[google] Use minimum cost circulation, not minimum cost flow to smooth profiles other minor fixes. (issue4536106)

Martin Thuresson martint@google.com
Thu Jun 2 18:28:00 GMT 2011


On Thu, Jun 2, 2011 at 11:17 AM, Xinliang David Li <davidxl@google.com> wrote:
> Counter overflow?

Infinite capacity should not be modified. If it is, there is a chance
for overflow as it is stored as
#define CAP_INFINITY INTTYPE_MAXIMUM (HOST_WIDEST_INT)

Martin

>
> David
>
> On Thu, Jun 2, 2011 at 11:12 AM, Martin Thuresson <martint@google.com> wrote:
>> On Thu, Jun 2, 2011 at 11:05 AM, Xinliang David Li <davidxl@google.com> wrote:
>>>
>>> Smoothing works for sample FDO and profile data from multi-threaded
>>> programs. You won't see any difference in SPEC.
>>
>> Dehao reported some performance improvements from the algorithmic improvements
>> he added in terms of extra fixup edges and handling of infinite capacity.
>>
>> Martin
>>
>>
>>
>>
>>>
>>> David
>>>
>>> On Thu, Jun 2, 2011 at 11:00 AM, Martin Thuresson <martint@google.com> wrote:
>>> > This patch from Neil Vachharajani and Dehao Chen improves mcf by using
>>> > minimum cost circulation instead of minimum cost flow to smooth profiles.
>>> > It also introduces a parameter for controlling running time of the algorithm.
>>> > This was what was originally presented in the academic work and handles
>>> > certain cases where the function entry and exit have incorrect profile
>>> > weights.
>>> >
>>> > For now, this is for google/main. Once I have collected performance results
>>> > from SPEC I will propose this patch for trunk as well.
>>> >
>>> > Bootstraps and no test regressions. Ok for google/main?
>>> >
>>> > 2011-06-02  Neil Vachharajani  <nvachhar@gmail.com>, Dehao Chen
>>> >        <danielcdh@gmail.com>
>>> >
>>> >        * gcc/doc/invoke.texi (min-mcf-cancel-iters): Document.
>>> >        * gcc/mcf.c (MAX_ITER): Use new param PARAM_MIN_MCF_CANCEL_ITERS.
>>> >        (edge_type): Add SINK_SOURCE_EDGE.
>>> >        (dump_fixup_edge): Handle SINK_SOURCE_EDGE.
>>> >        (create_fixup_graph): Make problem miminum cost circulation.
>>> >        (cancel_negative_cycle): Update handling of infinite capacity.
>>> >        (compute_residual_flow): Update handling of infinite capacity.
>>> >        (find_max_flow): Update handling of infinite capacity.
>>> >        (modify_sink_source_capacity): New function.
>>> >        (find_minimum_cost_flow): Make problem miminum cost circulation.
>>> >        Use param PARAM_MIN_MCF_CANCEL_ITERS.
>>> >        * gcc/params.def (PARAM_MIN_MCF_CANCEL_ITERS): Define.
>>> >
>>> > Index: gcc/doc/invoke.texi
>>> > ===================================================================
>>> > --- gcc/doc/invoke.texi (revision 174456)
>>> > +++ gcc/doc/invoke.texi (working copy)
>>> > @@ -8341,6 +8341,12 @@ whether the result of a complex multipli
>>> >
>>> >  The default is @option{-fno-cx-fortran-rules}.
>>> >
>>> > +@item min-mcf-cancel-iters
>>> > +The minimum number of iterations of negative cycle cancellation during
>>> > +MCF profile correction before early termination.  This parameter is
>>> > +only useful when using @option{-fprofile-correction}.
>>> > +
>>> > +
>>> >  @end table
>>> >
>>> >  The following options control optimizations that may improve
>>> > Index: gcc/mcf.c
>>> > ===================================================================
>>> > --- gcc/mcf.c   (revision 174456)
>>> > +++ gcc/mcf.c   (working copy)
>>> > @@ -52,6 +52,8 @@ along with GCC; see the file COPYING3.
>>> >  #include "langhooks.h"
>>> >  #include "tree.h"
>>> >  #include "gcov-io.h"
>>> > +#include "params.h"
>>> > +#include "diagnostic-core.h"
>>> >
>>> >  #include "profile.h"
>>> >
>>> > @@ -64,15 +66,18 @@ along with GCC; see the file COPYING3.
