[GHC] #13360: Add a flag to enable inferring HasCallStack constraints

[GHC]

#13360: Add a flag to enable inferring HasCallStack constraints
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        Reporter:  gridaphobe        |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by gridaphobe):

 Thanks for digging in to this! Now that we can see there is an impact for
 real-world code, I'd suggest stepping back to the simpler examples like
 the `loop` benchmark I linked earlier, to determine what's causing the
 overhead.

 HasCallStack **should** be equivalent to an extra argument that builds up
 a list of SrcLocs (technically `(String, SrcLoc)` pairs). So my first
 experiment would probably be to write another benchmark that implements
 HasCallStack in user code. These two versions should perform the same, do
 they? If not, maybe GHC is doing something silly when it generates the
 HasCallStack code.

 Next I would try shrinking the contents of the user-level CallStack,
 instead of `(String, SrcLoc)` make it a list of `()`. This should measure
 the cost of pushing a new item onto the stack. Perhaps that cost is
 proportional to the size of the item (I wouldn't think so since the item
 should be a thunk, but it's worth checking).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13360#comment:23>
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