[PATCH] Loop split upon semi-invariant condition (PR tree-optimization/89134)

Kyrill Tkachov kyrylo.tkachov@foss.arm.com
Wed Mar 13 09:43:00 GMT 2019 

Hi Feng,

On 3/13/19 1:56 AM, Feng Xue OS wrote:
> Richard,
>
>     Thanks for your comment. Yes, it is like kind of jump threading 
> with knowledge of loop structure. And what is rough time for GCC 10?
>
>

GCC 10 will be released once the number of P1 regressions gets down to 
zero. Past experience shows that it's around the April/May timeframe.

In the meantime my comment on the patch is that you should add some 
tests to the testsuite that showcase this transformation.

Thanks,

Kyrill


> Regards,
>
> Feng
>
>
> ________________________________
> From: Richard Biener <richard.guenther@gmail.com>
> Sent: Tuesday, March 12, 2019 4:31:49 PM
> To: Feng Xue OS
> Cc: gcc-patches@gcc.gnu.org
> Subject: Re: [PATCH] Loop split upon semi-invariant condition (PR 
> tree-optimization/89134)
>
> On Tue, Mar 12, 2019 at 7:20 AM Feng Xue OS 
> <fxue@os.amperecomputing.com> wrote:
> >
> > This patch is composed to implement a loop transformation on one of 
> its conditional statements, which we call it semi-invariant, in that 
> its computation is impacted in only one of its branches.
> >
> > Suppose a loop as:
> >
> >     void f (std::map<int, int> m)
> >     {
> >         for (auto it = m.begin (); it != m.end (); ++it) {
> >             /* if (b) is semi-invariant. */
> >             if (b) {
> >                 b = do_something();    /* Has effect on b */
> >             } else {
> > /* No effect on b */
> >             }
> >             statements;                      /* Also no effect on b */
> >         }
> >     }
> >
> > A transformation, kind of loop split, could be:
> >
> >     void f (std::map<int, int> m)
> >     {
> >         for (auto it = m.begin (); it != m.end (); ++it) {
> >             if (b) {
> >                 b = do_something();
> >             } else {
> >                 ++it;
> >                 statements;
> >                 break;
> >             }
> >             statements;
> >         }
> >
> >         for (; it != m.end (); ++it) {
> >             statements;
> >         }
> >     }
> >
> > If "statements" contains nothing, the second loop becomes an empty 
> one, which can be removed. (This part will be given in another patch). 
> And if "statements" are straight line instructions, we get an 
> opportunity to vectorize the second loop. In practice, this 
> optimization is found to improve some real application by %7.
> >
> > Since it is just a kind of loop split, the codes are mainly placed 
> in existing tree-ssa-loop-split module, and is controlled by 
> -fsplit-loop, and is enabled with -O3.
>
> Note the transform itself is jump-threading with the threading
> duplicating a whole CFG cycle.
>
> I didn't look at the patch details yet since this is suitable for GCC 
> 10 only.
>
> Thanks for implementing this.
> Richard.
>
> > Feng
> >
> >
> > diff --git a/gcc/ChangeLog b/gcc/ChangeLog
> > index 64bf6017d16..a6c2878d652 100644
> > --- a/gcc/ChangeLog
> > +++ b/gcc/ChangeLog
> > @@ -1,3 +1,23 @@
> > +2019-03-12  Feng Xue <fxue@os.amperecomputing.com>
> > +
> > +       PR tree-optimization/89134
> > +        * doc/invoke.texi (max-cond-loop-split-insns): Document new 
> --params.
> > +       (min-cond-loop-split-prob): Likewise.
> > +       * params.def: Add max-cond-loop-split-insns, 
> min-cond-loop-split-prob.
> > +       * passes.def (pass_cond_loop_split) : New pass.
> > +       * timevar.def (TV_COND_LOOP_SPLIT): New time variable.
> > +       * tree-pass.h (make_pass_cond_loop_split): New declaration.
> > +       * tree-ssa-loop-split.c (split_info): New class.
> > +       (find_vdef_in_loop, vuse_semi_invariant_p): New functions.
> > +       (ssa_semi_invariant_p, stmt_semi_invariant_p): Likewise.
> > +       (can_branch_be_excluded, get_cond_invariant_branch): Likewise.
> > +       (is_cond_in_hidden_loop, compute_added_num_insns): Likewise.
> > +       (can_split_loop_on_cond, mark_cond_to_split_loop): Likewise.
> > +       (split_loop_for_cond, tree_ssa_split_loops_for_cond): Likewise.
> > +       (pass_data_cond_loop_split): New variable.
> > +       (pass_cond_loop_split): New class.
> > +       (make_pass_cond_loop_split): New function.
> > +
> >  2019-03-11  Jakub Jelinek  <jakub@redhat.com>
> >
> >         PR middle-end/89655
> > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> > index df0883f2fc9..f5e09bd71fd 100644
> > --- a/gcc/doc/invoke.texi
> > +++ b/gcc/doc/invoke.texi
> > @@ -11316,6 +11316,14 @@ The maximum number of branches unswitched 
> in a single loop.
> >  @item lim-expensive
> >  The minimum cost of an expensive expression in the loop invariant 
> motion.
> >
> > +@item max-cond-loop-split-insns
> > +The maximum number of insns to be increased due to loop split on
> > +semi-invariant condition statement.
> > +
> > +@item min-cond-loop-split-prob
> > +The minimum threshold for probability of semi-invaraint condition
> > +statement to trigger loop split.
