Re: [PATCH][RTL-ifcvt] PR rtl-optimization/68841: Make sure one basic block doesn't clobber CC reg usage of the other

[Bernd Schmidt <bschmidt at redhat dot com>]

On 01/05/2016 03:22 PM, Kyrill Tkachov wrote:
> This works around the issue but we don't want to do perform the check
> for pairs of
> simple basic blocks because then we'll end up rejecting code that does
> things like:
> x = cond ? x + 1 : x - 1
> i.e. source of the set in both blocks reads and writes the same register.
> We can deal with this safely later on in the function since we rename
> the destinations
> of the two sets, so we don't want to reject this case here.

So we need to teach bbs_ok_for_cmove_arith that this is going to happen. 
How about the approach below? Still seems to fix the issue, and it looks 
like the CC set is present in the df info so everything should work as 
intended. Right?


Bernd

Index: gcc/ifcvt.c
===================================================================
--- gcc/ifcvt.c	(revision 232056)
+++ gcc/ifcvt.c	(working copy)
@@ -1866,11 +1866,13 @@ insn_valid_noce_process_p (rtx_insn *ins
 }
 
 
-/* Return true iff the registers that the insns in BB_A set do not
-   get used in BB_B.  */
+/* Return true iff the registers that the insns in BB_A set do not get
+   used in BB_B.  WILL_RENAME is true if we expect that BB_A consists
+   of a single single_set insn with a destination that the caller will
+   rename to a new pseudo.  */
 
 static bool
-bbs_ok_for_cmove_arith (basic_block bb_a, basic_block bb_b)
+bbs_ok_for_cmove_arith (basic_block bb_a, basic_block bb_b, bool will_rename)
 {
   rtx_insn *a_insn;
   bitmap bba_sets = BITMAP_ALLOC (&reg_obstack);
@@ -1887,13 +1889,15 @@ bbs_ok_for_cmove_arith (basic_block bb_a
 
       if (!sset_a)
 	{
+	  gcc_assert (!will_rename);
 	  BITMAP_FREE (bba_sets);
 	  return false;
 	}
 
       /* Record all registers that BB_A sets.  */
       FOR_EACH_INSN_DEF (def, a_insn)
-	bitmap_set_bit (bba_sets, DF_REF_REGNO (def));
+	if (!will_rename || DF_REF_REG (def) != SET_DEST (sset_a))
+	  bitmap_set_bit (bba_sets, DF_REF_REGNO (def));
     }
 
   rtx_insn *b_insn;
@@ -2082,9 +2086,9 @@ noce_try_cmove_arith (struct noce_if_inf
 	}
     }
 
-  if (then_bb && else_bb && !a_simple && !b_simple
-      && (!bbs_ok_for_cmove_arith (then_bb, else_bb)
-	  || !bbs_ok_for_cmove_arith (else_bb, then_bb)))
+  if (then_bb && else_bb
+      && (!bbs_ok_for_cmove_arith (then_bb, else_bb, a_simple)
+	  || !bbs_ok_for_cmove_arith (else_bb, then_bb, b_simple)))
     return FALSE;
 
   start_sequence ();