Re: [PATCH V3] Loop split upon semi-invariant condition (PR tree-optimization/89134)

[Feng Xue OS <fxue at os dot amperecomputing dot com>]

Hi Philipp,

   This is an updated patch based on comments form Michael, and if he think this is ok, we will merge it into trunk. Thanks,

Feng

________________________________________
From: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Sent: Tuesday, October 15, 2019 11:49 PM
To: Feng Xue OS
Cc: Michael Matz; Richard Biener; gcc-patches@gcc.gnu.org; Christoph Müllner; erick.ochoa@theobroma-systems.com
Subject: Re: [PATCH V3] Loop split upon semi-invariant condition (PR tree-optimization/89134)

Feng,

This looks good from our side and has shown useful (combined with the other 2 patches) in
our testing with SPEC2017.
Given that this looks final: what is the plan for getting this merged?

Thanks,
Philipp.

> On 12.09.2019, at 12:23, Feng Xue OS <fxue at os dot amperecomputing dot com> wrote:
>
> ---
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 1391a562c35..28981fa1048 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -11418,6 +11418,19 @@ 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
> +In a loop, if a branch of a conditional statement is selected since certain
> +loop iteration, any operand that contributes to computation of the conditional
> +expression remains unchanged in all following iterations, the statement is
> +semi-invariant, upon which we can do a kind of loop split transformation.
> +@option{max-cond-loop-split-insns} controls maximum number of insns to be
> +added due to loop split on semi-invariant conditional statement.
> +
> +@item min-cond-loop-split-prob
> +When FDO profile information is available, @option{min-cond-loop-split-prob}
> +specifies minimum threshold for probability of semi-invariant 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 13001a7bb2d..12bc8c26c9e 100644
> --- a/gcc/params.def
> +++ b/gcc/params.def
> @@ -386,6 +386,20 @@ 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 added 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-invariant 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/testsuite/g++.dg/tree-ssa/loop-cond-split-1.C b/gcc/testsuite/g++.dg/tree-ssa/loop-cond-split-1.C
> new file mode 100644
> index 00000000000..51f9da22fc7
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/tree-ssa/loop-cond-split-1.C
> @@ -0,0 +1,33 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O3 -fdump-tree-lsplit-details" } */
> +
> +#include <string>
> +#include <map>
> +
> +using namespace std;
> +
> +class  A
> +{
> +public:
> +  bool empty;
> +  void set (string s);
> +};
> +
> +class  B
> +{
> +  map<int, string> m;
> +  void f ();
> +};
> +
> +extern A *ga;
> +
> +void B::f ()
> +{
> +  for (map<int, string>::iterator iter = m.begin (); iter != m.end (); ++iter)
> +    {
> +      if (ga->empty)
> +        ga->set (iter->second);
> +    }
> +}
> +
> +/* { dg-final { scan-tree-dump-times "split loop 1 at branch" 1 "lsplit" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/loop-cond-split-1.c b/gcc/testsuite/gcc.dg/tree-ssa/loop-cond-split-1.c
> new file mode 100644
> index 00000000000..bbd522d6bcd
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/loop-cond-split-1.c
> @@ -0,0 +1,23 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O3 -fdump-tree-lsplit-details" } */
> +
> +__attribute__((pure)) __attribute__((noinline)) int inc (int i)
> +{
> +  return i + 1;
> +}
> +
> +extern int do_something (void);
> +extern int b;
> +
> +void test(int n)
> +{
> +  int i;
> +
> +  for (i = 0; i < n; i = inc (i))
> +    {
> +      if (b)
> +        b = do_something();
> +    }
> +}
> +
> +/* { dg-final { scan-tree-dump-times "split loop 1 at branch" 1 "lsplit" } } */
> diff --git a/gcc/tree-ssa-loop-split.c b/gcc/tree-ssa-loop-split.c
> index f5f083384bc..e4a1b6d2019 100644
> --- a/gcc/tree-ssa-loop-split.c
> +++ b/gcc/tree-ssa-loop-split.c
