There is good info in this thread … do add it to the ticket #15124
Simon
From: ghc-devs On Behalf Of Andreas Klebinger
Sent: 06 May 2018 20:59
To: Kavon Farvardin
Cc: ghc-devs@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 Farvardinmailto:kavon@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=gMqSinyL8ukfile:///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 Klebingermailto:klebinger.andreas@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.
