[gcc(refs/users/tnfchris/heads/gcc-14-early-break)] implement code motion for early break.

[gcc(refs/users/tnfchris/heads/gcc-14-early-break)] implement code motion for early break.

[Tamar Christina]

https://gcc.gnu.org/g:27fc08859dd9f360fa9e338450805bfa8dd049dd

commit 27fc08859dd9f360fa9e338450805bfa8dd049dd
Author: Tamar Christina <tamar.christina@arm.com>
Date:   Wed Jun 28 13:49:46 2023 +0100

    implement code motion for early break.
    
    When performing early break vectorization we need to be sure that the vector
    operations are safe to perform.  A simple example is e.g.
    
     for (int i = 0; i < N; i++)
     {
       vect_b[i] = x + i;
       if (vect_a[i]*2 != x)
         break;
       vect_a[i] = x;
     }
    
    where the store to vect_b is not allowed to be executed unconditionally since
    if we exit through the early break it wouldn't have been done for the full VF
    iteration.
    
    Effective the code motion determines:
      - is it safe/possible to vectorize the function
      - what updates to the VUSES should be performed if we do
      - Which statements need to be moved
      - Which statements can't be moved:
        * values that are live must be reachable through all exits
        * values that aren't single use and shared by the use/def chain of the cond
      - The final insertion point of the instructions.  In the cases we have
        multiple early exist statements this should be the one closest to the loop
        latch itself.
    
    After motion the loop above is:
    
     for (int i = 0; i < N; i++)
     {
       ... y = x + i;
       if (vect_a[i]*2 != x)
         break;
       vect_b[i] = y;
       vect_a[i] = x;
    
     }
    
    The operation is split into two, during data ref analysis we determine
    validity of the operation and generate a worklist of actions to perform if we
    vectorize.
    
    After peeling and just before statetement tranformation we replay this worklist
    which moves the statements and updates book keeping only in the main loop that's
    to be vectorized.  This includes updating of USES in exit blocks.
    
    Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
    
    Ok for master?
    
    Thanks,
    Tamar
    
    gcc/ChangeLog:
    
            * tree-vect-data-refs.cc (validate_early_exit_stmts): New.
            (vect_analyze_data_ref_dependences): Use it.
            * tree-vect-loop.cc (move_early_exit_stmts): New.
            (vect_transform_loop): Use it.
            * tree-vectorizer.h (LOOP_VINFO_EARLY_BRK_CONFLICT_STMTS,
            LOOP_VINFO_EARLY_BRK_DEST_BB, LOOP_VINFO_EARLY_BRK_VUSES): New.

Diff:
---
 gcc/tree-vect-data-refs.cc | 350 +++++++++++++++++++++++++++++++++++++++++++++
 gcc/tree-vect-loop.cc      |  44 ++++++
 gcc/tree-vectorizer.h      |  16 +++
 3 files changed, 410 insertions(+)

diff --git a/gcc/tree-vect-data-refs.cc b/gcc/tree-vect-data-refs.cc
index fcc950f528b..240bd7a8623 100644
--- a/gcc/tree-vect-data-refs.cc
+++ b/gcc/tree-vect-data-refs.cc
@@ -568,6 +568,278 @@ vect_analyze_data_ref_dependence (struct data_dependence_relation *ddr,
   return opt_result::success ();
 }
 
