Basic Block Layout in the NCG

Tue May 8 08:20:10 UTC 2018

There is good info in this thread … do add it to the ticket #15124

Simon

From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Andreas Klebinger
Sent: 06 May 2018 20:59
To: Kavon Farvardin <kavon at farvard.in>
Cc: ghc-devs at haskell.org
Subject: Re: Basic Block Layout in the NCG

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<file:///s:/www.youtube.com/watch%3fv=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#<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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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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