PR middle-end/44382: Tree reassociation improvement

gcc/

2011-09-06  Enkovich Ilya  <ilya.enkovich@intel.com>

	PR middle-end/44382
	* target.def (reassociation_width): New hook.

	* doc/tm.texi.in (reassociation_width): Likewise.

	* doc/tm.texi (reassociation_width): Likewise.

	* doc/invoke.texi (tree-reassoc-width): New param documented.

	* hooks.h (hook_int_uint_mode_1): New default hook.

	* hooks.c (hook_int_uint_mode_1): Likewise.

	* config/i386/i386.h (ix86_tune_indices): Add
	X86_TUNE_REASSOC_INT_TO_PARALLEL and
	X86_TUNE_REASSOC_FP_TO_PARALLEL.

	(TARGET_REASSOC_INT_TO_PARALLEL): New.
	(TARGET_REASSOC_FP_TO_PARALLEL): Likewise.

	* config/i386/i386.c (initial_ix86_tune_features): Add
	X86_TUNE_REASSOC_INT_TO_PARALLEL and
	X86_TUNE_REASSOC_FP_TO_PARALLEL.

	(ix86_reassociation_width) implementation of
	new hook for i386 target.

	* params.def (PARAM_TREE_REASSOC_WIDTH): New param added.

	* tree-ssa-reassoc.c (get_required_cycles): New function.
	(get_reassociation_width): Likewise.
	(swap_ops_for_binary_stmt): Likewise.
	(rewrite_expr_tree_parallel): Likewise.

	(rewrite_expr_tree): Refactored. Part of code moved into
	swap_ops_for_binary_stmt.

	(reassociate_bb): Now checks reassociation width to be used
	and call rewrite_expr_tree_parallel instead of rewrite_expr_tree
	if needed.

gcc/testsuite/

2011-09-06  Enkovich Ilya  <ilya.enkovich@intel.com>

	* gcc.dg/tree-ssa/pr38533.c (dg-options): Added option
	--param tree-reassoc-width=1.

	* gcc.dg/tree-ssa/reassoc-24.c: New test.
	* gcc.dg/tree-ssa/reassoc-25.c: Likewise.

From-SVN: r178602

15 files changed, 425 insertions, 49 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index f867f97..bf50b80 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,46 @@
+2011-09-06  Enkovich Ilya  <ilya.enkovich@intel.com>
+
+	PR middle-end/44382
+	* target.def (reassociation_width): New hook.
+
+	* doc/tm.texi.in (reassociation_width): Likewise.
+
+	* doc/tm.texi (reassociation_width): Likewise.
+
+	* doc/invoke.texi (tree-reassoc-width): New param documented.
+
+	* hooks.h (hook_int_uint_mode_1): New default hook.
+
+	* hooks.c (hook_int_uint_mode_1): Likewise.
+
+	* config/i386/i386.h (ix86_tune_indices): Add
+	X86_TUNE_REASSOC_INT_TO_PARALLEL and
+	X86_TUNE_REASSOC_FP_TO_PARALLEL.
+
+	(TARGET_REASSOC_INT_TO_PARALLEL): New.
+	(TARGET_REASSOC_FP_TO_PARALLEL): Likewise.
+
+	* config/i386/i386.c (initial_ix86_tune_features): Add
+	X86_TUNE_REASSOC_INT_TO_PARALLEL and
+	X86_TUNE_REASSOC_FP_TO_PARALLEL.
+
+	(ix86_reassociation_width) implementation of
+	new hook for i386 target.
+
+	* params.def (PARAM_TREE_REASSOC_WIDTH): New param added.
+
+	* tree-ssa-reassoc.c (get_required_cycles): New function.
+	(get_reassociation_width): Likewise.
+	(swap_ops_for_binary_stmt): Likewise.
+	(rewrite_expr_tree_parallel): Likewise.
+
+	(rewrite_expr_tree): Refactored. Part of code moved into
+	swap_ops_for_binary_stmt.
+
+	(reassociate_bb): Now checks reassociation width to be used
+	and call rewrite_expr_tree_parallel instead of rewrite_expr_tree
+	if needed.
+
 2011-09-06  Richard Guenther  <rguenther@suse.de>
 