> > +
> >  @item iv-consider-all-candidates-bound
> >  Bound on number of candidates for induction variables, below which
> >  all candidates are considered for each use in induction variable
> > diff --git a/gcc/params.def b/gcc/params.def
> > index 3f1576448be..2e067526958 100644
> > --- a/gcc/params.def
> > +++ b/gcc/params.def
> > @@ -386,6 +386,18 @@ DEFPARAM(PARAM_MAX_UNSWITCH_LEVEL,
> >         "The maximum number of unswitchings in a single loop.",
> >         3, 0, 0)
> >
> > +/* The maximum number of increased insns due to loop split on 
> semi-invariant
> > +   condition statement.  */
> > +DEFPARAM(PARAM_MAX_COND_LOOP_SPLIT_INSNS,
> > +       "max-cond-loop-split-insns",
> > +       "The maximum number of insns to be increased due to loop 
> split on semi-invariant condition statement.",
> > +       100, 0, 0)
> > +
> > +DEFPARAM(PARAM_MIN_COND_LOOP_SPLIT_PROB,
> > +       "min-cond-loop-split-prob",
> > +       "The minimum threshold for probability of semi-invaraint 
> condition statement to trigger loop split.",
> > +       30, 0, 100)
> > +
> >  /* The maximum number of insns in loop header duplicated by the 
> copy loop
> >     headers pass.  */
> >  DEFPARAM(PARAM_MAX_LOOP_HEADER_INSNS,
> > diff --git a/gcc/passes.def b/gcc/passes.def
> > index 446a7c48276..bde7f4c50c0 100644
> > --- a/gcc/passes.def
> > +++ b/gcc/passes.def
> > @@ -265,6 +265,7 @@ along with GCC; see the file COPYING3.  If not see
> >           NEXT_PASS (pass_tree_unswitch);
> >           NEXT_PASS (pass_scev_cprop);
> >           NEXT_PASS (pass_loop_split);
> > +         NEXT_PASS (pass_cond_loop_split);
> >           NEXT_PASS (pass_loop_versioning);
> >           NEXT_PASS (pass_loop_jam);
> >           /* All unswitching, final value replacement and splitting 
> can expose
> > diff --git a/gcc/timevar.def b/gcc/timevar.def
> > index 54154464a58..39f2df0e3ec 100644
> > --- a/gcc/timevar.def
> > +++ b/gcc/timevar.def
> > @@ -189,6 +189,7 @@ DEFTIMEVAR (TV_TREE_LOOP_IVCANON     , "tree 
> canonical iv")
> >  DEFTIMEVAR (TV_SCEV_CONST            , "scev constant prop")
> >  DEFTIMEVAR (TV_TREE_LOOP_UNSWITCH    , "tree loop unswitching")
> >  DEFTIMEVAR (TV_LOOP_SPLIT            , "loop splitting")
> > +DEFTIMEVAR (TV_COND_LOOP_SPLIT       , "loop splitting for conditions")
> >  DEFTIMEVAR (TV_LOOP_JAM              , "unroll and jam")
> >  DEFTIMEVAR (TV_COMPLETE_UNROLL       , "complete unrolling")
> >  DEFTIMEVAR (TV_TREE_PARALLELIZE_LOOPS, "tree parallelize loops")
> > diff --git a/gcc/tree-pass.h b/gcc/tree-pass.h
> > index 47be59b2a11..f441ba36871 100644
> > --- a/gcc/tree-pass.h
> > +++ b/gcc/tree-pass.h
> > @@ -367,6 +367,7 @@ extern gimple_opt_pass *make_pass_lim 
> (gcc::context *ctxt);
> >  extern gimple_opt_pass *make_pass_linterchange (gcc::context *ctxt);
> >  extern gimple_opt_pass *make_pass_tree_unswitch (gcc::context *ctxt);
> >  extern gimple_opt_pass *make_pass_loop_split (gcc::context *ctxt);
> > +extern gimple_opt_pass *make_pass_cond_loop_split (gcc::context *ctxt);
> >  extern gimple_opt_pass *make_pass_loop_jam (gcc::context *ctxt);
> >  extern gimple_opt_pass *make_pass_predcom (gcc::context *ctxt);
> >  extern gimple_opt_pass *make_pass_iv_canon (gcc::context *ctxt);
> > diff --git a/gcc/tree-ssa-loop-split.c b/gcc/tree-ssa-loop-split.c
> > index 999c9a30366..d287a0d7d4c 100644
> > --- a/gcc/tree-ssa-loop-split.c
> > +++ b/gcc/tree-ssa-loop-split.c
> > @@ -32,7 +32,9 @@ along with GCC; see the file COPYING3.  If not see
> >  #include "tree-ssa-loop.h"
> >  #include "tree-ssa-loop-manip.h"
> >  #include "tree-into-ssa.h"
> > +#include "tree-inline.h"
> >  #include "cfgloop.h"
> > +#include "params.h"
> >  #include "tree-scalar-evolution.h"
> >  #include "gimple-iterator.h"
> >  #include "gimple-pretty-print.h"
> > @@ -40,7 +42,9 @@ along with GCC; see the file COPYING3.  If not see
> >  #include "gimple-fold.h"
> >  #include "gimplify-me.h"
> >
> > -/* This file implements loop splitting, i.e. transformation of 
> loops like
> > +/* This file implements two kind of loop splitting.