>>> >  #define COST(k, w)      ((k) / mcf_ln ((w) + 2))
>>> >  /* Limit the number of iterations for cancel_negative_cycles() to ensure
>>> >    reasonable compile time.  */
>>> > -#define MAX_ITER(n, e)  10 + (1000000 / ((n) * (e)))
>>> > +#define MAX_ITER(n, e)  (PARAM_VALUE (PARAM_MIN_MCF_CANCEL_ITERS) + \
>>> > +                        (1000000 / ((n) * (e))))
>>> > +
>>> >  typedef enum
>>> >  {
>>> > -  INVALID_EDGE,
>>> > +  INVALID_EDGE = 0,
>>> >   VERTEX_SPLIT_EDGE,       /* Edge to represent vertex with w(e) = w(v).  */
>>> >   REDIRECT_EDGE,           /* Edge after vertex transformation.  */
>>> >   REVERSE_EDGE,
>>> >   SOURCE_CONNECT_EDGE,     /* Single edge connecting to single source.  */
>>> >   SINK_CONNECT_EDGE,       /* Single edge connecting to single sink.  */
>>> > +  SINK_SOURCE_EDGE,        /* Single edge connecting sink to source.  */
>>> >   BALANCE_EDGE,                    /* Edge connecting with source/sink: cp(e) = 0.  */
>>> >   REDIRECT_NORMALIZED_EDGE, /* Normalized edge for a redirect edge.  */
>>> >   REVERSE_NORMALIZED_EDGE   /* Normalized edge for a reverse edge.  */
>>> > @@ -250,6 +255,10 @@ dump_fixup_edge (FILE *file, fixup_graph
>>> >          fputs (" @SINK_CONNECT_EDGE", file);
>>> >          break;
>>> >
>>> > +       case SINK_SOURCE_EDGE:
>>> > +         fputs (" @SINK_SOURCE_EDGE", file);
>>> > +         break;
>>> > +
>>> >        case REVERSE_EDGE:
>>> >          fputs (" @REVERSE_EDGE", file);
>>> >          break;
>>> > @@ -465,7 +474,7 @@ create_fixup_graph (fixup_graph_type *fi
>>> >   double k_neg = 0;
>>> >   /* Vector to hold D(v) = sum_out_edges(v) - sum_in_edges(v).  */
>>> >   gcov_type *diff_out_in = NULL;
>>> > -  gcov_type supply_value = 1, demand_value = 0;
>>> > +  gcov_type supply_value = 0, demand_value = 0;
>>> >   gcov_type fcost = 0;
>>> >   int new_entry_index = 0, new_exit_index = 0;
>>> >   int i = 0, j = 0;
>>> > @@ -486,14 +495,15 @@ create_fixup_graph (fixup_graph_type *fi
>>> >     fnum_vertices_after_transform + n_edges + n_basic_blocks + 2;
>>> >
>>> >   /* In create_fixup_graph: Each basic block and edge can be split into 3
>>> > -     edges. Number of balance edges = n_basic_blocks. So after
>>> > -     create_fixup_graph:
>>> > -     max_edges = 4 * n_basic_blocks + 3 * n_edges
>>> > +     edges. Number of balance edges = n_basic_blocks - 1. And there is 1 edge
>>> > +     connecting new_entry and new_exit, and 2 edges connecting new_entry to
>>> > +     entry, and exit to new_exit. So after create_fixup_graph:
>>> > +     max_edges = 4 * n_basic_blocks + 3 * n_edges + 2
>>> >      Accounting for residual flow edges
>>> > -     max_edges = 2 * (4 * n_basic_blocks + 3 * n_edges)
>>> > -     = 8 * n_basic_blocks + 6 * n_edges
>>> > -     < 8 * n_basic_blocks + 8 * n_edges.  */
>>> > -  int fmax_num_edges = 8 * (n_basic_blocks + n_edges);
>>> > +     max_edges = 2 * (4 * n_basic_blocks + 3 * n_edges + 2)
>>> > +     = 8 * n_basic_blocks + 6 * n_edges + 4
>>> > +     < 8 * n_basic_blocks + 8 * n_edges + 8.  */
>>> > +  int fmax_num_edges = 8 * (n_basic_blocks + n_edges + 1);
>>> >
>>> >   /* Initial num of vertices in the fixup graph.  */
>>> >   fixup_graph->num_vertices = n_basic_blocks;
>>> > @@ -512,7 +522,7 @@ create_fixup_graph (fixup_graph_type *fi
>>> >
>>> >   /* Compute constants b, k_pos, k_neg used in the cost function calculation.
