[llvm-dev] Oddity w/MachineBlockPlacement and Loops

Xinliang David Li via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 23 21:48:47 PST 2016

Hi Phillip,
http://reviews.llvm.org/D10825 tries to fix similar issues. Looks like
there are missing cases. Can you create a small reproducer?

Cong has also improved loop rotation based on better cost model -- it is
not yet enabled (-mllvm -precise-rotation-cost=true to turn on). If
possible, can you also give it a try?



On Tue, Feb 23, 2016 at 5:19 PM, Philip Reames via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I'm getting some odd behavior out of MBP and was hoping someone knowledge
> of the code might be able to give some guidance.  Fair warning, I'm trying
> to describe a problem in code I don't really understand, so if something
> doesn't make sense, assume I misunderstood something.
> The problematic case I'm seeing is that cold blocks are being placed
> between the preheader and header of a hot loop.  This has the result of
> adding a bunch of cold code spread through out the code rather than grouped
> all together at the end of the function.
> From what I can tell tracing through the code, the critical decision that
> goes wrong is when we're visiting the preheader after forming a (correct)
> chain for the loop itself.  When selecting a successor to merge with, we
> appear to not be considering the loop even though the loop hasn't been
> rotated and the header would be ideal for fallthrough.
> In particular, we're printing the "(prob) (non-cold CFG conflict)" debug
> output message for the successor of the preheader which is the header.  If
> I'm reading this code correctly, it's identifying the fact there's a global
> more important predecessor for the header (i.e. the latch block), but it
> doesn't seem to be account for the fact that the latch block has already
> been combined into the header's chain.  Unless I'm misreading something, we
> *should* be able to merge the loop chain with the preheader chain in it's
> entirety right?
> At least one on example I've looked at, adding an early exit from
> BadCFGConflict loop when Pred and Succ are part of the same chain does
> appear to give the expected result, but I don't understand the code well
> enough to reason about whether that is generally a correct thing to do or
> not.
> Philip
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