 	PR tree-optimization/47025
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index a9c0aa7..555db59 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -2168,7 +2168,15 @@ static unsigned int initial_ix86_tune_features[X86_TUNE_LAST] = {
 
   /* X86_TUNE_AVX128_OPTIMAL: Enable 128-bit AVX instruction generation for
      the auto-vectorizer.  */
-  m_BDVER
+  m_BDVER,
+
+  /* X86_TUNE_REASSOC_INT_TO_PARALLEL: Try to produce parallel computations
+     during reassociation of integer computation.  */
+  m_ATOM,
+
+  /* X86_TUNE_REASSOC_FP_TO_PARALLEL: Try to produce parallel computations
+     during reassociation of fp computation.  */
+  m_ATOM
 };
 
 /* Feature tests against the various architecture variations.  */
@@ -34843,6 +34851,8 @@ ix86_enum_va_list (int idx, const char **pname, tree *ptree)
 #define TARGET_SCHED_DISPATCH has_dispatch
 #undef TARGET_SCHED_DISPATCH_DO
 #define TARGET_SCHED_DISPATCH_DO do_dispatch
+#undef TARGET_SCHED_REASSOCIATION_WIDTH
+#define TARGET_SCHED_REASSOCIATION_WIDTH ix86_reassociation_width
 
 /* The size of the dispatch window is the total number of bytes of
    object code allowed in a window.  */
@@ -35640,6 +35650,32 @@ has_dispatch (rtx insn, int action)
   return false;
 }
 
+/* Implementation of reassociation_width target hook used by
+   reassoc phase to identify parallelism level in reassociated
+   tree.  Statements tree_code is passed in OPC.  Arguments type
+   is passed in MODE.
+
+   Currently parallel reassociation is enabled for Atom
+   processors only and we set reassociation width to be 2
+   because Atom may issue up to 2 instructions per cycle.
+
+   Return value should be fixed if parallel reassociation is
+   enabled for other processors.  */
+
+static int
+ix86_reassociation_width (unsigned int opc ATTRIBUTE_UNUSED,
+			  enum machine_mode mode)
+{
+  int res = 1;
+
+  if (INTEGRAL_MODE_P (mode) && TARGET_REASSOC_INT_TO_PARALLEL)
+    res = 2;
+  else if (FLOAT_MODE_P (mode) && TARGET_REASSOC_FP_TO_PARALLEL)
+    res = 2;
+
+  return res;
+}
+
 /* ??? No autovectorization into MMX or 3DNOW until we can reliably
    place emms and femms instructions.  */
 
diff --git a/gcc/config/i386/i386.h b/gcc/config/i386/i386.h
index 47442a0e..21f5075 100644
--- a/gcc/config/i386/i386.h
+++ b/gcc/config/i386/i386.h
@@ -318,6 +318,8 @@ enum ix86_tune_indices {
   X86_TUNE_VECTORIZE_DOUBLE,
   X86_TUNE_SOFTWARE_PREFETCHING_BENEFICIAL,
   X86_TUNE_AVX128_OPTIMAL,
+  X86_TUNE_REASSOC_INT_TO_PARALLEL,
+  X86_TUNE_REASSOC_FP_TO_PARALLEL,
 
   X86_TUNE_LAST
 };
@@ -416,6 +418,11 @@ extern unsigned char ix86_tune_features[X86_TUNE_LAST];
 	ix86_tune_features[X86_TUNE_SOFTWARE_PREFETCHING_BENEFICIAL]
 #define TARGET_AVX128_OPTIMAL \
 	ix86_tune_features[X86_TUNE_AVX128_OPTIMAL]
+#define TARGET_REASSOC_INT_TO_PARALLEL \
+	ix86_tune_features[X86_TUNE_REASSOC_INT_TO_PARALLEL]
+#define TARGET_REASSOC_FP_TO_PARALLEL \
+	ix86_tune_features[X86_TUNE_REASSOC_FP_TO_PARALLEL]
+
 /* Feature tests against the various architecture variations.  */
 enum ix86_arch_indices {
   X86_ARCH_CMOVE,		/* || TARGET_SSE */
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index c5b19eb..b1ff187 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -9076,6 +9076,11 @@ The smallest number of different values for which it is best to use a
 jump-table instead of a tree of conditional branches.  If the value is
 0, use the default for the machine.  The default is 0.
 
