gcc/libcilkrts/runtime/cilk-abi-cilk-for.cpp

/* cilk-abi-cilk-for.cpp                  -*-C++-*-
 *
 *************************************************************************
 *
 *  Copyright (C) 2011-2016, Intel Corporation
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 *  PLEASE NOTE: This file is a downstream copy of a file mainitained in
 *  a repository at cilkplus.org. Changes made to this file that are not
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 *  
 *  We welcome your contributions to this open source project. Thank you
 *  for your assistance in helping us improve Cilk Plus.
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 **************************************************************************/

/* Implementation of cilk_for ABI.
 *
 * This file must be C++, not C, in order to handle C++ exceptions correctly
 * from within the body of the cilk_for loop
 */

#include "internal/abi.h"
#include "metacall_impl.h"
#include "global_state.h"

// Icky macros to determine if we're compiled with optimization.  Based on
// the declaration of __CILKRTS_ASSERT in common.h
#if defined(_WIN32)
# if defined (_DEBUG)
#   define CILKRTS_OPTIMIZED 0    // Assumes /MDd is always used with /Od
# else
#   define CILKRTS_OPTIMIZED 1
# endif // defined(_DEBUG)
#else
# if defined(__OPTIMIZE__)
#   define CILKRTS_OPTIMIZED 1
# else
#   define CILKRTS_OPTIMIZED 0
# endif
#endif

template <typename count_t>
static inline int grainsize(int req, count_t count)
{
    // A positive requested grain size comes from the user.  A very high grain
    // size risks losing parallelism, but the user told us what they want for
    // grainsize.  Who are we to argue?
    if (req > 0)
        return req;

    // At present, a negative requested grain size is treated the same way as
    // a zero grain size, i.e., the runtime computes the actual grainsize
    // using a hueristic.  In the future, the compiler may give us additional
    // information about the size of the cilk_for body by passing a negative
    // grain size.

    // Avoid generating a zero grainsize, even for empty loops.
    if (count < 1)
        return 1;

    global_state_t* g = cilkg_get_global_state();
    if (g->under_ptool)
    {
        // Grainsize = 1, when running under PIN, and when the grainsize has
        // not explicitly been set by the user.
        return 1;
    }
    else
    {
        // Divide loop count by 8 times the worker count and round up.
        const int Px8 = g->P * 8;
        count_t n = (count + Px8 - 1) / Px8;

        // 2K should be enough to amortize the cost of the cilk_for. Any
        // larger grainsize risks losing parallelism.
        if (n > 2048)
            return 2048;
        return (int) n;  // n <= 2048, so no loss of precision on cast to int
    }
}

/*
 * call_cilk_for_loop_body
 *
 * Centralizes the code to call the loop body.  The compiler should be
 * inlining this code
 *
 * low   - Low loop index we're considering in this portion of the algorithm
 * high  - High loop index we're considering in this portion of the algorithm
 * body  - lambda function for the cilk_for loop body
 * data  - data used by the lambda function
 * w     - __cilkrts_worker we're currently executing on
 * loop_root_pedigree - __cilkrts_pedigree node we generated for the root of
 *         the cilk_for loop to flatten out the internal nodes
 */
template <typename count_t, typename F>
inline static
void call_cilk_for_loop_body(count_t low, count_t high,
                             F body, void *data,
                             __cilkrts_worker *w,
                             __cilkrts_pedigree *loop_root_pedigree)
{
    // Cilkscreen should not report this call in a stack trace
    NOTIFY_ZC_INTRINSIC((char *)"cilkscreen_hide_call", 0);

    // The worker is only valid until the first spawn.  Fetch the
    // __cilkrts_stack_frame out of the worker, since it will be stable across
    // steals.  The sf pointer actually points to the *parent's*
    // __cilkrts_stack_frame, since this function is a non-spawning function
    // and therefore has no cilk stack frame of its own.
    __cilkrts_stack_frame *sf = w->current_stack_frame;

    // Save the pedigree node pointed to by the worker.  We'll need to restore
    // that when we exit since the spawn helpers in the cilk_for call tree
    // will assume that it's valid
    const __cilkrts_pedigree *saved_next_pedigree_node = w->pedigree.parent;

    // Add the leaf pedigree node to the chain. The parent is the root node
    // to flatten the tree regardless of the DAG branches in the cilk_for
    // divide-and-conquer recursion.
    //
    // The rank is initialized to the low index.  The user is
    // expected to call __cilkrts_bump_loop_rank at the end of the cilk_for
    // loop body.
    __cilkrts_pedigree loop_leaf_pedigree;

    loop_leaf_pedigree.rank = (uint64_t)low;
    loop_leaf_pedigree.parent = loop_root_pedigree;

    // The worker's pedigree always starts with a rank of 0
    w->pedigree.rank = 0;
    w->pedigree.parent = &loop_leaf_pedigree;

    // Call the compiler generated cilk_for loop body lambda function
    body(data, low, high);

    // The loop body may have included spawns, so we must refetch the worker
    // from the __cilkrts_stack_frame, which is stable regardless of which
    // worker we're executing on.
    w = sf->worker;

