Perl 4's special variable $* controls multi-line matching; it's been
  deprecated from Perl 5's inception, and is finally removed in Perl
  versions 5.9 (soon to be 5.10).

  Since GHC depends on Perl 5.6+ anyway, this patch introduces an
  equivalent effect to $* without using that special variable, by
  hooking into Perl's regex parsing process to add the /m flag.

  GHC only works with Perl versions 5.6 and later; however
  the current test greps in a rather non-future-proof way
  (explicitly against 5.6 and 5.8).
  
  We instead exploit the "v-string" change in 5.6, which
  displays the version string as "v5.x.y", rather than
  the earlier format "version 5.xxx_yyy".  Hence grepping
  against "v5" will make it future proof for a while.
  (At least until Perl 6 is installed as the system perl.)

We now have a separate bit on reporting bugs, via a different method, so
this section didn't really make sense any more.

We store .mix files in

 .hpc/<package>/<the.module.name>.mix

The package main the empty package (aka other naming passes), 
so Main just is stored in

 .hpc/Main.tix

This change in backwards compatable.

import GHC.Exts
main = print (C# (chr# 0xffffffff#))

This crashed at compile time (with or without hpc)
Tickled by pending cross-module hpc inlining patch.

It was returning False for type variables amongst other things, so
"instance C a" was telling us to use -XTypeSynonymInstances.

-XUnboxedTuples
-XExpressionSignaturesUnboxedTuples
-XTypeSynonymUnboxedTuples

Just looking at Opt_GlasgowExts is not sufficient to see if we need
to do an instance undecidability check. Rather than try to enumerate
all the extensions that may require us to do the check we now always
do it.