>>> >      b = sqrt(avg_vertex_weight(cfg)); k_pos = b; k_neg = 50b.  */
>>> > -  FOR_BB_BETWEEN (bb, ENTRY_BLOCK_PTR, NULL, next_bb)
>>> > +  FOR_ALL_BB (bb)
>>> >     total_vertex_weight += bb->count;
>>> >
>>> >   sqrt_avg_vertex_weight = mcf_sqrt (total_vertex_weight / n_basic_blocks);
>>> > @@ -526,10 +536,11 @@ create_fixup_graph (fixup_graph_type *fi
>>> >   if (dump_file)
>>> >     fprintf (dump_file, "\nVertex transformation:\n");
>>> >
>>> > -  FOR_BB_BETWEEN (bb, ENTRY_BLOCK_PTR, NULL, next_bb)
>>> > +  FOR_ALL_BB (bb)
>>> >   {
>>> >     /* v'->v'': index1->(index1+1).  */
>>> >     i = 2 * bb->index;
>>> > +
>>> >     fcost = (gcov_type) COST (k_pos, bb->count);
>>> >     add_fixup_edge (fixup_graph, i, i + 1, VERTEX_SPLIT_EDGE, bb->count,
>>> >                     fcost, CAP_INFINITY);
>>> > @@ -593,23 +604,45 @@ create_fixup_graph (fixup_graph_type *fi
>>> >
>>> >   new_entry_index = fixup_graph->new_entry_index = fixup_graph->num_vertices;
>>> >   fixup_graph->num_vertices++;
>>> > -  /* Set supply_value to 1 to avoid zero count function ENTRY.  */
>>> > -  add_fixup_edge (fixup_graph, new_entry_index, ENTRY_BLOCK, SOURCE_CONNECT_EDGE,
>>> > -                 1 /* supply_value */, 0, 1 /* supply_value */);
>>> > +  /* Set capacity to 0 initially, it will be updated after
>>> > +     supply_value is computed. */
>>> > +  add_fixup_edge (fixup_graph, new_entry_index, ENTRY_BLOCK,
>>> > +                 SOURCE_CONNECT_EDGE, 0 /* supply_value */, 0,
>>> > +                 0 /* supply_value */);
>>> > +  add_fixup_edge (fixup_graph, ENTRY_BLOCK, new_entry_index,
>>> > +                  SOURCE_CONNECT_EDGE, 0 /* supply_value */, 0,
>>> > +                  0 /* supply_value */);
>>> >
>>> > -  /* Create new exit with EXIT_BLOCK as single pred.  */
>>> > +
>>> > +  /* Set capacity to 0 initially, it will be updated after
>>> > +     demand_value is computed. */
>>> >   new_exit_index = fixup_graph->new_exit_index = fixup_graph->num_vertices;
>>> >   fixup_graph->num_vertices++;
>>> >   add_fixup_edge (fixup_graph, 2 * EXIT_BLOCK + 1, new_exit_index,
>>> >                   SINK_CONNECT_EDGE,
>>> >                   0 /* demand_value */, 0, 0 /* demand_value */);
>>> > +  add_fixup_edge (fixup_graph, new_exit_index, 2 * EXIT_BLOCK + 1,
>>> > +                  SINK_CONNECT_EDGE,
>>> > +                  0 /* demand_value */, 0, 0 /* demand_value */);
>>> > +
>>> > +
>>> > +  /* Create a back edge from the new_exit to the new_entry.
>>> > +     Initially, its capacity will be set to 0 so that it does not
>>> > +     affect max flow, but later its capacity will be changed to
>>> > +     infinity to cancel negative cycles. */
>>> > +  add_fixup_edge (fixup_graph, new_exit_index, new_entry_index,
>>> > +                 SINK_SOURCE_EDGE, 0, 0, 0);
>>> > +
>>> > +
>>> >
>>> >   /* Connect vertices with unbalanced D(v) to source/sink.  */
>>> >   if (dump_file)
>>> >     fprintf (dump_file, "\nD(v) balance:\n");
>>> > -  /* Skip vertices for ENTRY (0, 1) and EXIT (2,3) blocks, so start with i = 4.
