[llvm-dev] Oddity w/MachineBlockPlacement and Loops

Xinliang David Li via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 24 09:27:25 PST 2016

On Wed, Feb 24, 2016 at 9:21 AM, Philip Reames <listmail at philipreames.com>

> On 02/23/2016 09:48 PM, Xinliang David Li wrote:
> Hi Phillip,
> http://reviews.llvm.org/D10825 tries to fix similar issues. Looks like
> there are missing cases. Can you create a small reproducer?
> This doesn't quite look the same.  That review is about continuing the
> loop chain itself into the most profitable successor.  My case is about
> continuing the preheader's chain into the (previously formed) loop chain.

yes -- but it is in the same category -- the layout algorithm needs to
ignore blocks already in chain (not just the current chain, and there needs
to be a way to detect trivial chains).

> 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?
> Loop rotation is not the problem here.  The layout of the loop
> (header->latch->header cycle) is exactly what it should be.

yes -- I am just advertising the new layout strategy here :)



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