alpha alias tweeks

[Richard Henderson]

Here's a little bit to try and reduce the conflicts produced
by subword references, which use an AND on ev5 and earlier.

There are some remaining problems wrt alias sets, in that we
cannot use alias sets with AND at all at the moment -- the
early exit based on differing alias sets happens before any
tests for the additional global variable aliasing created by
the AND.  This primarily affects HImode, since QImode must 
always be considered to alias.  I'm not sure if it is worth
slowing down other ports for this one case.

On a related topic to the alias set thing, one thing I could
use down here is knowledge of how large the types or objects
involved.  For instance, 

	int i; short s;
	s = 1;
	i = 1;

may alias, but

	int i; short s[10];
	s[4] = 1;
	i = 1;

cannot.  It would also be nice if I could limit the additional
AND aliasing to global symbols.  Things allocated from the heap
and stack are always sufficiently aligned not to incurr such
additional aliasing.

Anyone have any objections to his patch?


r~


	* alias.c (base_alias_check): Accept new args for the modes of the
	two references.  Use them to determine if an AND can overlap.  Update
	all callers.  
	(memrefs_conflict_p): Assume sizes are aligned, and uses them
	to determine if an AND can overlap.

Index: alias.c
===================================================================
RCS file: /egcs/carton/cvsfiles/egcs/gcc/alias.c,v
retrieving revision 1.39
diff -u -p -r1.39 alias.c
--- alias.c	1998/08/25 12:01:52	1.39
+++ alias.c	1998/09/23 20:29:07
@@ -36,7 +36,8 @@ static int memrefs_conflict_p		PROTO((in
 					       HOST_WIDE_INT));
 static void record_set			PROTO((rtx, rtx));
 static rtx find_base_term		PROTO((rtx));
-static int base_alias_check		PROTO((rtx, rtx));
+static int base_alias_check		PROTO((rtx, rtx, enum machine_mode,
+					       enum machine_mode));
 static rtx find_base_value		PROTO((rtx));
 
 /* Set up all info needed to perform alias analysis on memory references.  */
@@ -596,8 +597,9 @@ find_base_term (x)
    objects, 1 if they might be pointers to the same object.  */
 
 static int
-base_alias_check (x, y)
+base_alias_check (x, y, x_mode, y_mode)
      rtx x, y;
+     enum machine_mode x_mode, y_mode;
 {
   rtx x_base = find_base_term (x);
   rtx y_base = find_base_term (y);
@@ -629,17 +631,23 @@ base_alias_check (x, y)
   if (rtx_equal_p (x_base, y_base))
     return 1;
 
-  /* The base addresses of the read and write are different
-     expressions.  If they are both symbols and they are not accessed
-     via AND, there is no conflict.  */
-  /* XXX: We can bring knowledge of object alignment and offset into 
-     play here.  For example, on alpha, "char a, b;" can alias one
-     another, though "char a; long b;" cannot.  Similarly, offsets
-     into strutures may be brought into play.  Given "char a, b[40];",
-     a and b[1] may overlap, but a and b[20] do not.  */
+  /* The base addresses of the read and write are different expressions. 
+     If they are both symbols and they are not accessed via AND, there is
+     no conflict.  We can bring knowledge of object alignment into play
+     here.  For example, on alpha, "char a, b;" can alias one another,
+     though "char a; long b;" cannot.  */
   if (GET_CODE (x_base) != ADDRESS && GET_CODE (y_base) != ADDRESS)
     {
-      return GET_CODE (x) == AND || GET_CODE (y) == AND;
+      if (GET_CODE (x) == AND && GET_CODE (y) == AND)
+	return 1;
+      if (GET_CODE (x) == AND
+	  && (GET_CODE (XEXP (x, 1)) != CONST_INT
+	      || GET_MODE_UNIT_SIZE (y_mode) < -INTVAL (XEXP (x, 1))))
+	return 1;
+      if (GET_CODE (y) == AND
+	  && (GET_CODE (XEXP (y, 1)) != CONST_INT
+	      || GET_MODE_UNIT_SIZE (x_mode) < -INTVAL (XEXP (y, 1))))
+	return 1;
     }
 