+@item tree-reassoc-width
+Set the maximum number of instructions executed in parallel in
+reassociated tree. This parameter overrides target dependent
+heuristics used by default if has non zero value.
+
 @end table
 @end table
 
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 7364aa1..335c1d1 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -6840,6 +6840,11 @@ the order of instructions is important for correctness when scheduling, but
 also the latencies of operations.
 @end deftypevr
 
+@deftypefn {Target Hook} int TARGET_SCHED_REASSOCIATION_WIDTH (unsigned int @var{opc}, enum machine_mode @var{mode})
+This hook is called by tree reassociator to determine a level of
+parallelism required in output calculations chain.
+@end deftypefn
+
 @node Sections
 @section Dividing the Output into Sections (Texts, Data, @dots{})
 @c the above section title is WAY too long.  maybe cut the part between
diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
index 4535fd6..6783826 100644
--- a/gcc/doc/tm.texi.in
+++ b/gcc/doc/tm.texi.in
@@ -6774,6 +6774,8 @@ in its second parameter.
 
 @hook TARGET_SCHED_EXPOSED_PIPELINE
 
+@hook TARGET_SCHED_REASSOCIATION_WIDTH
+
 @node Sections
 @section Dividing the Output into Sections (Texts, Data, @dots{})
 @c the above section title is WAY too long.  maybe cut the part between
diff --git a/gcc/hooks.c b/gcc/hooks.c
index 9025183..1ba44f9 100644
--- a/gcc/hooks.c
+++ b/gcc/hooks.c
@@ -161,6 +161,13 @@ default_can_output_mi_thunk_no_vcall (const_tree a ATTRIBUTE_UNUSED,
 }
 
 int
+hook_int_uint_mode_1 (unsigned int a ATTRIBUTE_UNUSED,
+		      enum machine_mode b ATTRIBUTE_UNUSED)
+{
+  return 1;
+}
+
+int
 hook_int_const_tree_0 (const_tree a ATTRIBUTE_UNUSED)
 {
   return 0;
diff --git a/gcc/hooks.h b/gcc/hooks.h
index 156d708..54ace24 100644
--- a/gcc/hooks.h
+++ b/gcc/hooks.h
@@ -68,6 +68,7 @@ extern void hook_void_tree_treeptr (tree, tree *);
 extern void hook_void_int_int (int, int);
 extern void hook_void_gcc_optionsp (struct gcc_options *);
 
+extern int hook_int_uint_mode_1 (unsigned int, enum machine_mode);
 extern int hook_int_const_tree_0 (const_tree);
 extern int hook_int_const_tree_const_tree_1 (const_tree, const_tree);
 extern int hook_int_rtx_0 (rtx);
diff --git a/gcc/params.def b/gcc/params.def
index e8372ed..c376c80 100644
--- a/gcc/params.def
+++ b/gcc/params.def
@@ -914,6 +914,13 @@ DEFPARAM (PARAM_ALLOW_STORE_DATA_RACES,
 	  "Allow new data races on stores to be introduced",
 	  1, 0, 1)
 
+/* Reassociation width to be used by tree reassoc optimization.  */
+DEFPARAM (PARAM_TREE_REASSOC_WIDTH,
+	  "tree-reassoc-width",
+	  "Set the maximum number of instructions executed in parallel in "
+	  "reassociated tree. If 0, use the target dependent heuristic.",
+	  0, 0, 0)
+
 
 /*
 Local variables:
diff --git a/gcc/target.def b/gcc/target.def
index 857f463..1e09ba7 100644
--- a/gcc/target.def
+++ b/gcc/target.def
@@ -913,6 +913,16 @@ the order of instructions is important for correctness when scheduling, but\n\
 also the latencies of operations.",
 bool, false)
 