>>> > -     diff_out_in[v''] will be 0, so skip v'' vertices, hence i += 2.  */
>>> > -  for (i = 4; i < new_entry_index; i += 2)
>>> > +
>>> > +  /* Skip vertices for ENTRY (0, 1) and EXIT (2,3) blocks, so start
>>> > +     with i = 4.  diff_out_in[v''] should be 0, but may not be due to
>>> > +     rounding error.  So here we consider all vertices.  */
>>> > +  for (i = 4; i < new_entry_index; i += 1)
>>> >     {
>>> >       if (diff_out_in[i] > 0)
>>> >        {
>>> > @@ -674,7 +707,6 @@ create_fixup_graph (fixup_graph_type *fi
>>> >              fprintf (dump_file, "------------------\n");
>>> >            }
>>> >
>>> > -         pfedge->cost /= 2;
>>> >          pfedge->norm_vertex_index = new_index;
>>> >          if (dump_file)
>>> >            {
>>> > @@ -684,7 +716,7 @@ create_fixup_graph (fixup_graph_type *fi
>>> >
>>> >          /* Add a new fixup edge: new_index->src.  */
>>> >          add_fixup_edge (fixup_graph, new_index, pfedge->src,
>>> > -                         REVERSE_NORMALIZED_EDGE, 0, r_pfedge->cost,
>>> > +                         REVERSE_NORMALIZED_EDGE, 0, 0,
>>> >                          r_pfedge->max_capacity);
>>> >          gcc_assert (fixup_graph->num_vertices <= fmax_num_vertices);
>>> >
>>> > @@ -692,7 +724,6 @@ create_fixup_graph (fixup_graph_type *fi
>>> >              ==> r_pfedge->src -> new_index.  */
>>> >          r_pfedge->dest = new_index;
>>> >          r_pfedge->type = REVERSE_NORMALIZED_EDGE;
>>> > -         r_pfedge->cost = pfedge->cost;
>>> >          r_pfedge->max_capacity = pfedge->max_capacity;
>>> >          if (dump_file)
>>> >            dump_fixup_edge (dump_file, fixup_graph, r_pfedge);
>>> > @@ -794,14 +825,12 @@ cancel_negative_cycle (fixup_graph_type
>>> >   bool found_cycle = false;
>>> >   int cycle_start = 0, cycle_end = 0;
>>> >   gcov_type sum_cost = 0, cycle_flow = 0;
>>> > -  int new_entry_index;
>>> >   bool propagated = false;
>>> >
>>> >   gcc_assert (fixup_graph);
>>> >   fnum_vertices = fixup_graph->num_vertices;
>>> >   fnum_edges = fixup_graph->num_edges;
>>> >   fedge_list = fixup_graph->edge_list;
>>> > -  new_entry_index = fixup_graph->new_entry_index;
>>> >
>>> >   /* Initialize.  */
>>> >   /* Skip ENTRY.  */
>>> > @@ -820,8 +849,6 @@ cancel_negative_cycle (fixup_graph_type
>>> >     for (i = 0; i < fnum_edges; i++)
>>> >       {
>>> >        pfedge = fedge_list + i;
>>> > -       if (pfedge->src == new_entry_index)
>>> > -         continue;
>>> >        if (pfedge->is_rflow_valid && pfedge->rflow
>>> >             && d[pfedge->src] != CAP_INFINITY
>>> >            && (d[pfedge->dest] > d[pfedge->src] + pfedge->cost))
>>> > @@ -843,8 +870,6 @@ cancel_negative_cycle (fixup_graph_type
>>> >   for (i = 0; i < fnum_edges; i++)
>>> >     {
>>> >       pfedge = fedge_list + i;
>>> > -      if (pfedge->src == new_entry_index)
>>> > -       continue;
>>> >       if (pfedge->is_rflow_valid && pfedge->rflow
>>> >           && d[pfedge->src] != CAP_INFINITY
>>> >          && (d[pfedge->dest] > d[pfedge->src] + pfedge->cost))
>>> > @@ -912,10 +937,12 @@ cancel_negative_cycle (fixup_graph_type
>>> >     {
>>> >       pfedge = find_fixup_edge (fixup_graph, cycle[k + 1], cycle[k]);
>>> >       r_pfedge = find_fixup_edge (fixup_graph, cycle[k], cycle[k + 1]);
>>> > -      pfedge->rflow -= cycle_flow;
>>> > +      if (pfedge->rflow != CAP_INFINITY)
>>> > +        pfedge->rflow -= cycle_flow;
>>> >       if (pfedge->type)
>>> >        pfedge->flow += cycle_flow;
>>> > -      r_pfedge->rflow += cycle_flow;
>>> > +      if (r_pfedge->rflow != CAP_INFINITY)
>>> > +        r_pfedge->rflow += cycle_flow;
>>> >       if (r_pfedge->type)
>>> >        r_pfedge->flow -= cycle_flow;
>>> >     }
>>> > @@ -945,7 +972,8 @@ compute_residual_flow (fixup_graph_type
>>> >   for (i = 0; i < fnum_edges; i++)
>>> >     {
>>> >       pfedge = fedge_list + i;
>>> > -      pfedge->rflow = pfedge->max_capacity - pfedge->flow;
>>> > +      pfedge->rflow = pfedge->max_capacity == CAP_INFINITY ?
