[PATCH] D19049: [BlockFrequencyInfo] Handling nested irreducible CFG with geometric series and top-down prorogation

[Duncan P. N. Exon Smith via llvm-commits]

> On 2016-Apr-12, at 21:23, Yuanfang Chen <cheny at udel.edu> wrote:
> 
> kula created this revision.
> kula added reviewers: dexonsmith, dnovillo.
> kula added a subscriber: llvm-commits.
> 
> This patch implements the TODOs in BlockFrequencyInfoImpl.h.

Great!  Thanks for working on this.

Do you have a concrete testcase where this is valuable?

> 1. handle nested irreducible loop.
> 2. true weight distribution among multi-heads of irreducible loop, instead of even split + adjustment by backedge later.
> 3. Use geometric series instead of loop scale for mass iteration. loop scale are still used, but for the purpose of scaling down local mass below 1.0.

The patch is pretty huge.  Is it possible to stage these changes somehow to make it easier to review?

It's going to take some time for me to page this algorithm back in.  The smaller each change is, the easier it'll be for me ;).

I haven't looked in detail at the patch yet, but just glancing I saw a few smaller NFC changes (like DEBUG output and variable names) that would should definitely be split out.

> 4. normal code path for reducible loop is not affected.
> 
> This patch passes all regression test. Several test cases are changed accordingly. I've changed the @nonentry_header case in irreducible.ll to use
> huge weight on the switch instruction to show that 'bottom' block should not be hot.
> 
> Method:
> 1. for loop a, found all SCCs, create SCC loops and adjust nodes in its parent node list accordingly.
> 2. In topological order, propagate mass on all SCCs where non-trivial SCCs incur resursion.
> 3. compute start term of geometric series.
> 4. for each header, compute its ratio of geometric series by propogation full mass starting from itself and other block hass empty mass. The cumulated backege mass is ratio.
> 5. find the max ratio among headers.
> 6. compute local scaled down mass for all headers with geometric series equation [a/(1-r)]. assume n is infinity. I've add a TODO in file to see if we should use [a*(1-r^n)/(1-r)].
> 7. Propagate with header mass to other blocks in loop.

What's the worst-case complexity?  How does it compare to the old algorithm?

> 
> http://reviews.llvm.org/D19049
> 
> Files:
>  include/llvm/Analysis/BlockFrequencyInfoImpl.h
>  lib/Analysis/BlockFrequencyInfoImpl.cpp
>  test/Analysis/BlockFrequencyInfo/irreducible.ll
> 
> <D19049.53516.patch>