+/* The following member value is a function that returns number
+   of operations reassociator should try to put in parallel for
+   statements of the given type.  By default 1 is used.  */
+DEFHOOK
+(reassociation_width,
+"This hook is called by tree reassociator to determine a level of\n\
+parallelism required in output calculations chain.",
+int, (unsigned int opc, enum machine_mode mode),
+hook_int_uint_mode_1)
+
 HOOK_VECTOR_END (sched)
 
 /* Functions relating to vectorization.  */
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index c1c0073..db1c7fd 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,11 @@
+2011-09-06  Enkovich Ilya  <ilya.enkovich@intel.com>
+
+	* gcc.dg/tree-ssa/pr38533.c (dg-options): Added option
+	--param tree-reassoc-width=1.
+
+	* gcc.dg/tree-ssa/reassoc-24.c: New test.
+	* gcc.dg/tree-ssa/reassoc-25.c: Likewise.
+
 2011-09-06  Richard Guenther  <rguenther@suse.de>
 
 	PR tree-optimization/48149
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr38533.c b/gcc/testsuite/gcc.dg/tree-ssa/pr38533.c
index e787227..a80a5a8 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/pr38533.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr38533.c
@@ -1,6 +1,6 @@
 /* PR middle-end/38533 */
 /* { dg-do compile } */
-/* { dg-options "-O2 -fdump-tree-reassoc1" } */
+/* { dg-options "-O2 --param tree-reassoc-width=1 -fdump-tree-reassoc1" } */
 
 #define A asm volatile ("" : "=r" (f) : "0" (0)); e |= f;
 #define B A A A A A A A A A A A
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/reassoc-24.c b/gcc/testsuite/gcc.dg/tree-ssa/reassoc-24.c
new file mode 100644
index 0000000..c871628
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/reassoc-24.c
@@ -0,0 +1,25 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 --param tree-reassoc-width=2 -fdump-tree-reassoc1" } */
+
+unsigned int
+foo (void)
+{
+  unsigned int a = 0;
+  unsigned int b;
+
+  asm volatile ("" : "=r" (b) : "0" (0));
+  a += b;
+  asm volatile ("" : "=r" (b) : "0" (0));
+  a += b;
+  asm volatile ("" : "=r" (b) : "0" (0));
+  a += b;
+  asm volatile ("" : "=r" (b) : "0" (0));
+  a += b;
+
+  return a;
+}
+
+/* Verify there are two pairs of __asm__ statements with no
+   intervening stmts.  */
+/* { dg-final { scan-tree-dump-times "__asm__\[^;\n]*;\n *__asm__" 2 "reassoc1"} } */
+/* { dg-final { cleanup-tree-dump "reassoc1" } } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/reassoc-25.c b/gcc/testsuite/gcc.dg/tree-ssa/reassoc-25.c
new file mode 100644
index 0000000..4ff66ef
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/reassoc-25.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 --param tree-reassoc-width=3 -fdump-tree-reassoc1-details" } */
+
+unsigned int
+foo (int a, int b, int c, int d)
+{
+  unsigned int s = 0;
+
+  s += a;
+  s += b;
+  s += c;
+  s += d;
+
+  return s;
+}
+
+/* Verify reassociation width was chosen to be 2.  */
+/* { dg-final { scan-tree-dump-times "Width = 2" 1 "reassoc1"} } */
+/* { dg-final { cleanup-tree-dump "reassoc1" } } */
diff --git a/gcc/tree-ssa-reassoc.c b/gcc/tree-ssa-reassoc.c
index 51f7ef8..03e0672 100644
--- a/gcc/tree-ssa-reassoc.c
+++ b/gcc/tree-ssa-reassoc.c
@@ -40,6 +40,8 @@ along with GCC; see the file COPYING3.  If not see
 #include "pointer-set.h"
 #include "cfgloop.h"
 #include "flags.h"
+#include "target.h"
+#include "params.h"
 
 /*  This is a simple global reassociation pass.  It is, in part, based
     on the LLVM pass of the same name (They do some things more/less
@@ -1617,6 +1619,62 @@ remove_visited_stmt_chain (tree var)
     }
 }
 