> @@ -32,7 +32,10 @@ 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 "tree-cfgcleanup.h"
> #include "cfgloop.h"
> +#include "params.h"
> #include "tree-scalar-evolution.h"
> #include "gimple-iterator.h"
> #include "gimple-pretty-print.h"
> @@ -40,7 +43,9 @@ 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 kinds of loop splitting.
> +
> +   One transformation of loops like:
>
>    for (i = 0; i < 100; i++)
>      {
> @@ -612,6 +617,722 @@ split_loop (class loop *loop1, class tree_niter_desc *niter)
>   return changed;
> }
>
> +/* Another transformation of loops like:
> +
> +   for (i = INIT (); CHECK (i); i = NEXT ())
> +     {
> +       if (expr (a_1, a_2, ..., a_n))  // expr is pure
> +         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 *> memory_stores;
> +
> +  /* Whether above memory stores vector has been filled.  */
> +  int need_init;
> +
> +  split_info () : bbs (NULL),  need_init (true) { }
> +
> +  ~split_info ()
> +    {
> +      if (bbs)
> +     free (bbs);
> +    }
> +};
> +
> +/* Find all statements with memory-write effect in LOOP, including memory
> +   store and non-pure function call, and keep those in a vector.  This work
> +   is only done 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->need_init = false;
> +
> +  /* 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 *> other_stores;
> +  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 towards the
> +      first, which ensures 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 an idea of of using them as shortcut
> +          terminators to memory alias analysis.  */
> +       if (gimple_code (stmt) == GIMPLE_CALL)
> +         info->memory_stores.safe_push (stmt);
> +       else
> +         other_stores.safe_push (stmt);
> +
> +       if (bitmap_set_bit (visited, SSA_NAME_VERSION (prev)))
> +         worklist.safe_push (prev);
> +     }
> +    } while (!worklist.is_empty ());
> +
> +    info->memory_stores.safe_splice (other_stores);
> +}
> +
> +
> +/* Given STMT, memory load or pure call statement, check whether it is impacted
> +   by some memory store in LOOP, excluding trace starting from SKIP_HEAD (the
> +   trace is composed of SKIP_HEAD and those basic block dominated by it, always
> +   corresponds to one branch of a conditional statement).  If SKIP_HEAD is
> +   NULL, all basic blocks of 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->need_init)
> +    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->memory_stores, i, store)
> +    {
> +      /* Skip basic blocks dominated by SKIP_HEAD, if non-NULL.  */
> +      if (skip_head
> +       && dominated_by_p (CDI_DOMINATORS, gimple_bb (store), skip_head))
> +     continue;
> +
> +      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 since
> +   certain iteration of LOOP, check whether an SSA name (NAME) remains
> +   unchanged in next interation.  We 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.  If non-
> +   NULL, SSA name to check is supposed to be defined before SKIP_HEAD.  */
> +
> +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;
> +
> +  if (gimple_code (def) == GIMPLE_PHI)
> +    {
> +      /* For PHI node that is not in loop header, its source operands should
> +      be defined inside the loop, which are seen as loop variant.  */
> +      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 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 the PHI node to definition
> +      of latch value, if excluding branch trace from 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);
> +
> +               /* Not consider redefinitions in excluded basic blocks.  */
> +               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, it is variant.  */
> +                   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 STMT is semi-invariant in LOOP, iff all its operands are
> +   semi-invariant.  Trace composed of basic block SKIP_HEAD and basic blocks
