Add a shallow_copy_rtvec function

[Richard Sandiford <rdsandiford@googlemail.com>]

The patch I'm about to post needs to copy an rtvec without copy_rtxing
the elements themselves.  This kind of copy is really the rtvec equivalent
of a shallow_copy_rtx, so here's a patch to add it under the name
shallow_copy_rtvec.

I searched gcc/ for existing cases where this might be useful, but the
only obvious one I could see was cselib_subst_to_values.  I think the
new structure is clearer, and since we're doing the "old != new"
comparisons anyway, we might as well avoid dirtying the cache in
cases where we know "old == new".

Tested on x86_64-linux-gnu.  OK to install?

Richard


gcc/
	* rtl.h (shallow_copy_rtvec): Declare.
	* rtl.c (shallow_copy_rtvec): New function.
	* cselib.c (cselib_subst_to_values): Use it.  Only modify an
	rtx field if the subrtx has changed.

Index: gcc/rtl.h
===================================================================
--- gcc/rtl.h	2009-10-22 21:00:52.000000000 +0100
+++ gcc/rtl.h	2009-10-22 21:03:37.000000000 +0100
@@ -1568,6 +1568,7 @@ extern rtx rtx_alloc_stat (RTX_CODE MEM_
 #define rtx_alloc(c) rtx_alloc_stat (c MEM_STAT_INFO)
 
 extern rtvec rtvec_alloc (int);
+extern rtvec shallow_copy_rtvec (rtvec);
 extern bool shared_const_p (const_rtx);
 extern rtx copy_rtx (rtx);
 extern void dump_rtx_statistics (void);
Index: gcc/rtl.c
===================================================================
--- gcc/rtl.c	2009-10-22 21:00:52.000000000 +0100
+++ gcc/rtl.c	2009-10-22 21:03:37.000000000 +0100
@@ -164,6 +164,20 @@ rtvec_alloc (int n)
   return rt;
 }
 
+/* Create a bitwise copy of VEC.  */
+
+rtvec
+shallow_copy_rtvec (rtvec vec)
+{
+  rtvec newvec;
+  int n;
+
+  n = GET_NUM_ELEM (vec);
+  newvec = rtvec_alloc (n);
+  memcpy (&newvec->elem[0], &vec->elem[0], sizeof (rtx) * n);
+  return newvec;
+}
+
 /* Return the number of bytes occupied by rtx value X.  */
 
 unsigned int
Index: gcc/cselib.c
===================================================================
--- gcc/cselib.c	2009-10-22 21:00:52.000000000 +0100
+++ gcc/cselib.c	2009-10-22 21:03:37.000000000 +0100
@@ -1422,30 +1422,31 @@ cselib_subst_to_values (rtx x)
 	{
 	  rtx t = cselib_subst_to_values (XEXP (x, i));
 
-	  if (t != XEXP (x, i) && x == copy)
-	    copy = shallow_copy_rtx (x);
-
-	  XEXP (copy, i) = t;
+	  if (t != XEXP (x, i))
+	    {
+	      if (x == copy)
+		copy = shallow_copy_rtx (x);
+	      XEXP (copy, i) = t;
+	    }
 	}
       else if (fmt[i] == 'E')
 	{
-	  int j, k;
+	  int j;
 
 	  for (j = 0; j < XVECLEN (x, i); j++)
 	    {
 	      rtx t = cselib_subst_to_values (XVECEXP (x, i, j));
 
-	      if (t != XVECEXP (x, i, j) && XVEC (x, i) == XVEC (copy, i))
+	      if (t != XVECEXP (x, i, j))
 		{
-		  if (x == copy)
-		    copy = shallow_copy_rtx (x);
-
-		  XVEC (copy, i) = rtvec_alloc (XVECLEN (x, i));
-		  for (k = 0; k < j; k++)
-		    XVECEXP (copy, i, k) = XVECEXP (x, i, k);
+		  if (XVEC (x, i) == XVEC (copy, i))
+		    {
+		      if (x == copy)
+			copy = shallow_copy_rtx (x);
+		      XVEC (copy, i) = shallow_copy_rtvec (XVEC (x, i));
+		    }
+		  XVECEXP (copy, i, j) = t;
 		}
-
-	      XVECEXP (copy, i, j) = t;
 	    }
 	}
     }