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See Note [shortcut call returns]

We do this in lots of other places (blockConcat, CmmLayoutStack, etc.)

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.

This gives the register allocator access to R1.., F1.., D1.. etc. for
the new code generator, and is a cheap way to eliminate all the extra
"x = R1" assignments that we get from copyIn.

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

   CmmTop -> CmmDecl
   CmmPgm -> CmmGroup

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.

- Block concat and branch-chain elimination were allowing a function call
  to return to the caller's entry point. But that doesn't leave anywhere
  for the infotable on the stack, since the SP on return must be the same
  as the SP on entry to the procedure.

Among others:
- Fixed Stg->C-- translation of let-no-escapes -- it's important to use the
  right continuation...
- Fixed infinite recursion in X86 backend (shortcutJump mishandled infinite loops)
- Fixed yet another wrong calling convention -- primops take args only in vanilla regs,
  but they may return results on the stack!
- Removed StackInfo from LGraph and Block -- now in LastCall and CmmZ
- Updated avail-variable and liveness code

o Fixed bug that emitted the copy-in code for closure entry
  in the wrong place -- at the initialization of the closure.
o Refactored some of the closure entry code.
o Added code to check that no LocalRegs are live-in to a procedure
   -- trip up some buggy programs earlier
o Fixed environment bindings for thunks
   -- we weren't (re)binding the free variables in a thunk
o Fixed a bug in proc-point splitting that dropped some updates
  to the entry block in a procedure.
o Fixed improper calls to code that generates CmmLit's for strings
o New invariant on cg_loc in CgIdInfo: the expression is always tagged
o Code to load free vars on entry to a thunk was (wrongly) placed before
  the heap check.
o Some of the StgCmm code was redundantly passing around Id's
  along with CgIdInfo's; no more.
o Initialize the LocalReg's that point to a closure before allocating and
  initializing the closure itself -- otherwise, we have problems with
  recursive closure bindings
o BlockEnv and BlockSet types are now abstract.
o Update frames:
  - push arguments in Old call area
  - keep track of the return sp in the FCode monad
  - keep the return sp in every call, tail call, and return
      (because it might be different at different call sites,
       e.g. tail calls to the gc after a heap check are performed
            before pushing the update frame)
  - set the sp appropriately on returns and tail calls
o Reduce call, tail call, and return to a single LastCall node
o Added slow entry code, using different calling conventions on entry and tail call
o More fixes to the calling convention code.
  The tricky stuff is all about the closure environment: it must be passed in R1,
  but in non-closures, there is no such argument, so we can't treat all arguments
  the same way: the closure environment is special. Maybe the right step forward
  would be to define a different calling convention for closure arguments.
o Let-no-escapes need to be emitted out-of-line -- otherwise, we drop code.
o Respect RTS requirement of word alignment for pointers
  My stack allocation can pack sub-word values into a single word on the stack,
  but it wasn't requiring word-alignment for pointers. It does now,
  by word-aligning both pointer registers and call areas.
o CmmLint was over-aggresively ruling out non-word-aligned memory references,
  which may be kosher now that we can spill small values into a single word.
o Wrong label order on a conditional branch when compiling switches.
o void args weren't dropped in many cases.
  To help prevent this kind of mistake, I defined a NonVoid wrapper,
  which I'm applying only to Id's for now, although there are probably
  other good candidates.
o A little code refactoring: separate modules for procpoint analysis splitting, 
  stack layout, and building infotables.
o Stack limit check: insert along with the heap limit check, using a symbolic
  constant (a special CmmLit), then replace it when the stack layout is known.
o Removed last node: MidAddToContext 
o Adding block id as a literal: means that the lowering of the calling conventions
  no longer has to produce labels early, which was inhibiting common-block elimination.
  Will also make it easier for the non-procpoint-splitting path.
o Info tables: don't try to describe the update frame!
o Over aggressive use of NonVoid!!!!
  Don't drop the non-void args before setting the type of the closure!!!
o Sanity checking:
  Added a pass to stub dead dead slots on the stack
  (only ~10 lines with the dataflow framework)
o More sanity checking:
  Check that incoming pointer arguments are non-stubbed.
  Note: these checks are still subject to dead-code removal, but they should
  still be quite helpful.
o Better sanity checking: why stop at function arguments?
  Instead, in mkAssign, check that _any_ assignment to a pointer type is non-null
  -- the sooner the crash, the easier it is to debug.
  Still need to add the debugging flag to turn these checks on explicitly.
o Fixed yet another calling convention bug.
  This time, the calls to the GC were wrong. I've added a new convention
  for GC calls and invoked it where appropriate.
  We should really straighten out the calling convention stuff:
    some of the code (and documentation) is spread across the compiler,
    and there's some magical use of the node register that should really
    be handled (not avoided) by calling conventions.
o Switch bug: the arms in mkCmmLitSwitch weren't returning to a single join point.
o Environment shadowing problem in Stg->Cmm:
  When a closure f is bound at the top-level, we should not bind f to the
  node register on entry to the closure.
  Why? Because if the body of f contains a let-bound closure g that refers
  to f, we want to make sure that it refers to the static closure for f.
  Normally, this would all be fine, because when we compile a closure,
  we rebind free variables in the environment. But f doesn't look like
  a free variable because it's a static value. So, the binding for f
  remains in the environment when we compile g, inconveniently referring
  to the wrong thing.
  Now, I bind the variable in the local environment only if the closure is not
  bound at the top level. It's still okay to make assumptions about the
  node holding the closure environment; we just won't find the binding
  in the environment, so code that names the closure will now directly
  get the label of the static closure, not the node register holding a
  pointer to the static closure.
o Don't generate bogus Cmm code containing SRTs during the STG -> Cmm pass!
  The tables made reference to some labels that don't exist when we compute and
  generate the tables in the back end.
o Safe foreign calls need some special treatment (at least until we have the integrated
  codegen). In particular:
  o they need info tables
  o they are not procpoints -- the successor had better be in the same procedure
  o we cannot (yet) implement the calling conventions early, which means we have
    to carry the calling-conv info all the way to the end
o We weren't following the old convention when registering a module.
  Now, we use update frames to push any new modules that have to be registered
  and enter the youngest one on the stack.
  We also use the update frame machinery to specify that the return should pop
  the return address off the stack.
o At each safe foreign call, an infotable must be at the bottom of the stack,
  and the TSO->sp must point to it.
o More problems with void args in a direct call to a function:
  We were checking the args (minus voids) to check whether the call was saturated,
  which caused problems when the function really wasn't saturated because it
  took an extra void argument.
o Forgot to distinguish integer != from floating != during Stg->Cmm
o Updating slotEnv and areaMap to include safe foreign calls
  The dataflow analyses that produce the slotEnv and areaMap give
  results for each basic block, but we also need the results for
  a safe foreign call, which is a middle node.
  After running the dataflow analysis, we have another pass that
  updates the results to includ any safe foreign calls.
o Added a static flag for the debugging technique that inserts
  instructions to stub dead slots on the stack and crashes when
  a stubbed value is loaded into a pointer-typed LocalReg.
o C back end expects to see return continuations before their call sites.
  Sorted the flowgraphs appropriately after splitting.
o PrimOp calling conventions are special -- unlimited registers, no stack
  Yet another calling convention...
o More void value problems: if the RHS of a case arm is a void-typed variable,
  don't try to return it.
o When calling some primOp, they may allocate memory; if so, we need to
  do a heap check when we return from the call.