>>> > +                      CAP_INFINITY : pfedge->max_capacity - pfedge->flow;
>>> >       pfedge->is_rflow_valid = true;
>>> >       add_rfixup_edge (fixup_graph, pfedge->dest, pfedge->src, pfedge->flow,
>>> >                       -pfedge->cost);
>>> > @@ -1070,20 +1098,22 @@ find_max_flow (fixup_graph_type *fixup_g
>>> >        {
>>> >          pfedge = find_fixup_edge (fixup_graph, bb_pred[u], u);
>>> >          r_pfedge = find_fixup_edge (fixup_graph, u, bb_pred[u]);
>>> > +
>>> > +          if (pfedge->rflow != CAP_INFINITY)
>>> > +           pfedge->rflow -= increment;
>>> > +          if (r_pfedge->rflow != CAP_INFINITY)
>>> > +           r_pfedge->rflow += increment;
>>> > +
>>> >          if (pfedge->type)
>>> >            {
>>> >              /* forward edge.  */
>>> >              pfedge->flow += increment;
>>> > -             pfedge->rflow -= increment;
>>> > -             r_pfedge->rflow += increment;
>>> >            }
>>> >          else
>>> >            {
>>> >              /* backward edge.  */
>>> >              gcc_assert (r_pfedge->type);
>>> > -             r_pfedge->rflow += increment;
>>> >              r_pfedge->flow -= increment;
>>> > -             pfedge->rflow -= increment;
>>> >            }
>>> >        }
>>> >
>>> > @@ -1302,6 +1332,60 @@ adjust_cfg_counts (fixup_graph_type *fix
>>> >  }
>>> >
>>> >
>>> > +/* Called before negative_cycle_cancellation, to form a cycle between
>>> > + * new_exit to new_entry in FIXUP_GRAPH with capacity MAX_FLOW. We
>>> > + * don't want the flow in the BALANCE_EDGE to be modified, so we set
>>> > + * the residural flow of those edges to 0 */
>>> > +
>>> > +static void
>>> > +modify_sink_source_capacity (fixup_graph_type *fixup_graph, gcov_type max_flow)
>>> > +{
>>> > +  fixup_edge_p edge, r_edge;
>>> > +  int i;
>>> > +  int entry = ENTRY_BLOCK;
>>> > +  int exit = 2 * EXIT_BLOCK + 1;
>>> > +  int new_entry = fixup_graph->new_entry_index;
>>> > +  int new_exit = fixup_graph->new_exit_index;
>>> > +
>>> > +  edge = find_fixup_edge (fixup_graph, new_entry, entry);
>>> > +  edge->max_capacity = CAP_INFINITY;
>>> > +  edge->rflow = CAP_INFINITY;
>>> > +
>>> > +  edge = find_fixup_edge (fixup_graph, entry, new_entry);
>>> > +  edge->max_capacity = CAP_INFINITY;
>>> > +  edge->rflow = CAP_INFINITY;
>>> > +
>>> > +  edge = find_fixup_edge (fixup_graph, exit, new_exit);
>>> > +  edge->max_capacity = CAP_INFINITY;
>>> > +  edge->rflow = CAP_INFINITY;
>>> > +
>>> > +  edge = find_fixup_edge (fixup_graph, new_exit, exit);
>>> > +  edge->max_capacity = CAP_INFINITY;
>>> > +  edge->rflow = CAP_INFINITY;
>>> > +
>>> > +  edge = find_fixup_edge (fixup_graph, new_exit, new_entry);
>>> > +  edge->max_capacity = CAP_INFINITY;
>>> > +  edge->flow = max_flow;
>>> > +  edge->rflow = CAP_INFINITY;
>>> > +
>>> > +  r_edge = find_fixup_edge (fixup_graph, new_entry, new_exit);
>>> > +  r_edge->rflow = max_flow;