+/* This function checks three consequtive operands in
+   passed operands vector OPS starting from OPINDEX and
+   swaps two operands if it is profitable for binary operation
+   consuming OPINDEX + 1 abnd OPINDEX + 2 operands.
+
+   We pair ops with the same rank if possible.
+
+   The alternative we try is to see if STMT is a destructive
+   update style statement, which is like:
+   b = phi (a, ...)
+   a = c + b;
+   In that case, we want to use the destructive update form to
+   expose the possible vectorizer sum reduction opportunity.
+   In that case, the third operand will be the phi node. This
+   check is not performed if STMT is null.
+
+   We could, of course, try to be better as noted above, and do a
+   lot of work to try to find these opportunities in >3 operand
+   cases, but it is unlikely to be worth it.  */
+
+static void
+swap_ops_for_binary_stmt (VEC(operand_entry_t, heap) * ops,
+			  unsigned int opindex, gimple stmt)
+{
+  operand_entry_t oe1, oe2, oe3;
+
+  oe1 = VEC_index (operand_entry_t, ops, opindex);
+  oe2 = VEC_index (operand_entry_t, ops, opindex + 1);
+  oe3 = VEC_index (operand_entry_t, ops, opindex + 2);
+
+  if ((oe1->rank == oe2->rank
+       && oe2->rank != oe3->rank)
+      || (stmt && is_phi_for_stmt (stmt, oe3->op)
+	  && !is_phi_for_stmt (stmt, oe1->op)
+	  && !is_phi_for_stmt (stmt, oe2->op)))
+    {
+      struct operand_entry temp = *oe3;
+      oe3->op = oe1->op;
+      oe3->rank = oe1->rank;
+      oe1->op = temp.op;
+      oe1->rank= temp.rank;
+    }
+  else if ((oe1->rank == oe3->rank
+	    && oe2->rank != oe3->rank)
+	   || (stmt && is_phi_for_stmt (stmt, oe2->op)
+	       && !is_phi_for_stmt (stmt, oe1->op)
+	       && !is_phi_for_stmt (stmt, oe3->op)))
+    {
+      struct operand_entry temp = *oe2;
+      oe2->op = oe1->op;
+      oe2->rank = oe1->rank;
+      oe1->op = temp.op;
+      oe1->rank= temp.rank;
+    }
+}
+
 /* Recursively rewrite our linearized statements so that the operators
    match those in OPS[OPINDEX], putting the computation in rank
    order.  */
@@ -1629,53 +1687,10 @@ rewrite_expr_tree (gimple stmt, unsigned int opindex,
   tree rhs2 = gimple_assign_rhs2 (stmt);
   operand_entry_t oe;
 
-  /* If we have three operands left, then we want to make sure the one
-     that gets the double binary op are the ones with the same rank.
-
-     The alternative we try is to see if this is a destructive
-     update style statement, which is like:
-     b = phi (a, ...)
-     a = c + b;
-     In that case, we want to use the destructive update form to
-     expose the possible vectorizer sum reduction opportunity.
-     In that case, the third operand will be the phi node.
-
-     We could, of course, try to be better as noted above, and do a
-     lot of work to try to find these opportunities in >3 operand
-     cases, but it is unlikely to be worth it.  */
+  /* If we have three operands left, then we want to make sure the ones
+     that get the double binary op are chosen wisely.  */
   if (opindex + 3 == VEC_length (operand_entry_t, ops))
-    {
-      operand_entry_t oe1, oe2, oe3;
-
-      oe1 = VEC_index (operand_entry_t, ops, opindex);
-      oe2 = VEC_index (operand_entry_t, ops, opindex + 1);
-      oe3 = VEC_index (operand_entry_t, ops, opindex + 2);
-
-      if ((oe1->rank == oe2->rank
-	   && oe2->rank != oe3->rank)
-	  || (is_phi_for_stmt (stmt, oe3->op)
-	      && !is_phi_for_stmt (stmt, oe1->op)
-	      && !is_phi_for_stmt (stmt, oe2->op)))
-	{
-	  struct operand_entry temp = *oe3;
-	  oe3->op = oe1->op;
-	  oe3->rank = oe1->rank;
-	  oe1->op = temp.op;
-	  oe1->rank= temp.rank;
-	}
-      else if ((oe1->rank == oe3->rank
-		&& oe2->rank != oe3->rank)
-	       || (is_phi_for_stmt (stmt, oe2->op)
-		   && !is_phi_for_stmt (stmt, oe1->op)
-		   && !is_phi_for_stmt (stmt, oe3->op)))
-	{
-	  struct operand_entry temp = *oe2;
-	  oe2->op = oe1->op;
-	  oe2->rank = oe1->rank;
-	  oe1->op = temp.op;
-	  oe1->rank= temp.rank;
-	}
-    }
+    swap_ops_for_binary_stmt (ops, opindex, stmt);
 
