[PATCH] Improve RTL ifcvt for empty else_bb (PR rtl-optimization/80491)

[Jakub Jelinek <jakub at redhat dot com>]

Hi!

The following patch is a partial fix for PR80491, an improvement for
if-conversion if there is empty else_bb.

What we do right now is in that case we only look at the immediately
preceeding (non-debug/non-note) instruction before cond_earliest
and if it is not the set of x, we just turn it into attempt to
use previous value of x rather than whatever it has been initialized to.

On the testcases from the PR, we have:
(insn 7 4 8 2 (set (reg:DI 97 [ _15+8 ])
        (const_int 0 [0])) 81 {*movdi_internal}
     (nil))
(insn 8 7 9 2 (set (reg:DI 104 [ a_11(D)->low ])
        (mem:DI (reg/v/f:DI 101 [ a ]) [2 a_11(D)->low+0 S8 A64])) 81 {*movdi_internal}
     (nil))
(insn 9 8 10 2 (set (reg:DI 105 [ b_12(D)->low ])
        (mem:DI (reg/v/f:DI 102 [ b ]) [2 b_12(D)->low+0 S8 A64])) 81 {*movdi_internal}
     (nil))
(insn 10 9 11 2 (parallel [
            (set (reg:CCC 17 flags)
                (compare:CCC (plus:DI (reg:DI 104 [ a_11(D)->low ])
                        (reg:DI 105 [ b_12(D)->low ]))
                    (reg:DI 104 [ a_11(D)->low ])))
            (set (reg:DI 103)
                (plus:DI (reg:DI 104 [ a_11(D)->low ])
                    (reg:DI 105 [ b_12(D)->low ])))
        ]) 316 {*adddi3_cc_overflow_1}
     (expr_list:REG_DEAD (reg:DI 105 [ b_12(D)->low ])
        (expr_list:REG_DEAD (reg:DI 104 [ a_11(D)->low ])
            (nil))))
(jump_insn 11 10 14 2 (set (pc)
        (if_then_else (ltu (reg:CCC 17 flags)
                (const_int 0 [0]))
            (label_ref 14)
            (pc))) 617 {*jcc_1}
     (expr_list:REG_DEAD (reg:CCC 17 flags)
        (int_list:REG_BR_PROB 4 (nil)))
 -> 14)
(code_label 14 11 35 3 3 (nil) [1 uses])
(note 35 14 15 3 [bb 3] NOTE_INSN_BASIC_BLOCK)
(insn 15 35 16 3 (set (reg:DI 97 [ _15+8 ])
        (const_int 1 [0x1])) 81 {*movdi_internal}
     (nil))
(code_label 16 15 36 4 2 (nil) [0 uses])
If insn 7 would come after insn 9 (which doesn't change the behavior, as
insn 8 and insn 9 don't clobber pseudo 97 and const_int 0 is constant),
we'd turn that into a setcc pattern, but otherwise we fail.

This patch let us search for x's setter earlier in the bb.
During testing I found that modified_in_p/modified_in_between_p don't
actually take into account calls that could change MEMs, so the patch
handles that too.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

During bootstrap/regtest it seems to trigger over 10000 times, but
my statistics collection only noted cases where it found an earlier setter
and the noce_process_if_block has been successful, has not attempted to
check if insn_b/set_b was NULL whether it would otherwise fail (but even
in cases where it doesn't fail perhaps with the patch it can generate
better code).

2017-04-25  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/80491
	* ifcvt.c (noce_process_if_block): When looking for x setter
	with missing else_bb, don't check only the insn right before
	cond_earliest, but look for the last insn that x is modified in
	within the same bb.

--- gcc/ifcvt.c.jj	2017-04-24 19:28:02.151975658 +0200
+++ gcc/ifcvt.c	2017-04-25 16:22:16.760928057 +0200
@@ -3440,7 +3440,36 @@ noce_process_if_block (struct noce_if_in
     }
   else
     {
-      insn_b = prev_nonnote_nondebug_insn (if_info->cond_earliest);
+      bool call_crossed = false;
+      insn_b = if_info->cond_earliest;
+      do
+	{
+	  insn_b = prev_nonnote_nondebug_insn (insn_b);
+	  if (!insn_b
+	      || (BLOCK_FOR_INSN (insn_b)
+		  != BLOCK_FOR_INSN (if_info->cond_earliest))
+	      || modified_in_p (x, insn_b))
+	    break;
+	  /* modified_in_p does not consider calls changing MEMs,
+	     so punt on calls if x or SET_SRC (set_b) are MEMs that
+	     could be changed by calls.  */
+	  if (CALL_P (insn_b))
+	    {
+	      call_crossed = true;
+	      if (MEM_P (x) && !MEM_READONLY_P (x))
+		break;
+	    }
+	}
+      while (1);
+      if (!call_crossed)
+	for (rtx_insn *insn_c = if_info->cond_earliest; insn_c != jump;
+	     insn_c = NEXT_INSN (insn_c))
+	  if (CALL_P (insn_c))
+	    {
+	      call_crossed = true;
+	      break;
+	    }
+
       /* We're going to be moving the evaluation of B down from above
 	 COND_EARLIEST to JUMP.  Make sure the relevant data is still
 	 intact.  */
@@ -3468,6 +3497,29 @@ noce_process_if_block (struct noce_if_in
 	  insn_b = NULL;
 	  set_b = NULL_RTX;
 	}
+      else if (call_crossed)
+	{
+	  subrtx_iterator::array_type array;
+	  /* Punt if there are any non-readonly MEMs and there are any
+	     calls in between insn_b and jump.  Even for
+	     RTL_CONST_OR_PURE_CALL_P calls the call argument stack slots
+	     might be modified, so unless we are prepared to carefully
+	     analyze what might be modified and what can't, just punt.  */
+	  FOR_EACH_SUBRTX (iter, array, SET_SRC (set_b), ALL)
+	    if (MEM_P (*iter) && !MEM_READONLY_P (*iter))
+	      {
+		insn_b = NULL;
+		set_b = NULL_RTX;
+		break;
+	      }
+	  FOR_EACH_SUBRTX (iter, array, x, ALL)
+	    if (MEM_P (*iter) && !MEM_READONLY_P (*iter))
+	      {
+		insn_b = NULL;
+		set_b = NULL_RTX;
+		break;
+	      }
+	}
     }
 
   /* If x has side effects then only the if-then-else form is safe to

	Jakub