+/* This function tries to validate whether an early break vectorization
+   is possible for the current instruction sequence. Returns True i
+   possible, otherwise False.
+
+   Requirements:
+     - Any memory access must be to a fixed size buffer.
+     - There must not be any loads and stores to the same object.
+     - Multiple loads are allowed as long as they don't alias.
+
+   NOTE:
+     This implemementation is very conservative. Any overlappig loads/stores
+     that take place before the early break statement gets rejected aside from
+     WAR dependencies.
+
+     i.e.:
+
+	a[i] = 8
+	c = a[i]
+	if (b[i])
+	  ...
+
+	is not allowed, but
+
+	c = a[i]
+	a[i] = 8
+	if (b[i])
+	  ...
+
+	is which is the common case.
+
+   Arguments:
+     - LOOP_VINFO: loop information for the current loop.
+     - CHAIN: Currently detected sequence of instructions that need to be moved
+	      if we are to vectorize this early break.
+     - FIXED: Sequences of SSA_NAMEs that must not be moved, they are reachable from
+	      one or more cond conditions.  If this set overlaps with CHAIN then FIXED
+	      takes precedence.  This deals with non-single use cases.
+     - LOADS: List of all loads found during traversal.
+     - BASES: List of all load data references found during traversal.
+     - GSTMT: Current position to inspect for validity.  The sequence
+	      will be moved upwards from this point.
+     - REACHING_VUSE: The dominating VUSE found so far.
+     - CURRENT_VDEF: The last VDEF we've seen.  These are updated in
+		      pre-order and updated in post-order after moving the
+		      instruction.  */
+
+static bool
+validate_early_exit_stmts (loop_vec_info loop_vinfo, hash_set<tree> *chain,
+			   hash_set<tree> *fixed, vec<tree> *loads,
+			   vec<data_reference *> *bases, tree *reaching_vuse,
+			   tree *current_vdef, gimple_stmt_iterator *gstmt,
+			   hash_map<tree, tree> *renames)
+{
+  if (gsi_end_p (*gstmt))
+    return true;
+
+  gimple *stmt = gsi_stmt (*gstmt);
+  if (gimple_has_ops (stmt))
+    {
+      tree dest = NULL_TREE;
+      /* Try to find the SSA_NAME being defined.  For Statements with an LHS
+	 use the LHS, if not, assume that the first argument of a call is the
+	 value being defined.  e.g. MASKED_LOAD etc.  */
+      if (gimple_has_lhs (stmt))
+	{
+	  if (is_gimple_assign (stmt))
+	    dest = gimple_assign_lhs (stmt);
+	  else if (const gcall *call = dyn_cast <const gcall *> (stmt))
+	    dest = gimple_call_lhs (call);
+	}
+      else if (const gcall *call = dyn_cast <const gcall *> (stmt))
+	dest = gimple_arg (call, 0);
+      else if (const gcond *cond = dyn_cast <const gcond *> (stmt))
+	{
+	  /* Operands of conds are ones we can't move.  */
+	  fixed->add (gimple_cond_lhs (cond));
+	  fixed->add (gimple_cond_rhs (cond));
+	}
+
+      bool move = false;
+
+      stmt_vec_info stmt_vinfo = loop_vinfo->lookup_stmt (stmt);
+      if (!stmt_vinfo)
+	{
+	   if (dump_enabled_p ())
+	     dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+			      "early breaks only supported. Unknown"
+			      " statement: %G", stmt);
+	   return false;
+	}
+
+      auto dr_ref = STMT_VINFO_DATA_REF (stmt_vinfo);
+      if (dr_ref)
+	{
+	   /* We currenly only support statically allocated objects due to
+	      not having first-faulting loads support or peeling for alignment
+	      support.  Compute the isize of the referenced object (it could be
+	      dynamically allocated).  */
+	   tree obj = DR_BASE_ADDRESS (dr_ref);
+	   if (!obj || TREE_CODE (obj) != ADDR_EXPR)
+	     {
+	       if (dump_enabled_p ())
+		 dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+				  "early breaks only supported on statically"
+				  " allocated objects.\n");
+	       return false;
+	     }
+
+	   tree refop = TREE_OPERAND (obj, 0);
+	   tree refbase = get_base_address (refop);
+	   if (!refbase || !DECL_P (refbase) || !DECL_SIZE (refbase)
+	       || TREE_CODE (DECL_SIZE (refbase)) != INTEGER_CST)
+	     {
+	       if (dump_enabled_p ())
+		 dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+				  "early breaks only supported on statically"
