re PR rtl-optimization/31455 (lower subreg causes a performance regression in…

re PR rtl-optimization/31455 (lower subreg causes a performance regression in the inner loop sometimes)

	PR rtl-optimization/31455
	* lower-subreg.c (find_decomposable_subregs): Don't decompose
	subregs which have a cast between modes which are not tieable.

From-SVN: r125265

gcc/ChangeLog

+2007-06-01  Ian Lance Taylor  <iant@google.com>
+
+	PR rtl-optimization/31455
+	* lower-subreg.c (find_decomposable_subregs): Don't decompose
+	subregs which have a cast between modes which are not tieable.
+
 2007-06-01  Uros Bizjak  <ubizjak@gmail.com>
 
 	* expr.c (force_operand) [DIV, MOD, UDIV, UMOD, ASHIFTRT]: Remove

gcc/lower-subreg.c

@@ -281,6 +281,18 @@ find_decomposable_subregs (rtx *px, void *data)
 	  bitmap_set_bit (decomposable_context, regno);
 	  return -1;
 	}
+
+      /* If this is a cast from one mode to another, where the modes
+	 have the same size, and they are not tieable, then mark this
+	 register as non-decomposable.  If we decompose it we are
+	 likely to mess up whatever the backend is trying to do.  */
+      if (outer_words > 1
+	  && outer_size == inner_size
+	  && !MODES_TIEABLE_P (GET_MODE (x), GET_MODE (inner)))
+	{
+	  bitmap_set_bit (non_decomposable_context, regno);
+	  return -1;
+	}
     }
   else if (REG_P (x))
     {