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Properly guard imports because they have to be precise on Windows and Darwin sets __PIC__ automatically

  
This patch implements pointer tagging as per our ICFP'07 paper "Faster
laziness using dynamic pointer tagging".  It improves performance by
10-15% for most workloads, including GHC itself.

The original patches were by Alexey Rodriguez Yakushev
<mrchebas@gmail.com>, with additions and improvements by me.  I've
re-recorded the development as a single patch.

The basic idea is this: we use the low 2 bits of a pointer to a heap
object (3 bits on a 64-bit architecture) to encode some information
about the object pointed to.  For a constructor, we encode the "tag"
of the constructor (e.g. True vs. False), for a function closure its
arity.  This enables some decisions to be made without dereferencing
the pointer, which speeds up some common operations.  In particular it
enables us to avoid costly indirect jumps in many cases.

More information in the commentary:

http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/HaskellExecution/PointerTagging

We needed to turn some inline C functions and C macros into either
real C functions or C-- macros.

This means we can avoid some StablePtrs, and also catch cases where
the AP_STACK has been evaluated (this can happen with :history, see
the hist001 test).

This is the result of Bernie Pope's internship work at MSR Cambridge,
with some subsequent improvements by me.  The main plan was to

 (a) Reduce the overhead for breakpoints, so we could enable 
     the feature by default without incurrent a significant penalty
 (b) Scatter more breakpoint sites throughout the code

Currently we can set a breakpoint on almost any subexpression, and the
overhead is around 1.5x slower than normal GHCi.  I hope to be able to
get this down further and/or allow breakpoints to be turned off.

This patch also fixes up :print following the recent changes to
constructor info tables.  (most of the :print tests now pass)

We now support single-stepping, which just enables all breakpoints.

  :step <expr>     executes <expr> with single-stepping turned on
  :step            single-steps from the current breakpoint

The mechanism is quite different to the previous implementation.  We
share code with the HPC (haskell program coverage) implementation now.
The coverage pass annotates source code with "tick" locations which
are tracked by the coverage tool.  In GHCi, each "tick" becomes a
potential breakpoint location.

Previously breakpoints were compiled into code that magically invoked
a nested instance of GHCi.  Now, a breakpoint causes the current
thread to block and control is returned to GHCi.

See the wiki page for more details and the current ToDo list:

  http://hackage.haskell.org/trac/ghc/wiki/NewGhciDebugger

We changed the convention a while ago so that BaseReg is returned to
the scheduler in R1, because BaseReg may change during the run of a
thread, e.g. during a foreign call.  A few places got missed,
mostly for very rare events.

Should fix concprog001, although I'm not able to reliably reproduce
the failure.

This primop ensures that the current computation is not being
duplicated, by calling threadPaused().  The idea is to use it inside
unsafePerformIO/unsafeInterleaveIO (see #986).

We recently discovered that they aren't a win any more, and just cost
code size.

This is a simplification & minor optimisation for GHCi

- infoPtr# :: a -> Addr#
- closurePayload# :: a -> (# Array b, ByteArr# #)

These prim ops provide the magic behind the ':print' command 

We can avoid using any other long long operations in PrimOps.cmm.
One more step towards compiling the RTS using the NCG.

Fixed version of an old patch by Simon Marlow. His description read:
 Also, now an arbitrarily short context switch interval may now be
 specified, as we increase the RTS ticker's resolution to match the
 requested context switch interval.  This also applies to +RTS -i (heap
 profiling) and +RTS -I (the idle GC timer).  +RTS -V is actually only
 required for increasing the resolution of the profile timer.

So that we can build the RTS with the NCG.

gmp.h #defines mpz_foo to __gmpz_foo, so the real ABI is __gmpz_foo,
so that is what we must invoke in order to be portable here.
Similarly for mpn --> __gmpn.

Most of the other users of the fptools build system have migrated to
Cabal, and with the move to darcs we can now flatten the source tree
without losing history, so here goes.

The main change is that the ghc/ subdir is gone, and most of what it
contained is now at the top level.  The build system now makes no
pretense at being multi-project, it is just the GHC build system.

No doubt this will break many things, and there will be a period of
instability while we fix the dependencies.  A straightforward build
should work, but I haven't yet fixed binary/source distributions.
Changes to the Building Guide will follow, too.

This gives some control over affinity, while we figure out the best
way to automatically schedule threads to make best use of the
available parallelism.