> > +
> > +   One transformation of loops like:
> >
> >     for (i = 0; i < 100; i++)
> >       {
> > @@ -670,6 +674,803 @@ tree_ssa_split_loops (void)
> >    return 0;
> >  }
> >
> > +
> > +/* Another transformation of loops like:
> > +
> > +   for (i = INIT (); CHECK (i); i = NEXT ())
> > +     {
> > +       if (expr (a_1, a_2, ..., a_n))
> > +         a_j = ...;  // change at least one a_j
> > +       else
> > +         S;          // not change any a_j
> > +     }
> > +
> > +   into:
> > +
> > +   for (i = INIT (); CHECK (i); i = NEXT ())
> > +     {
> > +       if (expr (a_1, a_2, ..., a_n))
> > +         a_j = ...;
> > +       else
> > +         {
> > +           S;
> > +           i = NEXT ();
> > +           break;
> > +         }
> > +     }
> > +
> > +   for (; CHECK (i); i = NEXT ())
> > +     {
> > +       S;
> > +     }
> > +
> > +   */
> > +
> > +/* Data structure to hold temporary information during loop split upon
> > +   semi-invariant conditional statement. */
> > +class split_info {
> > +public:
> > +  /* Array of all basic blocks in a loop, returned by 
> get_loop_body(). */
> > +  basic_block *bbs;
> > +
> > +  /* All memory store/clobber statements in a loop. */
> > +  auto_vec<gimple *> stores;
> > +
> > +  /* Whether above memory stores vector has been filled. */
> > +  bool set_stores;
> > +
> > +  /* Semi-invariant conditional statement, upon which to split loop. */
> > +  gcond *cond;
> > +
> > +  split_info () : bbs (NULL),  set_stores (false), cond (NULL) { }
> > +
> > +  ~split_info ()
> > +    {
> > +      if (bbs)
> > +        free (bbs);
> > +    }
> > +};
> > +
> > +/* Find all statements with memory-write effect in a loop, 
> including memory
> > +   store and non-pure function call, and keep those in a vector. 
> This work
> > +   is only done for one time, for the vector should be constant during
> > +   analysis stage of semi-invariant condition. */
> > +
> > +static void
> > +find_vdef_in_loop (struct loop *loop)
> > +{
> > +  split_info *info = (split_info *) loop->aux;
> > +  gphi *vphi = get_virtual_phi (loop->header);
> > +
> > +  /* Indicate memory store vector has been filled. */
> > +  info->set_stores = true;
> > +
> > +  /* If loop contains memory operation, there must be a virtual PHI 
> node in
> > +     loop header basic block. */
> > +  if (vphi == NULL)
> > +    return;
> > +
> > +  /* All virtual SSA names inside the loop are connected to be a cyclic
> > +     graph via virtual PHI nodes. The virtual PHI node in loop 
> header just
> > +     links the first and the last virtual SSA names, by using the 
> last as
> > +     PHI operand to define the first. */
> > +  const edge latch = loop_latch_edge (loop);
> > +  const tree first = gimple_phi_result (vphi);
> > +  const tree last = PHI_ARG_DEF_FROM_EDGE (vphi, latch);
> > +
> > +  /* The virtual SSA cyclic graph might consist of only one SSA 
> name, who
> > +     is defined by itself.
> > +
> > +        .MEM_1 = PHI <.MEM_2(loop entry edge), .MEM_1(latch edge)>
> > +
> > +     This means the loop contains only memory loads, so we can skip 
> it. */
> > +  if (first == last)
> > +    return;
> > +
> > +  auto_vec<gimple *> others;
> > +  auto_vec<tree> worklist;
> > +  auto_bitmap visited;
> > +
> > +  bitmap_set_bit (visited, SSA_NAME_VERSION (first));
> > +  bitmap_set_bit (visited, SSA_NAME_VERSION (last));
> > +  worklist.safe_push (last);
> > +
> > +  do
> > +    {
> > +      tree vuse = worklist.pop ();
> > +      gimple *stmt = SSA_NAME_DEF_STMT (vuse);
> > +
> > +      /* We mark the first and last SSA names as visited at the 
> beginning,
> > +         and reversely start the process from the last SSA name 
> toward the
> > +         first, which ensure that this do-while will not touch SSA 
> names
> > +         defined outside of the loop. */
> > +      gcc_assert (gimple_bb (stmt)
> > +                  && flow_bb_inside_loop_p (loop, gimple_bb (stmt)));
> > +
> > +      if (gimple_code (stmt) == GIMPLE_PHI)
> > +        {
> > +          gphi *phi = as_a <gphi *> (stmt);
> > +
> > +          for (unsigned i = 0; i < gimple_phi_num_args (phi); ++i)
> > +            {
> > +              tree arg = gimple_phi_arg_def (stmt, i);
> > +
> > +              if (bitmap_set_bit (visited, SSA_NAME_VERSION (arg)))
> > +                worklist.safe_push (arg);
> > +            }
> > +        }
> > +      else
> > +        {
> > +          tree prev = gimple_vuse (stmt);
> > +
> > +          /* Non-pure call statement is conservatively assumed to 
> impact
> > +             all memory locations. So place call statements ahead 
> of other
> > +             memory stores in the vector with the idea of of using 
> them as
> > +             shortcut terminators to memory alias analysis, kind of
> > +             optimization for compilation. */
> > +          if (gimple_code (stmt) == GIMPLE_CALL)
> > +            info->stores.safe_push (stmt);
> > +          else
> > +            others.safe_push (stmt);
> > +
> > +          if (bitmap_set_bit (visited, SSA_NAME_VERSION (prev)))
> > +            worklist.safe_push (prev);
> > +        }
> > +    } while (!worklist.is_empty ());
> > +
> > +    info->stores.safe_splice (others);
> > +}
> > +
> > +
> > +/* Given a memory load or pure call statement, check whether it is 
> impacted
> > +   by some memory store in the loop excluding those basic blocks 
> dominated
> > +   by SKIP_HEAD (those basic blocks always corresponds to one branch of
> > +   a conditional statement). If SKIP_HEAD is NULL, all basic blocks 
> of the
> > +   loop are checked. */
> > +
> > +static bool
> > +vuse_semi_invariant_p (struct loop *loop, gimple *stmt,
> > +                       const_basic_block skip_head)
> > +{
> > +  split_info *info = (split_info *) loop->aux;
> > +
> > +  /* Collect memory store/clobber statements if have not do that. */
> > +  if (!info->set_stores)
> > +    find_vdef_in_loop (loop);
> > +
> > +  tree rhs = is_gimple_assign (stmt) ? gimple_assign_rhs1 (stmt) : 
> NULL_TREE;
> > +  ao_ref ref;
> > +  gimple *store;
> > +  unsigned i;
> > +
> > +  ao_ref_init (&ref, rhs);
> > +
> > +  FOR_EACH_VEC_ELT (info->stores, i, store)
> > +    {
> > +      /* Skip those basic blocks dominated by SKIP_HEAD. */
> > +      if (skip_head
> > +          && dominated_by_p (CDI_DOMINATORS, gimple_bb (store), 
> skip_head))
> > +        continue;
> > +
> > +      /* For a pure call, it is assumed to be impacted by any 
> memory store.
