Basic Block Layout in the NCG

Sun May 6 19:58:31 UTC 2018

On branch probability:

I've actually created a patch to add more probabilities in the recent 
past which included
probabilities on CmmSwitches. Although it made little difference when 
only tagging error
branches.

Partially that also run into issues with code layout which is why I put 
that on ice for now.

The full patch is here:
https://phabricator.haskell.org/D4327

I think this really has to be done back to front as GHC currently throws
away all likelyhood information before we get to block layout.
Which makes it very hard to take advantage of this information.

Code Layout:

That seems exactly like the kind of pointers I was looking for!
I do wonder how well some of them aged. For example [5] and [6] use at 
most two way associative cache.
But as you said it should be at least a good starting point.

I will put your links into the ticket so they are easily found once I 
(or someone else!) has time to look
deeper into this.

Cheers
Andreas

> Kavon Farvardin <mailto:kavon at farvard.in>
> Sonntag, 6. Mai 2018 20:17
>> Does anyone have good hints for literature on basic block layout
>> algorithms?
>
> Here are some thoughts:
>
> * Branch Probability *
>
> Any good code layout algorithm should take branch probabilities into
> account.  From what I've seen, we already have a few likely-branch
> heuristics baked into the generation/transformation of Cmm, though
> perhaps it's worth doing more to add probabilities as in [1,3].  The
> richer type information in STG could come in handy.
>
> I think the first step in leveraging branch proability information is
> to use a Float to represent the magnitude of likeliness instead of a
> Bool.
>
> Target probabilities on CmmSwitches could also help create smarter
> SwitchPlans. Slides 20-21 in [2] demonstrate a lower-cost decision tree
> based on these probabilities.
>
>
> * Code Layout *
>
> The best all-in-one source for static code positioning I've seen is in
> [5], and might be a good starting point for exploring that space.  More
> importantly, [5] talks about function positioning, which is something I
> think we're missing.  A more sophisticated extension to [5]'s function
> positioning can be found in [6].
>
> Keeping in mind that LLVM is tuned to optimize loops within functions,
> at at high-level LLVM does the following [4]:
>
>      The algorithm works from the inner-most loop within a
>      function outward, and at each stage walks through the
>      basic blocks, trying to coalesce them into sequential
>      chains where allowed by the CFG (or demanded by heavy
>      probabilities). Finally, it walks the blocks in
>      topological order, and the first time it reaches a
>      chain of basic blocks, it schedules them in the
>      function in-order.
>
> There are also plenty of heuristics such as "tail duplication" to deal
> with diamonds and other odd cases in the CFG that are harder to layout.
>    Unfortunately, there don't seem to be any sources cited.  We may want
> to develop our own heuristics to modify the CFG for better layout as
> well.
>
>
>
> [1] Thomas Ball, James R. Larus. Branch Prediction for Free (https://do
> i.org/10.1145/173262.155119)
>
> [2] Hans Wennborg. The recent switch lowering improvements. (http://llv
> m.org/devmtg/2015-10/slides/Wennborg-SwitchLowering.pdf) See also: http
> s://www.youtube.com/watch?v=gMqSinyL8uk
>
> [3] James E. Smith. A study of branch prediction strategies (https://dl
> .acm.org/citation.cfm?id=801871)
>
> [4] http://llvm.org/doxygen/MachineBlockPlacement_8cpp_source.html
>
> [5] Karl Pettis, Robert C. Hansen. Profile guided code positioning. (ht
> tps://doi.org/10.1145/93542.93550)
>
> [6] Hashemi et al. Efficient procedure mapping using cache line
> coloring (https://doi.org/10.1145/258915.258931)
>
>
> ~kavon
>
>
> On Sat, 2018-05-05 at 21:23 +0200, Andreas Klebinger wrote:
>> Does anyone have good hints for literature on basic block layout
>> algorithms?
>> I've run into a few examples where the current algorithm falls apart
>> while working on Cmm.
>>
>> There is a trac ticket https://ghc.haskell.org/trac/ghc/ticket/15124#
>> ticket
>> where I tracked some of the issues I ran into.
>>
>> As it stands some cmm optimizations are far out weighted by
>> accidental changes they cause in the layout of basic blocks.
>>
>> The main problem seems to be that the current codegen only considers
>> the
>> last jump
>> in a basic block as relevant for code layout.
>>
>> This works well for linear chains of control flow but behaves badly
>> and
>> somewhat
>> unpredictable when dealing with branch heavy code where blocks have
>> more
>> than
>> one successor or calls.
>>
>> In particular if we have a loop
>>
>> A jmp B call C call D
>>
>> which we enter into at block B from Block E
>> we would like something like:
>>
>> E,B,C,D,A
>>
>> Which means with some luck C/D might be still in cache if we return
>> from
>> the call.
>>
>> However we can currently get:
>>
>> E,B,A,X,D,X,C
>>
>> where X are other unrelated blocks. This happens since call edges
>> are
>> invisible to the layout algorithm.
>> It even happens when we have (conditional) jumps from B  to C and C
>> to D
>> since these are invisible as well!
>>
>> I came across cases where inverting conditions lead to big
>> performance
>> losses since suddenly block layout
>> got all messed up. (~4% slowdown for the worst offenders).
>>
>> So I'm looking for solutions there.
>>
>> _______________________________________________
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>> ghc-devs at haskell.org
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> Andreas Klebinger <mailto:klebinger.andreas at gmx.at>
> Samstag, 5. Mai 2018 21:23
> Does anyone have good hints for literature on basic block layout 
> algorithms?
> I've run into a few examples where the current algorithm falls apart 
> while working on Cmm.
>
> There is a trac ticket 
> https://ghc.haskell.org/trac/ghc/ticket/15124#ticket
> where I tracked some of the issues I ran into.
>
> As it stands some cmm optimizations are far out weighted by
> accidental changes they cause in the layout of basic blocks.
>
> The main problem seems to be that the current codegen only considers 
> the last jump
> in a basic block as relevant for code layout.
>
> This works well for linear chains of control flow but behaves badly 
> and somewhat
> unpredictable when dealing with branch heavy code where blocks have 
> more than
> one successor or calls.
>
> In particular if we have a loop
>
> A jmp B call C call D
>
> which we enter into at block B from Block E
> we would like something like:
>
> E,B,C,D,A
>
> Which means with some luck C/D might be still in cache if we return 
> from the call.
>
> However we can currently get:
>
> E,B,A,X,D,X,C
>
> where X are other unrelated blocks. This happens since call edges are 
> invisible to the layout algorithm.
> It even happens when we have (conditional) jumps from B  to C and C to 
> D since these are invisible as well!
>
> I came across cases where inverting conditions lead to big performance 
> losses since suddenly block layout
> got all messed up. (~4% slowdown for the worst offenders).
>
> So I'm looking for solutions there.
>

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