+				  " allocated objects.\n");
+	       return false;
+	     }
+
+	   if (DR_IS_READ (dr_ref))
+	     {
+		loads->safe_push (dest);
+		bases->safe_push (dr_ref);
+	     }
+	   else if (DR_IS_WRITE (dr_ref))
+	     {
+		for (auto dr : bases)
+		  if (same_data_refs_base_objects (dr, dr_ref))
+		    {
+		      if (dump_enabled_p ())
+			  dump_printf_loc (MSG_MISSED_OPTIMIZATION,
+					   vect_location,
+					   "early breaks only supported,"
+					   " overlapping loads and stores found"
+					   " before the break statement.\n");
+		      return false;
+		    }
+		/* Any writes starts a new chain. */
+		move = true;
+	     }
+	}
+
+      /* If a statement if live and escapes the loop through usage in the loop
+	 epilogue then we can't move it since we need to maintain its
+	 reachability through all exits.  */
+      bool skip = false;
+      if (STMT_VINFO_LIVE_P (stmt_vinfo)
+	  && !(dr_ref && DR_IS_WRITE (dr_ref)))
+	{
+	  imm_use_iterator imm_iter;
+	  use_operand_p use_p;
+	  FOR_EACH_IMM_USE_FAST (use_p, imm_iter, dest)
+	    {
+	      basic_block bb = gimple_bb (USE_STMT (use_p));
+	      skip = bb == LOOP_VINFO_IV_EXIT (loop_vinfo)->dest;
+	      if (skip)
+		break;
+	    }
+	}
+
+      /* If we found the defining statement of a something that's part of the
+	 chain then expand the chain with the new SSA_VARs being used.  */
+      if (!skip && (chain->contains (dest) || move))
+	{
+	  move = true;
+	  for (unsigned x = 0; x < gimple_num_args (stmt); x++)
+	    {
+	      tree var = gimple_arg (stmt, x);
+	      if (TREE_CODE (var) == SSA_NAME)
+		{
+		  if (fixed->contains (dest))
+		    {
+		      move = false;
+		      fixed->add (var);
+		    }
+		  else
+		    chain->add (var);
+		}
+	      else
+		{
+		  use_operand_p use_p;
+		  ssa_op_iter iter;
+		  FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE)
+		    {
+		      tree op = USE_FROM_PTR (use_p);
+		      gcc_assert (TREE_CODE (op) == SSA_NAME);
+		      if (fixed->contains (dest))
+			{
+			  move = false;
+			  fixed->add (op);
+			}
+		      else
+			chain->add (op);
+		    }
+		}
+	    }
+
+	  if (dump_enabled_p ())
+	    {
+	      if (move)
+		dump_printf_loc (MSG_NOTE, vect_location,
+				"found chain %G", stmt);
+	      else
+		dump_printf_loc (MSG_NOTE, vect_location,
+				"ignored chain %G, not single use", stmt);
+	    }
+	}
+
+      if (move)
+	{
+	  if (dump_enabled_p ())
+	    dump_printf_loc (MSG_NOTE, vect_location,
+			     "==> recording stmt %G", stmt);
+
+	  for (tree ref : loads)
+	    if (stmt_may_clobber_ref_p (stmt, ref, true))
+	      {
+	        if (dump_enabled_p ())
+		  dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+				   "early breaks not supported as memory used"
+				   " may alias.\n");
+	        return false;
+	      }
+
+	  /* This statement is to be moved.  */
+	  LOOP_VINFO_EARLY_BRK_CONFLICT_STMTS (loop_vinfo).safe_push (stmt);
+
+	  /* If we've moved a VDEF, extract the defining MEM and update
+	     usages of it.   */
+	  tree vdef;
+	  if ((vdef = gimple_vdef (stmt)))
+	    {
+	      *current_vdef = vdef;
+	      *reaching_vuse = gimple_vuse (stmt);
+	    }
+	}
+    }
+
+  gsi_prev (gstmt);
+
+  if (!validate_early_exit_stmts (loop_vinfo, chain, fixed, loads, bases,
+				  reaching_vuse, current_vdef, gstmt, renames))
+    return false;
+
+  if (gimple_vuse (stmt)
+      && reaching_vuse && *reaching_vuse
+      && gimple_vuse (stmt) == *current_vdef)
+    {
+      tree new_vuse = *reaching_vuse;
+      tree *renamed = renames->get (new_vuse);
+      if (renamed)
+        new_vuse = *renamed;
+      LOOP_VINFO_EARLY_BRK_VUSES (loop_vinfo).safe_push ({stmt, new_vuse});
+      if (dump_enabled_p ())
+	  dump_printf_loc (MSG_NOTE, vect_location,
+			   "current_use: %T, new_use: %T, mem_ref: %G",
+			   *current_vdef, new_vuse, stmt);
+
+      if (!renamed)
+	{
+	  if (dump_enabled_p ())
+	    dump_printf_loc (MSG_NOTE, vect_location,
+			     "stored: %T -> %T\n", *current_vdef, new_vuse);
+
+	  renames->put (*current_vdef, new_vuse);
+	}
+    }
+
+  return true;
+}
+
 /* Function vect_analyze_data_ref_dependences.
 