> > +         For a memory load, use memory alias analysis to check that. */
> > +      if (!ref.ref || stmt_may_clobber_ref_p_1 (store, &ref))
> > +        return false;
> > +    }
> > +
> > +  return true;
> > +}
> > +
> > +/* Forward declaration */
> > +
> > +static bool
> > +stmt_semi_invariant_p (struct loop *loop, gimple *stmt,
> > +                       const_basic_block skip_head);
> > +
> > +/* Suppose one condition branch, led by SKIP_HEAD, is not executed 
> in certain
> > +   iteration, check whether an SSA name remains unchanged in next 
> interation.
> > +   We can call this characterisic as semi-invariantness. SKIP_HEAD 
> might be
> > +   NULL, if so, nothing excluded, all basic blocks and control 
> flows in the
> > +   loop will be considered. */
> > +
> > +static bool
> > +ssa_semi_invariant_p (struct loop *loop, const tree name,
> > +                      const_basic_block skip_head)
> > +{
> > +  gimple *def = SSA_NAME_DEF_STMT (name);
> > +  const_basic_block def_bb = gimple_bb (def);
> > +
> > +  /* An SSA name defined outside a loop is definitely 
> semi-invariant. */
> > +  if (!def_bb || !flow_bb_inside_loop_p (loop, def_bb))
> > +    return true;
> > +
> > +  /* This function is used to check semi-invariantness of a condition
> > +     statement, and SKIP_HEAD is always given as head of one of its
> > +     branches. So it implies that SSA name to check should be defined
> > +     before the conditional statement, and also before SKIP_HEAD. */
> > +
> > +  if (gimple_code (def) == GIMPLE_PHI)
> > +    {
> > +      /* In a normal loop, if a PHI node is located not in loop 
> header, all
> > +         its source operands should be defined inside the loop. As we
> > +         mentioned before, these source definitions are ahead of 
> SKIP_HEAD,
> > +         and will not be bypassed. Therefore, in each iteration, any of
> > +         these sources might be value provider to the SSA name, 
> which for
> > +         sure should not be seen as invariant. */
> > +      if (def_bb != loop->header || !skip_head)
> > +        return false;
> > +
> > +      const_edge latch = loop_latch_edge (loop);
> > +      tree from = PHI_ARG_DEF_FROM_EDGE (as_a <gphi *> (def), latch);
> > +
> > +      /* A PHI node in loop header always contains two source operands,
> > +         one is initial value, the other is the copy of last iteration
> > +         through loop latch, we call it latch value. From this PHI node
> > +         to definition of latch value, if excluding those basic blocks
> > +         dominated by SKIP_HEAD, there is no definition of other 
> version
> > +         of same variable, SSA name defined by the PHI node is
> > +         semi-invariant.
> > +
> > +                         loop entry
> > +                              |     .--- latch ---.
> > +                              |     |             |
> > +                              v     v             |
> > +                  x_1 = PHI <x_0, x_3>           |
> > +                           |                      |
> > +                           v                      |
> > +              .------- if (cond) -------.         |
> > +              |                         |         |
> > +              |                     [ SKIP ]      |
> > +              |                         |         |
> > +              |                     x_2 = ...     |
> > +              |                         |         |
> > +              '---- T ---->.<---- F ----'         |
> > +                           |                      |
> > +                           v                      |
> > +                  x_3 = PHI <x_1, x_2>            |
> > +                           |                      |
> > +                           '----------------------'
> > +
> > +        Suppose in certain iteration, execution flow in above graph 
> goes
> > +        through true branch, which means that one source value to 
> define
> > +        x_3 in false branch (x2) is skipped, x_3 only comes from 
> x_1, and
> > +        x_1 in next iterations is defined by x_3, we know that x_1 will
> > +        never changed if COND always chooses true branch from then 
> on. */
> > +
> > +      while (from != name)
> > +        {
> > +          /* A new value comes from a CONSTANT. */
> > +          if (TREE_CODE (from) != SSA_NAME)
> > +            return false;
> > +
> > +          gimple *stmt = SSA_NAME_DEF_STMT (from);
> > +          const_basic_block bb = gimple_bb (stmt);
> > +
> > +          /* A new value comes from outside of loop. */
> > +          if (!bb || !flow_bb_inside_loop_p (loop, bb))
