[llvm-commits] Cost Table, take 2

[David Tweed]

Hi,
I'd prefer to retain the cost table code as C++ rather than have it moved to a "build-time" table-gen file. However, I'm not actively submitting patches in that area at the moment and I'll defer to others' opinions on the issue.

Regards,
Dave

From: Chandler Carruth [mailto:chandlerc at google.com]
Sent: 24 January 2013 08:45
To: Renato Golin Linaro
Cc: LLVM Commits; Nadav Rotem; David Tweed; Evan Cheng
Subject: Re: Cost Table, take 2

At this point, I'm not really seeing the benefit of this patch.

It takes 2 structs and 2 functions, both of which are dead simple, and replaces them with 3 class templates in an inheritance tree plus an inner struct a constructor and one function that is significantly more complex. It doesn't make things more efficient in the process. It doesn't really make this a generic tool for table lookup, because the tables are not in fact generic. I think ultimately I would prefer the original code here. Maybe there is a massive simplification this provides to some future code you're considering that I don't see yet? Dunno...


Sorry for being a bit of a rain cloud here. To try to give some constructive ideas, I looked at the fundamental problem, and I see two designs that seem more workable:

1) A TargetCostTable class template in include/llvm/Target which accepts a T type parameter that can be either MVT, std::pair<MVT, MVT>, or std::tuple<MVT, MVT, ...>. Then the ISD and the cost value can be concrete things.

2) Teach tablegen to produce pre-sorted tables with lookup routines from a .td file in the target.

My suspicion is that #1 will end up being nearly as much code and complexity as the duplication between ARM and X86....

My suspicion is that *if* it is worth abstracting this stuff, we should just do #2 and make it really powerful. Then we can quickly build very large and detailed cost tables, and they will remain efficient and simple to use in each target.

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