[GHC] #3693: Show stack traces

[GHC]

#3693: Show stack traces
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        Reporter:  jpet              |            Owner:  Tarrasch
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:  7.10.1
       Component:  Runtime System    |          Version:  6.10.4
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by scpmw):

 > given a proposed Core-Core transformation, does the transformation meet
 the specfification or not?

 Simon covered the basic idea pretty well. From the transformation point of
 view, the general rule of thumb is that source notes want to stay on
 covered code (even if transformed), but don't care too much about new code
 getting added in ("soft scope"). So we can for example work with fusion by
 simply floating "upwards":

 {{{
 src<1.hs> foldr k z (src<2.hs> build g)
 ===> src<1.hs> src<2.hs> g k z
 }}}

 Cost-centres would disallow this kind of transformation, as the new stacks
 have little to do with the old one. Source notes on the other hand want
 the transformation to still happen, so we just annotate the new expression
 with everything that appeared on the deconstructed source expression. The
 paper goes into (way) more detail here, but the short version is that
 while not always optimal, this approach is always legal.

 The only special case so far is `let`- and `case`-floating, where we run
 into the same problems as cost-centres (and use pretty much the same
 solution). Additionally we decide to not care too much about allocation
 costs at a number of points where it would have made things too messy.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/3693#comment:65>
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