> > +            return false;
> > +
> > +          from = NULL_TREE;
> > +
> > +          if (gimple_code (stmt) == GIMPLE_PHI)
> > +            {
> > +              gphi *phi = as_a <gphi *> (stmt);
> > +
> > +              for (unsigned i = 0; i < gimple_phi_num_args (phi); ++i)
> > +                {
> > +                  const_edge e = gimple_phi_arg_edge (phi, i);
> > +
> > +                  /* Skip redefinition from basic blocks being 
> excluded. */
> > +                  if (!dominated_by_p (CDI_DOMINATORS, e->src, 
> skip_head))
> > +                    {
> > +                      /* There are more than one source operands 
> that can
> > +                         provide value to the SSA name. */
> > +                      if (from)
> > +                        return false;
> > +
> > +                      from = gimple_phi_arg_def (phi, i);
> > +                    }
> > +                }
> > +            }
> > +          else if (gimple_code (stmt) == GIMPLE_ASSIGN)
> > +            {
> > +              /* For simple value copy, check its rhs instead. */
> > +              if (gimple_assign_ssa_name_copy_p (stmt))
> > +                from = gimple_assign_rhs1 (stmt);
> > +            }
> > +
> > +          /* Any other kind of definition is deemed to introduce a 
> new value
> > +             to the SSA name. */
> > +          if (!from)
> > +            return false;
> > +        }
> > +        return true;
> > +    }
> > +
> > +  /* Value originated from volatile memory load or return of normal 
> (non-
> > +     const/pure) call should not be treated as constant in each 
> iteration. */
> > +  if (gimple_has_side_effects (def))
> > +    return false;
> > +
> > +  /* Check if any memory store may kill memory load at this place. */
> > +  if (gimple_vuse (def) && !vuse_semi_invariant_p (loop, def, 
> skip_head))
> > +    return false;
> > +
> > +  /* Check operands of definition statement of the SSA name. */
> > +  return stmt_semi_invariant_p (loop, def, skip_head);
> > +}
> > +
> > +/* Check whether a statement is semi-invariant, iff all its 
> operands are
> > +   semi-invariant. */
> > +
> > +static bool
> > +stmt_semi_invariant_p (struct loop *loop, gimple *stmt,
> > +                       const_basic_block skip_head)
> > +{
> > +  ssa_op_iter iter;
> > +  tree use;
> > +
> > +  /* Although operand of a statement might be SSA name, CONSTANT or 
> VARDECL,
> > +     here we only need to check SSA name operands. For VARDECL operand
> > +     involves memory load, check on VARDECL operand must have been done
> > +     prior to invocation of this function in ssa_semi_invariant_p. */
> > +  FOR_EACH_SSA_TREE_OPERAND (use, stmt, iter, SSA_OP_USE)
> > +    {
> > +      if (!ssa_semi_invariant_p (loop, use, skip_head))
> > +        return false;
> > +    }
> > +
> > +  return true;
> > +}
> > +
> > +/* Determine if unselect one branch of a conditional statement, 
> whether we
> > +   can exclude leading basic block of the branch and those basic blocks
> > +   dominated by the leading one. */
> > +
> > +static bool
> > +can_branch_be_excluded (basic_block branch_bb)
> > +{
> > +  if (single_pred_p (branch_bb))
> > +    return true;
> > +
> > +  edge e;
> > +  edge_iterator ei;
> > +
> > +  FOR_EACH_EDGE (e, ei, branch_bb->preds)
> > +    {
> > +      if (dominated_by_p (CDI_DOMINATORS, e->src, branch_bb))
> > +        continue;
> > +
> > +      if (dominated_by_p (CDI_DOMINATORS, branch_bb, e->src))
> > +        continue;
> > +
> > +       /* The branch can be reached through other path, not just 
> from the
> > +          conditional statement. */
> > +      return false;
> > +    }
> > +
> > +  return true;
> > +}
> > +
> > +/* Find out which branch of a conditional statement is invariant. That
> > +   is: once the branch is selected in certain loop iteration, any 
> operand
> > +   that contributes to computation of the conditional statement remains
> > +   unchanged in all following iterations. */
> > +
> > +static int
> > +get_cond_invariant_branch (struct loop *loop, gcond *cond)
> > +{
> > +  basic_block cond_bb = gimple_bb (cond);
> > +  basic_block targ_bb[2];
> > +  bool invar[2];
> > +  unsigned invar_checks;
> > +
> > +  for (unsigned i = 0; i < 2; i++)
> > +    {
> > +      targ_bb[i] = EDGE_SUCC (cond_bb, i)->dest;
> > +
> > +      /* One branch directs to loop exit, no need to perform loop 
> split upon
> > +         this conditional statement. Firstly, it is trivial if the exit
> > +         branch is semi-invariant, for the statement is just 
> loop-breaking.