    Examine all the data references in the loop, and make sure there do not
@@ -612,6 +884,84 @@ vect_analyze_data_ref_dependences (loop_vec_info loop_vinfo,
 	  return res;
       }
 
+  /* If we have early break statements in the loop, check to see if they
+     are of a form we can vectorizer.  */
+  if (LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
+    {
+      hash_set<tree> chain, fixed;
+      auto_vec<tree> loads;
+      auto_vec<data_reference *> bases;
+      hash_map<tree, tree> renames;
+      basic_block dest_bb = NULL;
+      tree vdef = NULL;
+      tree vuse = NULL;
+
+      if (dump_enabled_p ())
+	  dump_printf_loc (MSG_NOTE, vect_location,
+			   "loop contains multiple exits, analyzing"
+			   " statement dependencies.\n");
+
+      for (gcond *c : LOOP_VINFO_LOOP_CONDS (loop_vinfo))
+	{
+	  stmt_vec_info loop_cond_info = loop_vinfo->lookup_stmt (c);
+	  if (STMT_VINFO_TYPE (loop_cond_info) != loop_exit_ctrl_vec_info_type)
+	    continue;
+
+	  gimple *stmt = STMT_VINFO_STMT (loop_cond_info);
+	  gimple_stmt_iterator gsi = gsi_for_stmt (stmt);
+
+	  /* Initiaze the vuse chain with the one at the early break.  */
+	  if (!vuse)
+	    vuse = gimple_vuse (c);
+
+	  if (!validate_early_exit_stmts (loop_vinfo, &chain, &fixed, &loads,
+					  &bases, &vuse, &vdef, &gsi, &renames))
+	    return opt_result::failure_at (stmt,
+					   "can't safely apply code motion to "
+					   "dependencies of %G to vectorize "
+					   "the early exit.\n", stmt);
+
+	  /* Save destination as we go, BB are visited in order and the last one
+	     is where statements should be moved to.  */
+	  if (!dest_bb)
+	    dest_bb = gimple_bb (c);
+	  else
+	    {
+	      basic_block curr_bb = gimple_bb (c);
+	      if (dominated_by_p (CDI_DOMINATORS, curr_bb, dest_bb))
+		dest_bb = curr_bb;
+	    }
+	}
+
+      dest_bb = FALLTHRU_EDGE (dest_bb)->dest;
+      gcc_assert (dest_bb);
+      LOOP_VINFO_EARLY_BRK_DEST_BB (loop_vinfo) = dest_bb;
+
+      /* Do some renaming to update the uses chain.  */
+      for (unsigned i = 0; i < LOOP_VINFO_EARLY_BRK_VUSES (loop_vinfo).length (); i++)
+	{
+	  auto g = LOOP_VINFO_EARLY_BRK_VUSES (loop_vinfo)[i];
+	  tree *tmp = renames.get (g.second);
+	  if (tmp)
+	    LOOP_VINFO_EARLY_BRK_VUSES (loop_vinfo)[i]
+	      = std::make_pair (g.first, *tmp);
+	}
+
+      /* TODO: Remove? It's useful debug statement but may be too much.  */
+      for (auto g : LOOP_VINFO_EARLY_BRK_VUSES (loop_vinfo))
+	{
+	  if (dump_enabled_p ())
+	    dump_printf_loc (MSG_NOTE, vect_location,
+			     "overrode use: %T, mem_ref: %G",
+			     g.second, g.first);
+	}
+
+      if (dump_enabled_p ())
+	  dump_printf_loc (MSG_NOTE, vect_location,
+			   "recorded statements to be moved to BB %d\n",
+			   LOOP_VINFO_EARLY_BRK_DEST_BB (loop_vinfo)->index);
+    }
+
   return opt_result::success ();
 }
 
diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 43fcce8ad45..a4117eb980a 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -11188,6 +11188,45 @@ update_epilogue_loop_vinfo (class loop *epilogue, tree advance)
   epilogue_vinfo->shared->save_datarefs ();
 }
 
