[PATCH]: Stack alignment

["John David Anglin" <dave at hiauly1 dot hia dot nrc dot ca>]

> > When STACK_BOUNDARY is set to 64, there is one fill word in the frame
> > as a result of the starting frame offset and the need to align it to a
> > STACK_BOUNDARY.  If STACK_BOUNDARY were set to 512, we would have 15
> > fill words.
> 
> Eh?  Is the stack pointer aligned or not?

This turned out to be totally wrong.  Here is the actual problem.

Reload aligns the stack in each iteration.  It uses a default alignment
value of BIGGEST_ALIGNMENT.  If STARTING_FRAME_OFFSET % BIGGEST_ALIGNMENT
is not zero, the rounding in assign_stack_temp_for_type will cause the
frame size to be non-zero.  As a result, you will never have a non-empty
frame and code for doing the stack adjustments needs to be emitted in
the prologue and epilogue.

The enclosed patch changes assign_stack_temp_for_type to only do the
reload alignment when the current frame size is not zero.  With this
fix, I can save eight bytes in the stack offset on the 64-bit PA ports.

It has been tested with no regressions on hppa2.0w-hp-hpux11.11 and
hppa64-hp-hpux11.11.

Ok for trunk and 3.3?  The reason for installing on 3.3 is that I
would like to revert the change that I made to STARTING_FRAME_OFFSET
on the 64-bit ports when I found that we never had a non-empty frame.

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6605)

2003-02-17  John David Anglin  <dave.anglin@nrc-cnrc.gc.ca>

	* function.c (assign_stack_temp_for_type): Don't create a non-zero
	frame when doing a stack alignment if the current frame size is zero.

Index: function.c
===================================================================
RCS file: /cvsroot/gcc/gcc/gcc/function.c,v
retrieving revision 1.399
diff -u -3 -p -r1.399 function.c
--- function.c	1 Feb 2003 18:59:44 -0000	1.399
+++ function.c	17 Feb 2003 17:06:06 -0000
@@ -564,16 +564,28 @@ assign_stack_local_1 (mode, size, align,
   frame_off = STARTING_FRAME_OFFSET % frame_alignment;
   frame_phase = frame_off ? frame_alignment - frame_off : 0;
 
-  /* Round frame offset to that alignment.
-     We must be careful here, since FRAME_OFFSET might be negative and
-     division with a negative dividend isn't as well defined as we might
-     like.  So we instead assume that ALIGNMENT is a power of two and
-     use logical operations which are unambiguous.  */
+  /* Round the frame offset to the specified alignment unless the
+     frame size is zero and this is a frame alignment request.  When
+     STARTING_FRAME_OFFSET is non-zero, we don't want a frame alignment
+     request to create a frame if one doesn't already exist.  */
+  if (function->x_frame_offset
+      || mode != BLKmode
+      || size != 0)
+    {
+      /*  We must be careful here, since FRAME_OFFSET might be negative and
+	  division with a negative dividend isn't as well defined as we might
+	  like.  So we instead assume that ALIGNMENT is a power of two and
+	  use logical operations which are unambiguous.  */
 #ifdef FRAME_GROWS_DOWNWARD
-  function->x_frame_offset = FLOOR_ROUND (function->x_frame_offset - frame_phase, alignment) + frame_phase;
+      function->x_frame_offset
+	= (FLOOR_ROUND (function->x_frame_offset - frame_phase, alignment)
+	   + frame_phase);
 #else
-  function->x_frame_offset = CEIL_ROUND (function->x_frame_offset - frame_phase, alignment) + frame_phase;
+      function->x_frame_offset
+	= (CEIL_ROUND (function->x_frame_offset - frame_phase, alignment)
+	   + frame_phase);
 #endif
+    }
 
   /* On a big-endian machine, if we are allocating more space than we will use,
      use the least significant bytes of those that are allocated.  */