>>> > +
>>> > +  /* Find all the backwards residual edges corresponding to
>>> > +     BALANCE_EDGEs and set their residual flow to 0 to enforce a
>>> > +     minimum flow constraint on these edges. */
>>> > +  for (i = 4; i < new_entry; i += 1)
>>> > +    {
>>> > +      edge = find_fixup_edge (fixup_graph, i, new_entry);
>>> > +      if (edge)
>>> > +       edge->rflow = 0;
>>> > +      edge = find_fixup_edge (fixup_graph, new_exit, i);
>>> > +      if (edge)
>>> > +       edge->rflow = 0;
>>> > +    }
>>> > +}
>>> > +
>>> > +
>>> >  /* Implements the negative cycle canceling algorithm to compute a minimum cost
>>> >    flow.
>>> >  Algorithm:
>>> > @@ -1330,13 +1414,18 @@ find_minimum_cost_flow (fixup_graph_type
>>> >   int fnum_vertices;
>>> >   int new_exit_index;
>>> >   int new_entry_index;
>>> > +  gcov_type max_flow;
>>> >
>>> >   gcc_assert (fixup_graph);
>>> >   fnum_vertices = fixup_graph->num_vertices;
>>> >   new_exit_index = fixup_graph->new_exit_index;
>>> >   new_entry_index = fixup_graph->new_entry_index;
>>> >
>>> > -  find_max_flow (fixup_graph, new_entry_index, new_exit_index);
>>> > +  max_flow = find_max_flow (fixup_graph, new_entry_index, new_exit_index);
>>> > +
>>> > +  /* Adjust the fixup graph to translate things into a minimum cost
>>> > +     circulation problem. */
>>> > +  modify_sink_source_capacity (fixup_graph, max_flow);
>>> >
>>> >   /* Initialize the structures for find_negative_cycle().  */
>>> >   pred = (int *) xcalloc (fnum_vertices, sizeof (int));
>>> > @@ -1352,7 +1441,12 @@ find_minimum_cost_flow (fixup_graph_type
>>> >       iteration++;
>>> >       if (iteration > MAX_ITER (fixup_graph->num_vertices,
>>> >                                 fixup_graph->num_edges))
>>> > -        break;
>>> > +       {
>>> > +         inform (DECL_SOURCE_LOCATION (current_function_decl),
>>> > +                 "Exiting profile correction early to avoid excessive "
>>> > +                 "compile time");
>>> > +         break;
>>> > +       }
>>> >     }
>>> >
>>> >   if (dump_file)
>>> > Index: gcc/params.def
>>> > ===================================================================
>>> > --- gcc/params.def      (revision 174456)
>>> > +++ gcc/params.def      (working copy)
>>> > @@ -977,6 +977,12 @@ DEFPARAM (MIN_PARTITION_SIZE,
>>> >          "Size of minimal paritition for WHOPR (in estimated instructions)",
>>> >          1000, 0, 0)
>>> >
>>> > +DEFPARAM (PARAM_MIN_MCF_CANCEL_ITERS,
>>> > +         "min-mcf-cancel-iters",
>>> > +         "the minimum number of iterations of negative cycle cancellation "
>>> > +         "in MCF",
>>> > +         10, 1, 0)
>>> > +
>>> >  /* Diagnostic parameters.  */
>>> >
>>> >  DEFPARAM (CXX_MAX_NAMESPACES_FOR_DIAGNOSTIC_HELP,
>>> >
>>> > --
>>> > This patch is available for review at http://codereview.appspot.com/4536106
>>> >
>>
>



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