+/*  When vectorizing early break statements instructions that happen before
+    the early break in the current BB need to be moved to after the early
+    break.  This function deals with that and assumes that any validity
+    checks has already been performed.
+
+    While moving the instructions if it encounters a VUSE or VDEF it then
+    corrects the VUSES as it moves the statements along.  GDEST is the location
+    in which to insert the new statements.  */
+
+static void
+move_early_exit_stmts (loop_vec_info loop_vinfo)
+{
+  /* Move all stmts that need moving.  */
+  basic_block dest_bb = LOOP_VINFO_EARLY_BRK_DEST_BB (loop_vinfo);
+  gimple_stmt_iterator dest_gsi = gsi_start_bb (dest_bb);
+
+  for (gimple *stmt : LOOP_VINFO_EARLY_BRK_CONFLICT_STMTS (loop_vinfo))
+    {
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_NOTE, vect_location, "moving stmt %G", stmt);
+
+      gimple_stmt_iterator stmt_gsi = gsi_for_stmt (stmt);
+      gsi_move_before (&stmt_gsi, &dest_gsi);
+      gsi_prev (&dest_gsi);
+      update_stmt (stmt);
+    }
+
+  /* Update all the stmts with their new reaching VUSES.  */
+  for (auto p : LOOP_VINFO_EARLY_BRK_VUSES (loop_vinfo))
+    {
+      if (dump_enabled_p ())
+	  dump_printf_loc (MSG_NOTE, vect_location,
+			   "updating vuse %G", p.first);
+      unlink_stmt_vdef (p.first);
+      gimple_set_vuse (p.first, p.second);
+      update_stmt (p.first);
+    }
+}
+
 /* Function vect_transform_loop.
 
    The analysis phase has determined that the loop is vectorizable.
@@ -11326,6 +11365,11 @@ vect_transform_loop (loop_vec_info loop_vinfo, gimple *loop_vectorized_call)
       vect_schedule_slp (loop_vinfo, LOOP_VINFO_SLP_INSTANCES (loop_vinfo));
     }
 
+  /* Handle any code motion that we need to for early-break vectorization after
+     we've done peeling but just before we start vectorizing.  */
+  if (LOOP_VINFO_EARLY_BREAKS (loop_vinfo))
+    move_early_exit_stmts (loop_vinfo);
+
   /* FORNOW: the vectorizer supports only loops which body consist
      of one basic block (header + empty latch). When the vectorizer will
      support more involved loop forms, the order by which the BBs are
diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
index 1cc003c12e2..ec65b65b591 100644
--- a/gcc/tree-vectorizer.h
+++ b/gcc/tree-vectorizer.h
@@ -919,6 +919,19 @@ public:
      analysis.  */
   vec<_loop_vec_info *> epilogue_vinfos;
 
+  /* Used to store the list of statements needing to be moved if doing early
+     break vectorization as they would violate the scalar loop semantics if
+     vectorized in their current location.  */
+  auto_vec<gimple *> early_break_conflict;
+
+  /* The final basic block where to move statements to.  In the case of
+     multiple exits this could be pretty far away.  */
+  basic_block early_break_dest_bb;
+
+  /* Statements whose VUSES need updating if early break vectorization is to
+     happen.  */
+  auto_vec<std::pair<gimple*, tree>> early_break_vuses;
+
 } *loop_vec_info;
 
 /* Access Functions.  */
@@ -972,6 +985,9 @@ public:
 #define LOOP_VINFO_REDUCTION_CHAINS(L)     (L)->reduction_chains
 #define LOOP_VINFO_PEELING_FOR_GAPS(L)     (L)->peeling_for_gaps
 #define LOOP_VINFO_PEELING_FOR_NITER(L)    (L)->peeling_for_niter
+#define LOOP_VINFO_EARLY_BRK_CONFLICT_STMTS(L) (L)->early_break_conflict
+#define LOOP_VINFO_EARLY_BRK_DEST_BB(L)    (L)->early_break_dest_bb
+#define LOOP_VINFO_EARLY_BRK_VUSES(L)      (L)->early_break_vuses
 #define LOOP_VINFO_LOOP_CONDS(L)           (L)->conds
 #define LOOP_VINFO_LOOP_IV_COND(L)         (L)->loop_iv_cond
 #define LOOP_VINFO_NO_DATA_DEPENDENCIES(L) (L)->no_data_dependencies