   /* If one address is a stack reference there can be no alias:
@@ -811,18 +819,24 @@ memrefs_conflict_p (xsize, x, ysize, y, 
       }
 
   /* Treat an access through an AND (e.g. a subword access on an Alpha)
-     as an access with indeterminate size.
-     ??? Could instead convert an n byte reference at (and x y) to an
-     n-y byte reference at (plus x y). */
+     as an access with indeterminate size.  Assume that references 
+     besides AND are aligned, so if the size of the other reference is
+     at least as large as the alignment, assume no other overlap.  */
   if (GET_CODE (x) == AND && GET_CODE (XEXP (x, 1)) == CONST_INT)
-    return memrefs_conflict_p (-1, XEXP (x, 0), ysize, y, c);
+    {
+      if (ysize < -INTVAL (XEXP (x, 1)))
+	xsize = -1;
+      return memrefs_conflict_p (xsize, XEXP (x, 0), ysize, y, c);
+    }
   if (GET_CODE (y) == AND && GET_CODE (XEXP (y, 1)) == CONST_INT)
     {
-      /* XXX: If we are indexing far enough into the array/structure, we
+      /* ??? If we are indexing far enough into the array/structure, we
 	 may yet be able to determine that we can not overlap.  But we 
 	 also need to that we are far enough from the end not to overlap
-	 a following reference, so we do nothing for now.  */
-      return memrefs_conflict_p (xsize, x, -1, XEXP (y, 0), c);
+	 a following reference, so we do nothing with that for now.  */
+      if (xsize < -INTVAL (XEXP (y, 1)))
+	ysize = -1;
+      return memrefs_conflict_p (xsize, x, ysize, XEXP (y, 0), c);
     }
 
   if (CONSTANT_P (x))
@@ -917,15 +931,15 @@ true_dependence (mem, mem_mode, x, varie
   if (RTX_UNCHANGING_P (x) && ! RTX_UNCHANGING_P (mem))
     return 0;
 
-  if (! base_alias_check (XEXP (x, 0), XEXP (mem, 0)))
+  if (mem_mode == VOIDmode)
+    mem_mode = GET_MODE (mem);
+
+  if (! base_alias_check (XEXP (x, 0), XEXP (mem, 0), GET_MODE (x), mem_mode))
     return 0;
 
   x_addr = canon_rtx (XEXP (x, 0));
   mem_addr = canon_rtx (XEXP (mem, 0));
 
-  if (mem_mode == VOIDmode)
-    mem_mode = GET_MODE (mem);
-
   if (! memrefs_conflict_p (GET_MODE_SIZE (mem_mode), mem_addr,
 			    SIZE_FOR_MODE (x), x_addr, 0))
     return 0;
@@ -974,7 +987,8 @@ anti_dependence (mem, x)
   if (RTX_UNCHANGING_P (mem))
     return 0;
 
-  if (! base_alias_check (XEXP (x, 0), XEXP (mem, 0)))
+  if (! base_alias_check (XEXP (x, 0), XEXP (mem, 0), GET_MODE (x),
+			  GET_MODE (mem)))
     return 0;
 
   x = canon_rtx (x);
@@ -1008,7 +1022,8 @@ output_dependence (mem, x)
   if (MEM_VOLATILE_P (x) && MEM_VOLATILE_P (mem))
     return 1;
 
-  if (! base_alias_check (XEXP (x, 0), XEXP (mem, 0)))
+  if (! base_alias_check (XEXP (x, 0), XEXP (mem, 0), GET_MODE (x),
+			  GET_MODE (mem)))
     return 0;
 
   x = canon_rtx (x);