> > +         Secondly, if the opposite branch is semi-invariant, it 
> means that
> > +         the statement is real loop-invariant, which is covered by loop
> > +         unswitch. */
> > +      if (!flow_bb_inside_loop_p (loop, targ_bb[i]))
> > +        return -1;
> > +    }
> > +
> > +  invar_checks = 0;
> > +
> > +  for (unsigned i = 0; i < 2; i++)
> > +    {
> > +      invar[!i] = false;
> > +
> > +      if (!can_branch_be_excluded (targ_bb[i]))
> > +        continue;
> > +
> > +      /* Given a semi-invariant branch, if its opposite branch 
> dominates
> > +         loop latch, it and its following trace will only be 
> executed in
> > +         final iteration of loop, namely it is not part of repeated 
> body
> > +         of the loop. Similar to the above case that the branch is loop
> > +         exit, no need to split loop. */
> > +      if (dominated_by_p (CDI_DOMINATORS, loop->latch, targ_bb[i]))
> > +        continue;
> > +
> > +      invar[!i] = stmt_semi_invariant_p (loop, cond, targ_bb[i]);
> > +      invar_checks++;
> > +    }
> > +
> > +  /* With both branches being invariant (handled by loop unswitch) or
> > +     variant is not what we want. */
> > +  if (invar[0] ^ !invar[1])
> > +    return -1;
> > +
> > +  /* Found a real loop-invariant condition, do nothing. */
> > +  if (invar_checks < 2 && stmt_semi_invariant_p (loop, cond, NULL))
> > +    return -1;
> > +
> > +  return invar[1];
> > +}
> > +
> > +/* Return TRUE is conditional statement in a normal loop is also inside
> > +   a nested non-recognized loop, such as an irreducible loop. */
> > +
> > +static bool
> > +is_cond_in_hidden_loop (const struct loop *loop, basic_block cond_bb,
> > +                        int branch)
> > +{
> > +  basic_block branch_bb = EDGE_SUCC (cond_bb, branch)->dest;
> > +
> > +  if (cond_bb == loop->header || branch_bb == loop->latch)
> > +    return false;
> > +
> > +  basic_block *bbs = ((split_info *) loop->aux)->bbs;
> > +  auto_vec<basic_block> worklist;
> > +
> > +  for (unsigned i = 0; i < loop->num_nodes; i++)
> > +    bbs[i]->flags &= ~BB_REACHABLE;
> > +
> > +  /* Mark latch basic block as visited to be end point for 
> reachablility
> > +     traversal. */
> > +  loop->latch->flags |= BB_REACHABLE;
> > +
> > +  gcc_assert (flow_bb_inside_loop_p (loop, branch_bb));
> > +
> > +  /* Start from specified branch, the opposite branch is ignored for it
> > +     will not be executed. */
> > +  branch_bb->flags |= BB_REACHABLE;
> > +  worklist.safe_push (branch_bb);
> > +
> > +  do
> > +    {
> > +      basic_block bb = worklist.pop ();
> > +      edge e;
> > +      edge_iterator ei;
> > +
> > +      FOR_EACH_EDGE (e, ei, bb->succs)
> > +        {
> > +          basic_block succ_bb = e->dest;
> > +
> > +          if (succ_bb == cond_bb)
> > +            return true;
> > +
> > +          if (!flow_bb_inside_loop_p (loop, succ_bb))
> > +            continue;
> > +
> > +          if (succ_bb->flags & BB_REACHABLE)
> > +            continue;
> > +
> > +          succ_bb->flags |= BB_REACHABLE;
> > +          worklist.safe_push (succ_bb);
> > +        }
> > +    } while (!worklist.is_empty ());
> > +
> > +  return false;
> > +}
> > +
> > +
> > +/* Calculate increased code size measured by estimated insn number if
> > +   applying loop split upon certain branch of a conditional 
> statement. */
> > +
> > +static int
> > +compute_added_num_insns (struct loop *loop, const_basic_block cond_bb,
> > +                         int branch)
> > +{
> > +  const_basic_block targ_bb_var = EDGE_SUCC (cond_bb, !branch)->dest;
> > +  basic_block *bbs = ((split_info *) loop->aux)->bbs;
> > +  int num = 0;
> > +
> > +  for (unsigned i = 0; i < loop->num_nodes; i++)
> > +    {
> > +      /* Do no count basic blocks only in opposite branch. */
> > +      if (dominated_by_p (CDI_DOMINATORS, bbs[i], targ_bb_var))
> > +        continue;
> > +
> > +      for (gimple_stmt_iterator gsi = gsi_start_bb (bbs[i]); 
> !gsi_end_p (gsi);
> > +           gsi_next (&gsi))
> > +        num += estimate_num_insns (gsi_stmt (gsi), &eni_size_weights);
> > +    }
> > +
> > +  return num;
> > +}
> > +
> > +/* Return true if it is eligible and profitable to perform loop 
> split upon
> > +   a conditional statement. */
> > +
> > +static bool
> > +can_split_loop_on_cond (struct loop *loop, gcond *cond)
> > +{
> > +  int branch = get_cond_invariant_branch (loop, cond);
> > +
> > +  if (branch < 0)
> > +    return false;
> > +
> > +  basic_block cond_bb = gimple_bb (cond);
> > +
> > +  /* Add a threshold for increased code size to disable loop split. */
> > +  if (compute_added_num_insns (loop, cond_bb, branch) >
> > +      PARAM_VALUE (PARAM_MAX_COND_LOOP_SPLIT_INSNS))
> > +    return false;
> > +
> > +  /* In each interation, conditional statement candidate should be
> > +     executed only once. */
> > +  if (is_cond_in_hidden_loop (loop, cond_bb, branch))
> > +    return false;
> > +
> > +  profile_probability prob = EDGE_SUCC (cond_bb, branch)->probability;
> > +
> > +  /* When accurate profile information is available, and execution
> > +     frequency of the branch is too low, just let it go. */
> > +  if (prob.reliable_p ())
> > +    {
> > +      int thres = PARAM_VALUE (PARAM_MIN_COND_LOOP_SPLIT_PROB);
> > +
> > +      if (prob < profile_probability::always ().apply_scale (thres, 
> 100))
> > +        return false;
> > +    }
> > +
> > +  /* Temporarily keep branch index in conditional statement. */
> > +  gimple_set_plf (cond, GF_PLF_1, branch);
> > +  return true;
> > +}
> > +
> > +/* Traverse all conditional statements in a loop, to find out a good
> > +   candidate upon which we can do loop split. */
> > +
> > +static bool
> > +mark_cond_to_split_loop (struct loop *loop)
> > +{
> > +  split_info *info = new split_info ();
> > +  basic_block *bbs = info->bbs = get_loop_body (loop);
> > +
> > +  /* Allocate an area to keep temporary info, and associate its address
> > +     with loop aux field. */
> > +  loop->aux = info;
> > +
> > +  for (unsigned i = 0; i < loop->num_nodes; i++)
> > +    {
> > +      basic_block bb = bbs[i];
> > +
> > +      /* Skip statement in inner recognized loop, because we want that
> > +         conditional statement executes at most once in each 
> iteration. */
> > +      if (bb->loop_father != loop)
> > +        continue;
> > +
> > +      /* Actually this check is not a must constraint. With it, we can
> > +         ensure conditional statement will execute at least once in
> > +         each iteration. */
> > +      if (!dominated_by_p (CDI_DOMINATORS, loop->latch, bb))
> > +        continue;
> > +
> > +      gimple *last = last_stmt (bb);
> > +
> > +      if (!last || gimple_code (last) != GIMPLE_COND)
> > +        continue;
> > +
> > +      gcond *cond = as_a <gcond *> (last);
> > +
> > +      if (can_split_loop_on_cond (loop, cond))
> > +        {
> > +          info->cond = cond;
> > +          return true;
> > +        }
> > +    }
> > +
> > +  delete info;
> > +  loop->aux = NULL;
> > +
> > +  return false;
> > +}
> > +
> > +/* Given a loop with a chosen conditional statement candidate, 
> perform loop
> > +   split transformation illustrated as the following graph.
