Basic Block Layout in the NCG

[Kavon Farvardin]

> 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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