[PATCH] Remove inefficient branchless conditional negate optimization

["Wilco Dijkstra" <wdijkstr at arm dot com>]

Several GCC versions ago a conditional negate optimization was introduced as a workaround for
PR45685. However the branchless expansion for conditional negate is extremely inefficient on most
targets (5 sequentially dependent instructions rather than 2 on AArch64). Since the underlying issue
has been resolved (the example in PR45685 no longer generates a branch on x64), remove the
workaround so that conditional negates are treated in exactly the same way as conditional invert,
add, subtract, and, orr, xor etc. Simple example:

int f(int x) { if (x > 3) x = -x; return x; }

Before:
        cmp     w0, 3
        cset    w2, gt
        neg     w1, w2
        eor     w0, w0, w1
        add     w0, w2, w0
        ret

        xorl    %ecx, %ecx
        cmpl    $3, %edi
        setg    %cl
        movl    %ecx, %edx
        negl    %edx
        xorl    %edx, %edi
        leal    (%rcx,%rdi), %eax
        ret

After:
        cmp     w0, 4
        csneg   w0, w0, w0, lt
        ret

        movl    %edi, %edx
        movl    %edi, %eax
        negl    %edx
        cmpl    $4, %edi
        cmovge  %edx, %eax
        ret

ChangeLog: 
2015-02-26  Wilco Dijkstra  wdijkstr@arm.com

	* gcc/tree-ssa-phiopt.c (neg_replacement): Remove.
	(tree_ssa_phiopt_worker): Remove negate optimization.

---
 gcc/tree-ssa-phiopt.c | 156 --------------------------------------------------
 1 file changed, 156 deletions(-)

diff --git a/gcc/tree-ssa-phiopt.c b/gcc/tree-ssa-phiopt.c
index bad546d..38a59ab 100644
--- a/gcc/tree-ssa-phiopt.c
+++ b/gcc/tree-ssa-phiopt.c
@@ -96,8 +96,6 @@ static bool minmax_replacement (basic_block, basic_block,
 				edge, edge, gimple, tree, tree);
 static bool abs_replacement (basic_block, basic_block,
 			     edge, edge, gimple, tree, tree);
-static bool neg_replacement (basic_block, basic_block,
-			     edge, edge, gimple, tree, tree);
 static bool cond_store_replacement (basic_block, basic_block, edge, edge,
 				    hash_set<tree> *);
 static bool cond_if_else_store_replacement (basic_block, basic_block, basic_block);
@@ -209,23 +207,6 @@ tree_ssa_phiopt_worker (bool do_store_elim, bool do_hoist_loads)
     /* Calculate the set of non-trapping memory accesses.  */
     nontrap = get_non_trapping ();
 
-  /* The replacement of conditional negation with a non-branching
-     sequence is really only a win when optimizing for speed and we
-     can avoid transformations by gimple if-conversion that result
-     in poor RTL generation.
-
-     Ideally either gimple if-conversion or the RTL expanders will
-     be improved and the code to emit branchless conditional negation
-     can be removed.  */
-  bool replace_conditional_negation = false;
-  if (!do_store_elim)
-    replace_conditional_negation
-      = ((!optimize_size && optimize >= 2)
-	 || (((flag_tree_loop_vectorize || cfun->has_force_vectorize_loops)
-	      && flag_tree_loop_if_convert != 0)
-	     || flag_tree_loop_if_convert == 1
-	     || flag_tree_loop_if_convert_stores == 1));
-
   /* Search every basic block for COND_EXPR we may be able to optimize.
 
      We walk the blocks in order that guarantees that a block with
@@ -380,9 +361,6 @@ tree_ssa_phiopt_worker (bool do_store_elim, bool do_hoist_loads)
 	    cfgchanged = true;
 	  else if (abs_replacement (bb, bb1, e1, e2, phi, arg0, arg1))
 	    cfgchanged = true;
-	  else if (replace_conditional_negation
-		   && neg_replacement (bb, bb1, e1, e2, phi, arg0, arg1))
-	    cfgchanged = true;
 	  else if (minmax_replacement (bb, bb1, e1, e2, phi, arg0, arg1))
 	    cfgchanged = true;
 	}
@@ -1294,140 +1272,6 @@ abs_replacement (basic_block cond_bb, basic_block middle_bb,
   return true;
 }
 