> > +
> > +               .-------T------ if (true) ------F------.
> > +               |                    .---------------. |
> > +               |                    |               | |
> > +               v                    |               v v
> > +          pre-header                | pre-header
> > +               | .------------.     | | .------------.
> > +               | |            |     | | |            |
> > +               | v            |     | | v            |
> > +             header           |     | header           |
> > +               |              |     | |              |
> > +       [ bool r = cond; ]     |     | |              |
> > +               |              |     | |              |
> > +      .---- if (r) -----.     |     |        .--- if (true) ---.     |
> > +      |                 |     |     | |                 |     |
> > +  invariant             |     |     | invariant             |     |
> > +      |                 |     |     | |                 |     |
> > +      '---T--->.<---F---'     |     | '---T--->.<---F---'     |
> > +               |              |    / |              |
> > +             stmts            |   / stmts            |
> > +               |              |  / |              |
> > +              / \             | /                    / \             |
> > +     .-------*   *       [ if (!r) ] .-------*   *            |
> > +     |           |            | |           |            |
> > +     |         latch          |             | latch          |
> > +     |           |            | |           |            |
> > +     |           '------------' |           '------------'
> > +     '------------------------. .-----------'
> > +             loop1            | | loop2
> > +                              v v
> > +                             exits
> > +
> > +   In the graph, loop1 represents the part derived from original 
> one, and
> > +   loop2 is duplicated using loop_version (), which corresponds to 
> the part
> > +   of original one being splitted out. In loop1, a new bool 
> temporary (r)
> > +   is introduced to keep value of the condition result. In original 
> latch
> > +   edge of loop1, we insert a new conditional statement whose value 
> comes
> > +   from previous temporary (r), one of its branch goes back to 
> loop1 header
> > +   as a latch edge, and the other branch goes to loop2 pre-header as an
> > +   entry edge. And also in loop2, we abandon the variant branch of the
> > +   conditional statement candidate by setting a constant bool 
> condition,
> > +   based on which branch is semi-invariant. */
> > +
> > +static bool
> > +split_loop_for_cond (struct loop *loop1)
> > +{
> > +  split_info *info = (split_info *) loop1->aux;
> > +  gcond *cond = info->cond;
> > +  basic_block cond_bb = gimple_bb (cond);
> > +  int branch = gimple_plf (cond, GF_PLF_1);
> > +  bool true_invar = !!(EDGE_SUCC (cond_bb, branch)->flags & 
> EDGE_TRUE_VALUE);
> > +
> > +  if (dump_file && (dump_flags & TDF_DETAILS))
> > +   {
> > +     fprintf (dump_file, "In %s(), split loop %d at branch<%s>, BB 
> %d\n",
> > +              current_function_name (), loop1->num,
> > +              true_invar ? "T" : "F", cond_bb->index);
> > +     print_gimple_stmt (dump_file, cond, 0, TDF_SLIM | TDF_VOPS);
> > +   }
> > +
> > +  initialize_original_copy_tables ();
> > +
> > +  struct loop *loop2 = loop_version (loop1, boolean_true_node, NULL,
> > + profile_probability::always (),
> > + profile_probability::never (),
> > + profile_probability::always (),
> > + profile_probability::always (),
> > +                                     true);
> > +  if (!loop2)
> > +    {
> > +      free_original_copy_tables ();
> > +      return false;
> > +    }
> > +
> > +  /* Generate a bool type temporary to hold result of the condition. */
> > +  tree tmp = make_ssa_name (boolean_type_node);
> > +  gimple_stmt_iterator gsi = gsi_last_bb (cond_bb);
> > +  gimple *stmt = gimple_build_assign (tmp,
> > +                                      gimple_cond_code (cond),
> > +                                      gimple_cond_lhs (cond),
> > +                                      gimple_cond_rhs (cond));
> > +
> > +  gsi_insert_before (&gsi, stmt, GSI_NEW_STMT);
> > +  gimple_cond_set_condition (cond, EQ_EXPR, tmp, boolean_true_node);
> > +  update_stmt (cond);
> > +
> > +  /* Replace the condition in loop2 with a bool constant to let pass
> > +     manager remove the variant branch after current pass finishes. */
> > +  basic_block cond_bb_copy = get_bb_copy (cond_bb);
> > +  gcond *cond_copy = as_a<gcond *> (last_stmt (cond_bb_copy));
> > +