In addition to the primitive, there is also:
 
  GHC.Conc.forkOnIO :: Int -> IO () -> IO ThreadId

where 'forkOnIO i m' creates a thread on Capability (i `rem` N), where
N is the number of available Capabilities set by +RTS -N.

Threads forked by forkOnIO do not automatically migrate when there are
free Capabilities, like normal threads do.  Still, if you're using
forkOnIO exclusively, it's a good idea to do +RTS -qm to disable work
pushing anyway (work pushing takes too much time when the run queues
are large, this is something we need to fix).

This relates to the recent introduction of clean/dirty TSOs, and the
consqeuent write barriers required.  We were missing some write
barriers in the takeMVar/putMVar family of primops, when performing
the take/put directly on another TSO.

Fixes #705, and probably some test failures.

We now have more stg_ap entry points: stg_ap_*_fast, which take
arguments in registers according to the platform calling convention.
This is faster if the function being called is evaluated and has the
right arity, which is the common case (see the eval/apply paper for
measurements).  

We still need the stg_ap_*_info entry points for stack-based
application, such as an overflows when a function is applied to too
many argumnets.  The stg_ap_*_fast functions actually just check for
an evaluated function, and if they don't find one, push the args on
the stack and invoke stg_ap_*_info.  (this might be slightly slower in
some cases, but not the common case).

fix one incorrect case, and made several more accurate

atomicModifyMutVar# was re-using the storage manager mutex (sm_mutex)
to get its atomicity guarantee in SMP mode. But recently the addition
of a call to dirty_MUT_VAR() to implement the read barrier lead to a
rare deadlock case, because dirty_MUT_VAR() very occasionally needs to
allocate a new block to chain on the mutable list, which requires
sm_mutex.

rather than recordMutableGen(), the former works better in SMP

tryPutMVarzh_fast: make it work in the non-full case.

Merge to STABLE.

Improve the GC behaviour of IORefs (see Ticket #650).

This is a small change to the way IORefs interact with the GC, which
should improve GC performance for programs with plenty of IORefs.

Previously we had a single closure type for mutable variables,
MUT_VAR.  Mutable variables were *always* on the mutable list in older
generations, and always traversed on every GC.

Now, we have two closure types: MUT_VAR_CLEAN and MUT_VAR_DIRTY.  The
latter is on the mutable list, but the former is not.  (NB. this
differs from MUT_ARR_PTRS_CLEAN and MUT_ARR_PTRS_DIRTY, both of which
are on the mutable list).  writeMutVar# now implements a write
barrier, by calling dirty_MUT_VAR() in the runtime, that does the
necessary modification of MUT_VAR_CLEAN into MUT_VAR_DIRY, and adding
to the mutable list if necessary.

This results in some pretty dramatic speedups for GHC itself.  I've
just measureed a 30% overall speedup compiling a 31-module program
(anna) with the default heap settings :-D

Improve the GC behaviour of IOArrays/STArrays

See Ticket #650

This is a small change to the way mutable arrays interact with the GC,
that can have a dramatic effect on performance, and make tricks with
unsafeThaw/unsafeFreeze redundant.  Data.HashTable should be faster
now (I haven't measured it yet).

We now have two mutable array closure types, MUT_ARR_PTRS_CLEAN and
MUT_ARR_PTRS_DIRTY.  Both are on the mutable list if the array is in
an old generation.  writeArray# sets the type to MUT_ARR_PTRS_DIRTY.
The garbage collector can set the type to MUT_ARR_PTRS_CLEAN if it
finds that no element of the array points into a younger generation
(discovering this required a small addition to evacuate(), but rough
tests indicate that it doesn't measurably affect performance).

NOTE: none of this affects unboxed arrays (IOUArray/STUArray), only
boxed arrays (IOArray/STArray).

We could go further and extend the DIRTY bit to be per-block rather
than for the whole array, but for now this is an easy improvement.

Raise the (new) exception NestedAtomically when atomically is nested
(using unsafePerformIO).  This is a small improvement over crashing.

Small performance improvement to STM: reduce the size of an atomically
frame from 3 words to 2 words by combining the "waiting" boolean field
with the info pointer, i.e. having two separate info tables/return
addresses for an atomically frame, one for the normal case and one for
the waiitng case.

Re-use temporary storage in the STM implementation

Fix a crash in STM; we were releasing ownership of the transaction too
early in stmWait(), so a TSO could be woken up before we had finished
putting it to sleep properly.

Fix some problems with array thawing/freezing and the GC.