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Instead of relying on common-block-elimination to share return
continuations in the common case (case-alternative heap checks) we do
it explicitly.  This isn't hard to do, is more robust, and saves some
compilation time.  Full commentary in Note [sharing continuations].

Proc-point splitting is only required by backends that do not support
having proc-points within a code block (that is, everything except the
native backend, i.e. LLVM and C).

Not doing proc-point splitting saves some compilation time, and might
produce slightly better code in some cases.

All the flags that 'ways' imply are now dynamic

To help with this, we now also pass DynFlags around inside the SpecM
monad.

A thunk with no free variables was not getting blackholed when
-feager-blackholing was on, but we were nevertheless pushing the
stg_bh_upd_frame version of the update frame that expects to see a
black hole.

I fixed this twice for good measure:

 - we now call blackHoleOnEntry when pushing the update frame to check
   whether the closure was actually blackholed, and so that we use the
   same predicate in both places

 - we now black hole thunks even if they have no free variables.
   These only occur when optimisation is off, but presumably if you say
   -feager-blackholing then that's what you want to happen.

We were using the SRT information generated by the computeSRTs pass to
decide whether to add a static link field to a constructor or not, and
this broke when I disabled computeSRTs for the new code generator.  So
I've hacked it for now to only rely on the SRT information generated
by CoreToStg.

Also:
 - improvements to code generation: push slow-call continuations
   on the stack instead of generating explicit continuations

 - remove unused CmmInfo wrapper type (replace with CmmInfoTable)

 - squash Area and AreaId together, remove now-unused RegSlot

 - comment out old unused stack-allocation code that no longer
   compiles after removal of RegSlot

We only use it for "compiler" sources, i.e. not for libraries.
Many modules have a -fno-warn-tabs kludge for now.

User visible changes
====================

Profilng
--------

Flags renamed (the old ones are still accepted for now):

  OLD            NEW
  ---------      ------------
  -auto-all      -fprof-auto
  -auto          -fprof-exported
  -caf-all       -fprof-cafs

New flags:

  -fprof-auto              Annotates all bindings (not just top-level
                           ones) with SCCs

  -fprof-top               Annotates just top-level bindings with SCCs

  -fprof-exported          Annotates just exported bindings with SCCs

  -fprof-no-count-entries  Do not maintain entry counts when profiling
                           (can make profiled code go faster; useful with
                           heap profiling where entry counts are not used)

Cost-centre stacks have a new semantics, which should in most cases
result in more useful and intuitive profiles.  If you find this not to
be the case, please let me know.  This is the area where I have been
experimenting most, and the current solution is probably not the
final version, however it does address all the outstanding bugs and
seems to be better than GHC 7.2.

Stack traces
------------

+RTS -xc now gives more information.  If the exception originates from
a CAF (as is common, because GHC tends to lift exceptions out to the
top-level), then the RTS walks up the stack and reports the stack in
the enclosing update frame(s).

Result: +RTS -xc is much more useful now - but you still have to
compile for profiling to get it.  I've played around a little with
adding 'head []' to GHC itself, and +RTS -xc does pinpoint the problem
quite accurately.

I plan to add more facilities for stack tracing (e.g. in GHCi) in the
future.

Coverage (HPC)
--------------

 * derived instances are now coloured yellow if they weren't used
 * likewise record field names
 * entry counts are more accurate (hpc --fun-entry-count)
 * tab width is now correct (markup was previously off in source with
   tabs)

Internal changes
================

In Core, the Note constructor has been replaced by

        Tick (Tickish b) (Expr b)

which is used to represent all the kinds of source annotation we
support: profiling SCCs, HPC ticks, and GHCi breakpoints.

Depending on the properties of the Tickish, different transformations
apply to Tick.  See CoreUtils.mkTick for details.

Tickets
=======

This commit closes the following tickets, test cases to follow:

  - Close #2552: not a bug, but the behaviour is now more intuitive
    (test is T2552)

  - Close #680 (test is T680)

  - Close #1531 (test is result001)

  - Close #949 (test is T949)

  - Close #2466: test case has bitrotted (doesn't compile against current
    version of vector-space package)

See Note [atomic CAFs] in rts/sm/Storage.c

And some knock-on changes

1c2f8953 (symptom was broken
biographical profiling, see #5451).

Simon Peyton Jones authored Aug 22, 2011 and Simon Marlow committed Aug 25, 2011

* Move CgRep (private to old codgen) from SMRep to ClosureInfo
* Avoid using CgRep in new codegen
* Move SMRep and Bitmap from codeGen/ to cmm/

When allocating new objects on the heap, we previously returned
a CmmExpr containing the heap pointer as well as the tag expression,
which would be added to the code graph upon first usage.  Unfortunately,
this meant that untagged heap pointers living in registers might
be spilled to the stack, where they interacted poorly with garbage
collection (we saw this bug specifically with the compacting garbage
collector.)

This fix immediately tags the register containing the heap pointer,
so that unless we have extremely unfriendly spill code, the new pointer
will never be spilled to the stack untagged.

An alternate solution might have been to modify allocDynClosure to
tag the pointer upon the initial register allocation, but not all
invocations of allocDynClosure tag the resulting pointer, and
threading the consequent CgIdInfo for the cases that did would have
been annoying.

This changes the new code generator to make use of the Hoopl package
for dataflow analysis.  Hoopl is a new boot package, and is maintained
in a separate upstream git repository (as usual, GHC has its own
lagging darcs mirror in http://darcs.haskell.org/packages/hoopl).

During this merge I squashed recent history into one patch.  I tried
to rebase, but the history had some internal conflicts of its own
which made rebase extremely confusing, so I gave up. The history I
squashed was:

  - Update new codegen to work with latest Hoopl
  - Add some notes on new code gen to cmm-notes
  - Enable Hoopl lag package.
  - Add SPJ note to cmm-notes
  - Improve GC calls on new code generator.

Work in this branch was done by:
   - Milan Straka <fox@ucw.cz>
   - John Dias <dias@cs.tufts.edu>
   - David Terei <davidterei@gmail.com>

Edward Z. Yang <ezyang@mit.edu> merged in further changes from GHC HEAD
and fixed a few bugs.

The type of the CmmLabel ctor is now
  CmmLabel :: PackageId -> FastString -> CmmLabelInfo -> CLabel
  
 - When you construct a CmmLabel you have to explicitly say what
   package it is in. Many of these will just use rtsPackageId, but
   I've left it this way to remind people not to pretend labels are
   in the RTS package when they're not. 
   
 - When parsing a Cmm file, labels that are not defined in the 
   current file are assumed to be in the RTS package. 
   
   Labels imported like
      import label
   are assumed to be in a generic "foreign" package, which is different
   from the current one.
   
   Labels imported like
      import "package-name" label
   are marked as coming from the named package.
   
   This last one is needed for the integer-gmp library as we want to
   refer to labels that are not in the same compilation unit, but
   are in the same non-rts package.
   
   This should help remove the nasty #ifdef __PIC__ stuff from
   integer-gmp/cbits/gmp-wrappers.cmm
   

Code simplifications due to call/return separation; some improvements to how node argument is managed

We used to use StaticFlags