   /* The final recursion case for this function is that you have
      exactly two operations left.
@@ -1760,6 +1775,178 @@ rewrite_expr_tree (gimple stmt, unsigned int opindex,
   rewrite_expr_tree (SSA_NAME_DEF_STMT (rhs1), opindex + 1, ops, moved);
 }
 
+/* Find out how many cycles we need to compute statements chain.
+   OPS_NUM holds number os statements in a chain.  CPU_WIDTH is a
+   maximum number of independent statements we may execute per cycle.  */
+
+static int
+get_required_cycles (int ops_num, int cpu_width)
+{
+  int res;
+  int elog;
+  unsigned int rest;
+
+  /* While we have more than 2 * cpu_width operands
+     we may reduce number of operands by cpu_width
+     per cycle.  */
+  res = ops_num / (2 * cpu_width);
+
+  /* Remained operands count may be reduced twice per cycle
+     until we have only one operand.  */
+  rest = (unsigned)(ops_num - res * cpu_width);
+  elog = exact_log2 (rest);
+  if (elog >= 0)
+    res += elog;
+  else
+    res += floor_log2 (rest) + 1;
+
+  return res;
+}
+
+/* Returns an optimal number of registers to use for computation of
+   given statements.  */
+
+static int
+get_reassociation_width (int ops_num, enum tree_code opc,
+			 enum machine_mode mode)
+{
+  int param_width = PARAM_VALUE (PARAM_TREE_REASSOC_WIDTH);
+  int width;
+  int width_min;
+  int cycles_best;
+
+  if (param_width > 0)
+    width = param_width;
+  else
+    width = targetm.sched.reassociation_width (opc, mode);
+
+  if (width == 1)
+    return width;
+
+  /* Get the minimal time required for sequence computation.  */
+  cycles_best = get_required_cycles (ops_num, width);
+
+  /* Check if we may use less width and still compute sequence for
+     the same time.  It will allow us to reduce registers usage.
+     get_required_cycles is monotonically increasing with lower width
+     so we can perform a binary search for the minimal width that still
+     results in the optimal cycle count.  */
+  width_min = 1;
+  while (width > width_min)
+    {
+      int width_mid = (width + width_min) / 2;
+
+      if (get_required_cycles (ops_num, width_mid) == cycles_best)
+	width = width_mid;
+      else if (width_min < width_mid)
+	width_min = width_mid;
+      else
+	break;
+    }
+
+  return width;
+}
+
+/* Recursively rewrite our linearized statements so that the operators
+   match those in OPS[OPINDEX], putting the computation in rank
+   order and trying to allow operations to be executed in
+   parallel.  */
+
+static void
+rewrite_expr_tree_parallel (gimple stmt, int width,
+			    VEC(operand_entry_t, heap) * ops)
+{
+  enum tree_code opcode = gimple_assign_rhs_code (stmt);
+  int op_num = VEC_length (operand_entry_t, ops);
+  int stmt_num = op_num - 1;
+  gimple *stmts = XALLOCAVEC (gimple, stmt_num);
+  int op_index = op_num - 1;
+  int stmt_index = 0;
+  int ready_stmts_end = 0;
+  int i = 0;
+  tree last_rhs1 = gimple_assign_rhs1 (stmt);
+  tree lhs_var;
+
+  /* We start expression rewriting from the top statements.
+     So, in this loop we create a full list of statements
+     we will work with.  */
