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

Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 14 11:54:18 PDT 2017

Going back to the original problem. Why shouldn't
SimplifyUsingDistributiveLaws handle both these cases?

For the bitwise test case if you distribute you get
(~A | A) & (B | A)

The left side of that simplifies to all 1s which is the identify value for
and. So even though the right side doesn't simplify it's a win.


On Fri, Jul 14, 2017 at 11:46 AM, Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> 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.
>  -Hal
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
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> llvm-dev at lists.llvm.org
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