/* -----------------------------------------------------------------------------
 *
 * (c) The GHC Team, 1998-2012
 *
 * Code for starting, stopping and restarting threads.
 *
 * This file is written in a subset of C--, extended with various
 * features specific to GHC.  It is compiled by GHC directly.  For the
 * syntax of .cmm files, see the parser in ghc/compiler/cmm/CmmParse.y.
 *
 * ---------------------------------------------------------------------------*/

#include "Cmm.h"

/*
 * This module contains the two entry points and the final exit point
 * to/from the Haskell world.  We can enter either by:
 *
 *   a) returning to the address on the top of the stack, or
 *   b) entering the closure on the top of the stack
 *
 * the function stg_stop_thread_entry is the final exit for a
 * thread: it is the last return address on the stack.  It returns
 * to the scheduler marking the thread as finished.
 */

#define CHECK_SENSIBLE_REGS()                           \
    ASSERT(Hp != 0);                                    \
    ASSERT(HpAlloc == 0);                               \
    ASSERT(Sp != 0);                                    \
    ASSERT(SpLim != 0);                                 \
    ASSERT(SpLim - WDS(RESERVED_STACK_WORDS) <= Sp);

/* -----------------------------------------------------------------------------
   Returning from the STG world.
   -------------------------------------------------------------------------- */

INFO_TABLE_RET(stg_stop_thread, STOP_FRAME,
               W_ info_ptr,
               PROF_HDR_FIELDS(W_,p1,p2))
/* no return list: explicit stack layout */
{
    /*
       The final exit.

       The top-top-level closures (e.g., "main") are of type "IO a".
       When entered, they perform an IO action and return an 'a' in R1.

       We save R1 on top of the stack where the scheduler can find it,
       tidy up the registers and return to the scheduler.

       We Leave the stack looking like this:

                +----------------+
                |      -------------------> return value
                +----------------+
                | stg_enter_info |
                +----------------+

       The stg_enter_info is just a dummy info table so that the
       garbage collector can understand the stack (there must always
       be an info table on top of the stack).
    */

    /*
       Here we setup the stack unwinding annotation necessary to allow
       debuggers to find their way back to the C stack.

       This is a bit fiddly as we assume the layout of the stack prepared
       for us by StgRun. Note that in most cases StgRun is written in assembler
       and therefore has no associated unwind information. For this reason we
       need to identify the platform stack pointer and return address values for
       the StgRun's caller.
     */
#if defined(x86_64_HOST_ARCH)
    // offset of 8 in MachSp value due to return address
    unwind MachSp = MachSp + RESERVED_C_STACK_BYTES + STG_RUN_STACK_FRAME_SIZE + 8,
           UnwindReturnReg = W_[MachSp + RESERVED_C_STACK_BYTES + STG_RUN_STACK_FRAME_SIZE];
#else
    // FIXME: Fill in for other platforms
    unwind MachSp = return,
           UnwindReturnReg = return;
#endif

    Sp = Sp + SIZEOF_StgStopFrame - WDS(2);
    Sp(1) = R1;
    Sp(0) = stg_enter_info;

    StgTSO_what_next(CurrentTSO) = ThreadComplete::I16;

    SAVE_THREAD_STATE();

    /* The return code goes in BaseReg->rRet, and BaseReg is returned in R1 */
    StgRegTable_rRet(BaseReg) = ThreadFinished;
    R1 = BaseReg;

    jump StgReturn [R1];
}

/* -----------------------------------------------------------------------------
   Start a thread from the scheduler by returning to the address on
   the top of the stack.  This is used for all entries to STG code
   from C land.

   On the way back, we (usually) pass through stg_returnToSched which saves
   the thread's state away nicely.
   -------------------------------------------------------------------------- */

stg_returnToStackTop /* no args: explicit stack layout */
{
  LOAD_THREAD_STATE();
  CHECK_SENSIBLE_REGS();
  jump %ENTRY_CODE(Sp(0)) [];
}

stg_returnToSched /* no args: explicit stack layout */
{
  W_ r1;
  r1 = R1; // foreign calls may clobber R1
  SAVE_THREAD_STATE();
  foreign "C" threadPaused(MyCapability() "ptr", CurrentTSO);
  R1 = r1;
  jump StgReturn [R1];
}

// A variant of stg_returnToSched that doesn't call threadPaused() on the
// current thread.  This is used for switching from compiled execution to the
// interpreter, where calling threadPaused() on every switch would be too
// expensive.
//
// See Note [avoiding threadPaused] in Interpreter.c
//
stg_returnToSchedNotPaused /* no args: explicit stack layout */
{
  SAVE_THREAD_STATE();
  jump StgReturn [R1];
}

// A variant of stg_returnToSched, but instead of returning directly to the
// scheduler, we jump to the code fragment pointed to by R2.  This lets us
// perform some final actions after making the thread safe, such as unlocking
// the MVar on which we are about to block in SMP mode.
stg_returnToSchedButFirst /* no args: explicit stack layout */
{
  W_ r1, r2, r3;
  r1 = R1;
  r2 = R2;
  r3 = R3;
  SAVE_THREAD_STATE();
  // foreign calls may clobber R1/R2/.., so we save them above
  foreign "C" threadPaused(MyCapability() "ptr", CurrentTSO);
  R1 = r1;
  R2 = r2;
  R3 = r3;
  jump R2 [R1,R3];
}

stg_threadFinished /* no args: explicit stack layout */
{
  StgRegTable_rRet(BaseReg) = ThreadFinished;
  R1 = BaseReg;
  jump StgReturn [R1];
}

/* -----------------------------------------------------------------------------
    Strict IO application - performing an IO action and entering its result.

    rts_evalIO() lets you perform Haskell IO actions from outside of
    Haskell-land, returning back to you their result. Want this result
    to be evaluated to WHNF by that time, so that we can easily get at
    the int/char/whatever using the various get{Ty} functions provided
    by the RTS API.

    stg_forceIO takes care of this, performing the IO action and entering
    the results that comes back.

    ------------------------------------------------------------------------- */

INFO_TABLE_RET(stg_forceIO, RET_SMALL, P_ info_ptr)
    return (P_ ret)
{
    ENTER(ret);
}