+  stmts[stmt_num - 1] = stmt;
+  for (i = stmt_num - 2; i >= 0; i--)
+    stmts[i] = SSA_NAME_DEF_STMT (gimple_assign_rhs1 (stmts[i+1]));
+
+  lhs_var = create_tmp_reg (TREE_TYPE (last_rhs1), NULL);
+  add_referenced_var (lhs_var);
+
+  for (i = 0; i < stmt_num; i++)
+    {
+      tree op1, op2;
+
+      /* Determine whether we should use results of
+	 already handled statements or not.  */
+      if (ready_stmts_end == 0
+	  && (i - stmt_index >= width || op_index < 1))
+	ready_stmts_end = i;
+
+      /* Now we choose operands for the next statement.  Non zero
+	 value in ready_stmts_end means here that we should use
+	 the result of already generated statements as new operand.  */
+      if (ready_stmts_end > 0)
+	{
+	  op1 = gimple_assign_lhs (stmts[stmt_index++]);
+	  if (ready_stmts_end > stmt_index)
+	    op2 = gimple_assign_lhs (stmts[stmt_index++]);
+	  else if (op_index >= 0)
+	    op2 = VEC_index (operand_entry_t, ops, op_index--)->op;
+	  else
+	    {
+	      gcc_assert (stmt_index < i);
+	      op2 = gimple_assign_lhs (stmts[stmt_index++]);
+	    }
+
+	  if (stmt_index >= ready_stmts_end)
+	    ready_stmts_end = 0;
+	}
+      else
+	{
+	  if (op_index > 1)
+	    swap_ops_for_binary_stmt (ops, op_index - 2, NULL);
+	  op2 = VEC_index (operand_entry_t, ops, op_index--)->op;
+	  op1 = VEC_index (operand_entry_t, ops, op_index--)->op;
+	}
+
+      /* If we emit the last statement then we should put
+	 operands into the last statement.  It will also
+	 break the loop.  */
+      if (op_index < 0 && stmt_index == i)
+	i = stmt_num - 1;
+
+      if (dump_file && (dump_flags & TDF_DETAILS))
+	{
+	  fprintf (dump_file, "Transforming ");
+	  print_gimple_stmt (dump_file, stmts[i], 0, 0);
+	}
+
+      /* We keep original statement only for the last one.  All
+	 others are recreated.  */
+      if (i == stmt_num - 1)
+	{
+	  gimple_assign_set_rhs1 (stmts[i], op1);
+	  gimple_assign_set_rhs2 (stmts[i], op2);
+	  update_stmt (stmts[i]);
+	}
+      else
+	stmts[i] = build_and_add_sum (lhs_var, op1, op2, opcode);
+
+      if (dump_file && (dump_flags & TDF_DETAILS))
+	{
+	  fprintf (dump_file, " into ");
+	  print_gimple_stmt (dump_file, stmts[i], 0, 0);
+	}
+    }
+
+  remove_visited_stmt_chain (last_rhs1);
+}
+
 /* Transform STMT, which is really (A +B) + (C + D) into the left
    linear form, ((A+B)+C)+D.
    Recurse on D if necessary.  */
@@ -2282,7 +2469,21 @@ reassociate_bb (basic_block bb)
 		    }
 		}
 	      else
-		rewrite_expr_tree (stmt, 0, ops, false);
+		{
+		  enum machine_mode mode = TYPE_MODE (TREE_TYPE (lhs));
+		  int ops_num = VEC_length (operand_entry_t, ops);
+		  int width = get_reassociation_width (ops_num, rhs_code, mode);
+
+		  if (dump_file && (dump_flags & TDF_DETAILS))
+		    fprintf (dump_file,
+			     "Width = %d was chosen for reassociation\n", width);
+
+		  if (width > 1
+		      && VEC_length (operand_entry_t, ops) > 3)
+		    rewrite_expr_tree_parallel (stmt, width, ops);
+		  else
+		    rewrite_expr_tree (stmt, 0, ops, false);
+		}
 
 	      VEC_free (operand_entry_t, heap, ops);
 	    }