    // Restore the pedigree chain. It must be valid because the spawn helpers
    // generated by the cilk_for implementation will access it.
    w->pedigree.parent = saved_next_pedigree_node;
}

/* capture_spawn_arg_stack_frame
 *
 * Efficiently get the address of the caller's __cilkrts_stack_frame.  The
 * preconditons are that 'w' is the worker at the time of the call and
 * 'w->current_stack_frame' points to the __cilkrts_stack_frame within the
 * spawn helper.  This function should be called only within the argument list
 * of a function that is being spawned because that is the only situation in
 * which these preconditions hold.  This function returns the worker
 * (unchanged) after storing the captured stack frame pointer is stored in the
 * sf argument.
 *
 * The purpose of this function is to get the caller's stack frame in a
 * context where the caller's worker is known but its stack frame is not
 * necessarily initialized.  The "shrink wrap" optimization delays
 * initializing the contents of a spawning function's '__cilkrts_stack_frame'
 * as well as the 'current_stack_frame' pointer within the worker.  By calling
 * this function within a spawning function's argument list, we can ensure
 * that these initializations have occured but that a detach (which would
 * invalidate the worker pointer in the caller) has not yet occured.  Once the
 * '__cilkrts_stack_frame' has been retrieved in this way, it is stable for the
 * remainder of the caller's execution, and becomes an efficient way to get
 * the worker (much more efficient than calling '__cilkrts_get_tls_worker()'),
 * even after a spawn or sync.
 */
inline __cilkrts_worker* 
capture_spawn_arg_stack_frame(__cilkrts_stack_frame* &sf, __cilkrts_worker* w)
{
    // Get current stack frame
    sf = w->current_stack_frame;
#ifdef __INTEL_COMPILER
#   if __INTEL_COMPILER <= 1300 && __INTEL_COMPILER_BUILD_DATE < 20130101
    // In older compilers 'w->current_stack_frame' points to the
    // spawn-helper's stack frame.  In newer compiler's however, it points
    // directly to the pointer's stack frame.  (This change was made to avoid
    // having the spawn helper in the frame list when evaluating function
    // arguments, thus avoiding corruption when those arguments themselves
    // contain cilk_spawns.)
    
    // w->current_stack_frame is the spawn helper's stack frame.
    // w->current_stack_frame->call_parent is the caller's stack frame.
    sf = sf->call_parent;
#   endif
#endif
    return w;
}

/*
 * cilk_for_recursive
 *
 * Templatized function to implement the recursive divide-and-conquer
 * algorithm that's how we implement a cilk_for.
 *
 * low   - Low loop index we're considering in this portion of the algorithm
 * high  - High loop index we're considering in this portion of the algorithm
 * body  - lambda function for the cilk_for loop body
 * data  - data used by the lambda function
 * grain - grain size (0 if it should be computed)
 * w     - __cilkrts_worker we're currently executing on
 * loop_root_pedigree - __cilkrts_pedigree node we generated for the root of
 *         the cilk_for loop to flatten out the internal nodes
 */
template <typename count_t, typename F>
static
void cilk_for_recursive(count_t low, count_t high,
                        F body, void *data, int grain,
                        __cilkrts_worker *w,
                        __cilkrts_pedigree *loop_root_pedigree)
{
tail_recurse:
    // Cilkscreen should not report this call in a stack trace
    // This needs to be done everytime the worker resumes
    NOTIFY_ZC_INTRINSIC((char *)"cilkscreen_hide_call", 0);

    count_t count = high - low;
    // Invariant: count > 0, grain >= 1
    if (count > grain)
    {
        // Invariant: count >= 2
        count_t mid = low + count / 2;
        // The worker is valid only until the first spawn and is expensive to
        // retrieve (using '__cilkrts_get_tls_worker') after the spawn.  The
        // '__cilkrts_stack_frame' is more stable, but isn't initialized until
        // the first spawn.  Thus, we want to grab the address of the
        // '__cilkrts_stack_frame' after it is initialized but before the
        // spawn detaches.  The only place we can do that is within the
        // argument list of the spawned function, hence the call to
        // capture_spawn_arg_stack_frame().
        __cilkrts_stack_frame *sf;
#if defined(__GNUC__) && ! defined(__INTEL_COMPILER) && ! defined(__clang__)
        // The current version of gcc initializes the sf structure eagerly.
        // We can take advantage of this fact to avoid calling
        // `capture_spawn_arg_stack_frame` when compiling with gcc.
        // Remove this if the "shrink-wrap" optimization is implemented.
        sf = w->current_stack_frame;
        _Cilk_spawn cilk_for_recursive(low, mid, body, data, grain, w,
                                       loop_root_pedigree);
#else        
        _Cilk_spawn cilk_for_recursive(low, mid, body, data, grain,
                                       capture_spawn_arg_stack_frame(sf, w),
                                       loop_root_pedigree);
#endif
        w = sf->worker;
        low = mid;

        goto tail_recurse;
    }

    // Call the cilk_for loop body lambda function passed in by the compiler to
    // execute one grain
    call_cilk_for_loop_body(low, high, body, data, w, loop_root_pedigree);
}

static void noop() { }

/*
 * cilk_for_root
 *
 * Templatized function to implement the top level of a cilk_for loop.
 *
 * body  - lambda function for the cilk_for loop body
 * data  - data used by the lambda function
 * count - trip count for loop
 * grain - grain size (0 if it should be computed)
 */
template <typename count_t, typename F>
static void cilk_for_root(F body, void *data, count_t count, int grain)
{
    // Cilkscreen should not report this call in a stack trace
    NOTIFY_ZC_INTRINSIC((char *)"cilkscreen_hide_call", 0);

