant)
So  I'm just going to leave this stuff alone since it's pretty clear people
just view me as being either obstructionist or unrealistic.

I don't see anything happening until we are willing to push more, and i
don't see that happening until instcombine is even worse than it is now.
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