This merge does not turn on the new codegen (which only compiles
a select few programs at this point),
but it does introduce some changes to the old code generator.

The high bits:
1. The Rep Swamp patch is finally here.
   The highlight is that the representation of types at the
   machine level has changed.
   Consequently, this patch contains updates across several back ends.
2. The new Stg -> Cmm path is here, although it appears to have a
   fair number of bugs lurking.
3. Many improvements along the CmmCPSZ path, including:
   o stack layout
   o some code for infotables, half of which is right and half wrong
   o proc-point splitting

o Moved BlockId stuff to a new file to avoid module recursion
o Defined stack areas for parameter-passing locations and spill slots
o Part way through replacing copy in and copy out nodes
  - added movement instructions for stack pointer
  - added movement instructions for call and return parameters
    (but not with the proper calling conventions)
o Inserting spills and reloads for proc points is now procpoint-aware
  (it was relying on the presence of a CopyIn node as a proxy for
   procpoint knowledge)
o Changed ZipDataflow to expect AGraphs (instead of being polymorphic in
   the type of graph)

Several changes in this patch, partially bug fixes, partially new code:
o bug fixes in ZipDataflow
   - added some checks to verify that facts converge
   - removed some erroneous checks of convergence on entry nodes
   - added some missing applications of transfer functions
o changed dataflow clients to use ZipDataflow, making ZipDataflow0 obsolete
o eliminated DFA monad (no need for separate analysis and rewriting monads with ZipDataflow)
o started stack layout changes
   - no longer generating CopyIn and CopyOut nodes (not yet fully expunged though)
   - still not using proper calling conventions
o simple new optimizations:
   - common block elimination
      -- have not yet tried to move the Adams opt out of CmmProcPointZ
   - block concatenation
o piped optimization fuel up to the HscEnv
   - can be limited by a command-line flag
   - not tested, and probably not yet properly used by clients
o added unique supply to FuelMonad, also lifted unique supply to DFMonad

  -- LastBranch no longer takes parameters
  -- LastJump and LastReturn no longer carry CmmActuals;
     instead, those are carried by a CopyOut in the same basic block

We now set the flags once and for all in compiler/Makefile.

This patch combines two changes:
  1. As requested by SimonPJ, the redundancy inherent in having
     LastCall bear actual parameters has been removed.  The actual
     parameters are now carried by a separate CopyOut node.
  2. The internal (to zipper) representation of calls has changed;
     the representation of calling conventions is more orthogonal,
     and there is now no such thing as a 'safe' or 'final' call
     to a CallishMachOp.   This change has affected the interface
     to MkZipCfgCmm, which now provides a static guarantee.  Simon's
     new upstream code will be affected; I've patched the existing
     code in CmmCvt (which becomes ever hairier).
  

Interface MkZipCfgCmm should now be sufficient for all construction
needs, though some identifiers are re-exported from (and explained in)
MkZipCfg.  ZipCfgCmmRep should be used only by modules involved in
analysis, optimization, or translation of Cmm programs.