> > +  if (true_invar)
> > +    gimple_cond_make_true (cond_copy);
> > +  else
> > +    gimple_cond_make_false (cond_copy);
> > +
> > +  update_stmt (cond_copy);
> > +
> > +  /* Insert a new conditional statement on latch edge of loop1. This
> > +     statement acts as a switch to transfer execution from loop1 to
> > +     loop2, when loop1 enters into invariant state. */
> > +  basic_block latch_bb = split_edge (loop_latch_edge (loop1));
> > +  basic_block break_bb = split_edge (single_pred_edge (latch_bb));
> > +  gimple *break_cond = gimple_build_cond (EQ_EXPR, tmp, 
> boolean_true_node,
> > +                                          NULL_TREE, NULL_TREE);
> > +
> > +  gsi = gsi_last_bb (break_bb);
> > +  gsi_insert_after (&gsi, break_cond, GSI_NEW_STMT);
> > +
> > +  edge to_loop1 = single_succ_edge (break_bb);
> > +  edge to_loop2 = make_edge (break_bb, loop_preheader_edge 
> (loop2)->src, 0);
> > +
> > +  to_loop1->flags &= ~EDGE_FALLTHRU;
> > +
> > +  if (true_invar)
> > +    {
> > +      to_loop1->flags |= EDGE_FALSE_VALUE;
> > +      to_loop2->flags |= EDGE_TRUE_VALUE;
> > +    }
> > +  else
> > +    {
> > +      to_loop1->flags |= EDGE_TRUE_VALUE;
> > +      to_loop2->flags |= EDGE_FALSE_VALUE;
> > +    }
> > +
> > +  update_ssa (TODO_update_ssa);
> > +
> > +  /* Due to introduction of a control flow edge from loop1 latch to 
> loop2
> > +     pre-header, we should update PHIs in loop2 to reflect this 
> connection
> > +     between loop1 and loop2. */
> > +  connect_loop_phis (loop1, loop2, to_loop2);
> > +
> > +  free_original_copy_tables ();
> > +
> > +  rewrite_into_loop_closed_ssa_1 (NULL, 0, SSA_OP_USE, loop1);
> > +
> > +  return true;
> > +}
> > +
> > +/* Main entry point to perform loop splitting for suitable 
> if-conditions
> > +   in all loops. */
> > +
> > +static unsigned int
> > +tree_ssa_split_loops_for_cond (void)
> > +{
> > +  struct loop *loop;
> > +  auto_vec<struct loop *> loop_list;
> > +  bool changed = false;
> > +  unsigned i;
> > +
> > +  FOR_EACH_LOOP (loop, LI_INCLUDE_ROOT)
> > +    loop->aux = NULL;
> > +
> > +  /* Go through all loops starting from innermost. */
> > +  FOR_EACH_LOOP (loop, LI_FROM_INNERMOST)
> > +    {
> > +      /* Put loop in a list if found a conditional statement 
> candidate in
> > +         the loop. This is stage for analysis, no change anything 
> in the
> > +         function. */
> > +      if (!loop->aux
> > +          && !optimize_loop_for_size_p (loop)
> > +          && mark_cond_to_split_loop (loop))
> > +        loop_list.safe_push (loop);
> > +
> > +      /* If any of our inner loops was split, don't split us,
> > +         and mark our containing loop as having had splits as well. */
> > +      loop_outer (loop)->aux = loop->aux;
> > +    }
> > +
> > +  FOR_EACH_VEC_ELT (loop_list, i, loop)
> > +    {
> > +      /* Extract selected loop and perform loop split. This is 
> stage for
> > +         transformation. */
> > +      changed |= split_loop_for_cond (loop);
> > +
> > +      delete (split_info *) loop->aux;
> > +    }
> > +
> > +  FOR_EACH_LOOP (loop, LI_INCLUDE_ROOT)
> > +    loop->aux = NULL;
> > +
> > +  if (changed)
> > +    return TODO_cleanup_cfg;
> > +  return 0;
> > +}
> > +
> > +
> >  /* Loop splitting pass.  */
> >
> >  namespace {
> > @@ -716,3 +1517,48 @@ make_pass_loop_split (gcc::context *ctxt)
> >  {
> >    return new pass_loop_split (ctxt);
> >  }
> > +
> > +namespace {
> > +
> > +const pass_data pass_data_cond_loop_split =
> > +{
> > +  GIMPLE_PASS, /* type */
> > +  "cond_lsplit", /* name */
> > +  OPTGROUP_LOOP, /* optinfo_flags */
> > +  TV_COND_LOOP_SPLIT, /* tv_id */
> > +  PROP_cfg, /* properties_required */
> > +  0, /* properties_provided */
> > +  0, /* properties_destroyed */
> > +  0, /* todo_flags_start */
> > +  0, /* todo_flags_finish */
> > +};
> > +
> > +class pass_cond_loop_split : public gimple_opt_pass
> > +{
> > +public:
> > +  pass_cond_loop_split (gcc::context *ctxt)
> > +    : gimple_opt_pass (pass_data_cond_loop_split, ctxt)
> > +  {}
> > +
> > +  /* opt_pass methods: */
> > +  virtual bool gate (function *) { return flag_split_loops != 0; }
> > +  virtual unsigned int execute (function *);
> > +
> > +}; // class pass_cond_loop_split
> > +
> > +unsigned int
> > +pass_cond_loop_split::execute (function *fun)
> > +{
> > +  if (number_of_loops (fun) <= 1)
> > +    return 0;
> > +
> > +  return tree_ssa_split_loops_for_cond ();
> > +}
> > +
> > +} // anon namespace
> > +
> > +gimple_opt_pass *
> > +make_pass_cond_loop_split (gcc::context *ctxt)
> > +{
> > +  return new pass_cond_loop_split (ctxt);
> > +}

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