> +   dominated by it are excluded from the loop.  */
> +
> +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.  This is because check on
> +     VARDECL operands, which involve memory loads, must have been done
> +     prior to invocation of this function in vuse_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 when conditional statement never transfers execution to one of its
> +   branch, whether we can remove the branch's leading basic block (BRANCH_BB)
> +   and those basic blocks dominated by BRANCH_BB.  */
> +
> +static bool
> +branch_removable_p (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 from opposite branch, or from some
> +       statement not dominated by the conditional statement.  */
> +      return false;
> +    }
> +
> +  return true;
> +}
> +
> +/* Find out which branch of a conditional statement (COND) is invariant in the
> +   execution context of LOOP.  That is: once the branch is selected in certain
> +   iteration of the loop, any operand that contributes to computation of the
> +   conditional statement remains unchanged in all following iterations.  */
> +
> +static edge
> +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 to break loop.  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 NULL;
> +    }
> +
> +  invar_checks = 0;
> +
> +  for (unsigned i = 0; i < 2; i++)
> +    {
> +      invar[!i] = false;
> +
> +      if (!branch_removable_p (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 NULL;
> +
> +  /* Found a real loop-invariant condition, do nothing.  */
> +  if (invar_checks < 2 && stmt_semi_invariant_p (loop, cond, NULL))
> +    return NULL;
> +
> +  return EDGE_SUCC (cond_bb, (unsigned) invar[1]);
> +}
> +
> +/* Calculate increased code size measured by estimated insn number if applying
> +   loop split upon certain branch (BRANCH_EDGE) of a conditional statement.  */
> +
> +static int
> +compute_added_num_insns (struct loop *loop, const_edge branch_edge)
> +{
> +  basic_block cond_bb = branch_edge->src;
> +  unsigned branch = EDGE_SUCC (cond_bb, 1) == branch_edge;
> +  basic_block opposite_bb = 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], opposite_bb))
> +     continue;
> +
> +      num += estimate_num_insns_seq (bb_seq (bbs[i]), &eni_size_weights);
> +    }
> +
> +  /* It is unnecessary to evaluate expression of the conditional statement
> +     in new loop that contains only invariant branch.  This expresion should
> +     be constant value (either true or false).  Exclude code size of insns
> +     that contribute to computation of the expression.  */
> +
> +  auto_vec<gimple *> worklist;
> +  hash_set<gimple *> removed;
> +  gimple *stmt = last_stmt (cond_bb);
> +
> +  worklist.safe_push (stmt);
> +  removed.add (stmt);
> +  num -= estimate_num_insns (stmt, &eni_size_weights);
> +
> +  do
> +    {
> +      ssa_op_iter opnd_iter;
> +      use_operand_p opnd_p;
> +
> +      stmt = worklist.pop ();
> +      FOR_EACH_PHI_OR_STMT_USE (opnd_p, stmt, opnd_iter, SSA_OP_USE)
> +     {
> +       tree opnd = USE_FROM_PTR (opnd_p);
> +
> +       if (TREE_CODE (opnd) != SSA_NAME || SSA_NAME_IS_DEFAULT_DEF (opnd))
> +         continue;
> +
> +       gimple *opnd_stmt = SSA_NAME_DEF_STMT (opnd);
> +       use_operand_p use_p;
> +       imm_use_iterator use_iter;
> +
> +       if (removed.contains (opnd_stmt)
> +           || !flow_bb_inside_loop_p (loop, gimple_bb (opnd_stmt)))
> +         continue;
> +
> +       FOR_EACH_IMM_USE_FAST (use_p, use_iter, opnd)
> +         {
> +              gimple *use_stmt = USE_STMT (use_p);
> +
> +           if (!is_gimple_debug (use_stmt) && !removed.contains (use_stmt))
> +             {
> +               opnd_stmt = NULL;
> +               break;
> +             }
> +         }
> +
> +       if (opnd_stmt)
> +         {
> +           worklist.safe_push (opnd_stmt);
> +           removed.add (opnd_stmt);
> +           num -= estimate_num_insns (opnd_stmt, &eni_size_weights);