-/*  The function neg_replacement replaces conditional negation with
-    equivalent straight line code.  Returns TRUE if replacement is done,
-    otherwise returns FALSE.
-
-    COND_BB branches around negation occuring in MIDDLE_BB.
-
-    E0 and E1 are edges out of COND_BB.  E0 reaches MIDDLE_BB and
-    E1 reaches the other successor which should contain PHI with
-    arguments ARG0 and ARG1.
-
-    Assuming negation is to occur when the condition is true,
-    then the non-branching sequence is:
-
-       result = (rhs ^ -cond) + cond
-
-    Inverting the condition or its result gives us negation
-    when the original condition is false.  */
-
-static bool
-neg_replacement (basic_block cond_bb, basic_block middle_bb,
-		 edge e0 ATTRIBUTE_UNUSED, edge e1,
-		 gimple phi, tree arg0, tree arg1)
-{
-  gimple new_stmt, cond;
-  gimple_stmt_iterator gsi;
-  gimple assign;
-  edge true_edge, false_edge;
-  tree rhs, lhs;
-  enum tree_code cond_code;
-  bool invert = false;
-
-  /* This transformation performs logical operations on the
-     incoming arguments.  So force them to be integral types.   */
-  if (!INTEGRAL_TYPE_P (TREE_TYPE (arg0)))
-    return false;
-
-  /* OTHER_BLOCK must have only one executable statement which must have the
-     form arg0 = -arg1 or arg1 = -arg0.  */
-
-  assign = last_and_only_stmt (middle_bb);
-  /* If we did not find the proper negation assignment, then we can not
-     optimize.  */
-  if (assign == NULL)
-    return false;
-
-  /* If we got here, then we have found the only executable statement
-     in OTHER_BLOCK.  If it is anything other than arg0 = -arg1 or
-     arg1 = -arg0, then we can not optimize.  */
-  if (gimple_code (assign) != GIMPLE_ASSIGN)
-    return false;
-
-  lhs = gimple_assign_lhs (assign);
-
-  if (gimple_assign_rhs_code (assign) != NEGATE_EXPR)
-    return false;
-
-  rhs = gimple_assign_rhs1 (assign);
-
-  /* The assignment has to be arg0 = -arg1 or arg1 = -arg0.  */
-  if (!(lhs == arg0 && rhs == arg1)
-      && !(lhs == arg1 && rhs == arg0))
-    return false;
-
-  /* The basic sequence assumes we negate when the condition is true.
-     If we need the opposite, then we will either need to invert the
-     condition or its result.  */
-  extract_true_false_edges_from_block (cond_bb, &true_edge, &false_edge);
-  invert = false_edge->dest == middle_bb;
-
-  /* Unlike abs_replacement, we can handle arbitrary conditionals here.  */
-  cond = last_stmt (cond_bb);
-  cond_code = gimple_cond_code (cond);
-
-  /* If inversion is needed, first try to invert the test since
-     that's cheapest.  */
-  if (invert)
-    {
-      bool honor_nans = HONOR_NANS (gimple_cond_lhs (cond));
-      enum tree_code new_code = invert_tree_comparison (cond_code, honor_nans);
-
-      /* If invert_tree_comparison was successful, then use its return
-	 value as the new code and note that inversion is no longer
-	 needed.  */
-      if (new_code != ERROR_MARK)
-	{
-	  cond_code = new_code;
-	  invert = false;
-	}
-    }
-
-  tree cond_val = make_ssa_name (boolean_type_node);
-  new_stmt = gimple_build_assign (cond_val, cond_code,
-				  gimple_cond_lhs (cond),
-				  gimple_cond_rhs (cond));
-  gsi = gsi_last_bb (cond_bb);
-  gsi_insert_before (&gsi, new_stmt, GSI_NEW_STMT);
-
-  /* If we still need inversion, then invert the result of the
-     condition.  */
-  if (invert)
-    {
-      tree tmp = make_ssa_name (boolean_type_node);
-      new_stmt = gimple_build_assign (tmp, BIT_XOR_EXPR, cond_val,
-				      boolean_true_node);
-      gsi_insert_after (&gsi, new_stmt, GSI_NEW_STMT);
-      cond_val = tmp;
-    }
-
-  /* Get the condition in the right type so that we can perform
-     logical and arithmetic operations on it.  */
-  tree cond_val_converted = make_ssa_name (TREE_TYPE (rhs));
-  new_stmt = gimple_build_assign (cond_val_converted, NOP_EXPR, cond_val);
-  gsi_insert_after (&gsi, new_stmt, GSI_NEW_STMT);
-
-  tree neg_cond_val_converted = make_ssa_name (TREE_TYPE (rhs));
-  new_stmt = gimple_build_assign (neg_cond_val_converted, NEGATE_EXPR,
-				  cond_val_converted);
-  gsi_insert_after (&gsi, new_stmt, GSI_NEW_STMT);
-
-  tree tmp = make_ssa_name (TREE_TYPE (rhs));
-  new_stmt = gimple_build_assign (tmp, BIT_XOR_EXPR, rhs,
-				  neg_cond_val_converted);
-  gsi_insert_after (&gsi, new_stmt, GSI_NEW_STMT);
-
-  tree new_lhs = make_ssa_name (TREE_TYPE (rhs));
-  new_stmt = gimple_build_assign (new_lhs, PLUS_EXPR, tmp, cond_val_converted);
-  gsi_insert_after (&gsi, new_stmt, GSI_NEW_STMT);
-
-  replace_phi_edge_with_variable (cond_bb, e1, phi, new_lhs);
-
-  /* Note that we optimized this PHI.  */
-  return true;
-}
-
 /* Auxiliary functions to determine the set of memory accesses which
    can't trap because they are preceded by accesses to the same memory
    portion.  We do that for MEM_REFs, so we only need to track
-- 
1.9.1