    // Pedigree computation:
    //
    //    If the last pedigree node on entry to the _Cilk_for has value X,
    //    then at the start of each iteration of the loop body, the value of
    //    the last pedigree node should be 0, the value of the second-to-last
    //    node should equal the loop counter, and the value of the
    //    third-to-last node should be X.  On return from the _Cilk_for, the
    //    value of the last pedigree should be incremented to X+2. The
    //    pedigree within the loop is thus flattened, such that the depth of
    //    recursion does not affect the results either inside or outside of
    //    the loop.  Note that the pedigree after the loop exists is the same
    //    as if a single spawn and sync were executed within this function.

    // TBD: Since the shrink-wrap optimization was turned on in the compiler,
    // it is not possible to get the current stack frame without actually
    // forcing a call to bind-thread.  This spurious spawn is a temporary
    // stopgap until the correct intrinsics are added to give us total control
    // over frame initialization.
    _Cilk_spawn noop();

    // Fetch the current worker.  From that we can get the current stack frame
    // which will be constant even if we're stolen
    __cilkrts_worker *w = __cilkrts_get_tls_worker();
    __cilkrts_stack_frame *sf = w->current_stack_frame;

    // Decrement the rank by one to undo the pedigree change from the
    // _Cilk_spawn
    --w->pedigree.rank;

    // Save the current worker pedigree into loop_root_pedigree, which will be
    // the root node for our flattened pedigree.
    __cilkrts_pedigree loop_root_pedigree = w->pedigree;

    // Don't splice the loop_root node in yet.  It will be done when we
    // call the loop body lambda function
//    w->pedigree.rank = 0;
//    w->pedigree.next = &loop_root_pedigree;

    /* Spawn is necessary at top-level to force runtime to start up.
     * Runtime must be started in order to call the grainsize() function.
     */
    int gs = grainsize(grain, count);
    cilk_for_recursive((count_t) 0, count, body, data, gs, w,
                       &loop_root_pedigree);

    // Need to refetch the worker after calling a spawning function.
    w = sf->worker;

    // Restore the pedigree in the worker.
    w->pedigree = loop_root_pedigree;

    // Bump the worker pedigree.
    ++w->pedigree.rank;

    // Implicit sync will increment the pedigree leaf rank again, for a total
    // of two increments.  If the noop spawn above is removed, then we'll need
    // to re-enable the following code:
//     // If this is an optimized build, then the compiler will have optimized
//     // out the increment of the worker's pedigree in the implied sync.  We
//     // need to add one to make the pedigree_loop test work correctly.
// #if CILKRTS_OPTIMIZED
//     ++sf->worker->pedigree.rank;
// #endif
}

// Use extern "C" to suppress name mangling of __cilkrts_cilk_for_32 and
// __cilkrts_cilk_for_64.
extern "C" {

/*
 * __cilkrts_cilk_for_32
 *
 * Implementation of cilk_for for 32-bit trip counts (regardless of processor
 * word size).  Assumes that the range is 0 - count.
 *
 * body  - lambda function for the cilk_for loop body
 * data  - data used by the lambda function
 * count - trip count for loop
 * grain - grain size (0 if it should be computed)
 */

CILK_ABI_THROWS_VOID __cilkrts_cilk_for_32(__cilk_abi_f32_t body, void *data,
                                            cilk32_t count, int grain)
{
    // Cilkscreen should not report this call in a stack trace
    NOTIFY_ZC_INTRINSIC((char *)"cilkscreen_hide_call", 0);

    // Check for an empty range here as an optimization - don't need to do any
    // __cilkrts_stack_frame initialization
    if (count > 0)
        cilk_for_root(body, data, count, grain);
}

/*
 * __cilkrts_cilk_for_64
 *
 * Implementation of cilk_for for 64-bit trip counts (regardless of processor
 * word size).  Assumes that the range is 0 - count.
 *
 * body  - lambda function for the cilk_for loop body
 * data  - data used by the lambda function
 * count - trip count for loop
 * grain - grain size (0 if it should be computed)
 */
CILK_ABI_THROWS_VOID __cilkrts_cilk_for_64(__cilk_abi_f64_t body, void *data,
                                            cilk64_t count, int grain)
{
    // Cilkscreen should not report this call in a stack trace
    NOTIFY_ZC_INTRINSIC((char *)"cilkscreen_hide_call", 0);

    // Check for an empty range here as an optimization - don't need to do any
    // __cilkrts_stack_frame initialization
    if (count > 0)
        cilk_for_root(body, data, count, grain);
}

} // end extern "C"

/* End cilk-abi-cilk-for.cpp */