> +         }
> +     }
> +    } while (!worklist.is_empty ());
> +
> +  gcc_assert (num >= 0);
> +  return num;
> +}
> +
> +/* Find out loop-invariant branch of a conditional statement (COND) if it has,
> +   and check whether it is eligible and profitable to perform loop split upon
> +   this branch in LOOP.  */
> +
> +static edge
> +get_cond_branch_to_split_loop (struct loop *loop, gcond *cond)
> +{
> +  edge invar_branch = get_cond_invariant_branch (loop, cond);
> +
> +  if (!invar_branch)
> +    return NULL;
> +
> +  profile_probability prob = invar_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 NULL;
> +    }
> +
> +  /* Add a threshold for increased code size to disable loop split.  */
> +  if (compute_added_num_insns (loop, invar_branch)
> +      > PARAM_VALUE (PARAM_MAX_COND_LOOP_SPLIT_INSNS))
> +    return NULL;
> +
> +  return invar_branch;
> +}
> +
> +/* Given a loop (LOOP1) with a loop-invariant branch (INVAR_BRANCH) of some
> +   conditional statement, 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
> +do_split_loop_on_cond (struct loop *loop1, edge invar_branch)
> +{
> +  basic_block cond_bb = invar_branch->src;
> +  bool true_invar = !!(invar_branch->flags & EDGE_TRUE_VALUE);
> +  gcond *cond = as_a <gcond *> (last_stmt (cond_bb));
> +
> +  gcc_assert (cond_bb->loop_father == loop1);
> +
> +  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);
> +
> +  basic_block cond_bb_copy = get_bb_copy (cond_bb);
> +  gcond *cond_copy = as_a<gcond *> (last_stmt (cond_bb_copy));
> +
> +  /* Replace the condition in loop2 with a bool constant to let PassManager
> +     remove the variant branch after current pass completes.  */
> +  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;
> +  to_loop1->flags |= true_invar ? EDGE_FALSE_VALUE : EDGE_TRUE_VALUE;
> +  to_loop2->flags |= true_invar ? EDGE_TRUE_VALUE : 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;
> +}
> +
> +/* Traverse all conditional statements in LOOP, to find out a good candidate
> +   upon which we can do loop split.  */
> +
> +static bool
> +split_loop_on_cond (struct loop *loop)
> +{
> +  split_info *info = new split_info ();
> +  basic_block *bbs = info->bbs = get_loop_body (loop);
> +  bool do_split = false;
> +
> +  /* 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];
> +
> +      /* We only consider conditional statement, which be executed at most once
> +      in each iteration of the loop.  So skip statements in inner loops.  */
> +      if ((bb->loop_father != loop) || (bb->flags & BB_IRREDUCIBLE_LOOP))
> +     continue;
> +
> +      /* Actually this check is not a must constraint. With it, we can ensure
> +      conditional statement will always be executed 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);
> +      edge branch_edge = get_cond_branch_to_split_loop (loop, cond);
> +
> +      if (branch_edge)
> +     {
> +       do_split_loop_on_cond (loop, branch_edge);
> +       do_split = true;
> +       break;
> +     }
> +    }
> +
> +  delete info;
> +  loop->aux = NULL;
> +
> +  return do_split;
> +}
> +
> /* Main entry point.  Perform loop splitting on all suitable loops.  */
>
> static unsigned int
> @@ -662,6 +1383,32 @@ tree_ssa_split_loops (void)
>       }
>     }
>
> +  if (changed)
> +    {
> +      cleanup_tree_cfg ();
> +      changed = false;
> +    }
> +
> +  /* Perform loop splitting for suitable if-conditions in all loops.  */
> +  FOR_EACH_LOOP (loop, LI_INCLUDE_ROOT)
> +    loop->aux = NULL;
> +
> +  FOR_EACH_LOOP (loop, LI_FROM_INNERMOST)
> +    {
> +      if (loop->aux)
> +        {
> +       loop_outer (loop)->aux = loop;
> +       continue;
> +     }
> +
> +      if (!optimize_loop_for_size_p (loop)
> +       && split_loop_on_cond (loop))
> +     {
> +       loop_outer (loop)->aux = loop;
> +       changed = true;
> +     }
> +    }
> +
>   FOR_EACH_LOOP (loop, LI_INCLUDE_ROOT)
>     loop->aux = NULL;
>
> --
> 2.17.1
>