[llvm-dev] [GlobalISel] Prioritizing long patterns in combiner over short ones

[Amara Emerson via llvm-dev]

+Daniel.

Daniel, do you remember if there was a strong reason for choosing to go top-down on the combiner?

Amara

> On Dec 17, 2020, at 2:14 AM, Jay Foad via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> I am really surprised that "the combiner runs top-down". Surely it's
> better to combine smaller sub-expressions before combining larger
> expressions? Or is there actually a good reason to keep the top-down
> order??
> 
> DAGCombiner also runs mostly top-down (but it can also add nodes to
> the worklist in a bottom-up direction for certain cases). The top-down
> order causes problems when you have deep expressions trees that could
> be simplified away to nothing. Outer combines see these subexpression
> unsimplified. They can use recursive helper functions like
> computeKnownBits to try to analyse the unsimplified expression, but
> the recursive helper functions typically have a max recursion depth of
> 6. which is easy to hit.
> 
> DAGCombiner is so embedded now that it is hard to change the basic
> top-down order, but I would hope we could change the GlobalISel
> combiner.
> 
> Jay.
> 
> On Tue, 15 Dec 2020 at 15:09, Dominik Montada via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> 
>> Hi,
>> 
>> I'm currently writing target specific combiners with GlobalISel. I have
>> a case where a sub-node of a larger pattern also matches another,
>> smaller combiner pattern. Because the combiner runs top-down, the
>> smaller pattern is matched before the larger pattern has a chance to be
>> matched. Do I have to teach my larger pattern to handle this case or is
>> there a better way to do this?
>> 
>> More importantly, are there any plans to improve this behavior?
>> 
>> Cheers,
>> 
>> Dominik
>> 
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