Re: Fix 21166

[Nathan Sidwell <nathan at codesourcery dot com>]

Richard Henderson wrote:

> Gimme a new patch with exactly what you have in mind and I'll
> try to poke holes in it.

ok, here's a new version.  It takes advantage of your observation that
alignment might be constrained by something else in the struct.

I'm slightly uncomfortable, from a language abstractness POV, that the
packedness of one field can depend on the alignment constraints of another
field.  But then, specifying packing puts us squarely in implementation
defined land.

booted & tested on i686-pc-linux-gnu.

nathan
--
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

2005-05-10  Nathan Sidwell  <nathan@codesourcery.com>

	* stor-layout.c (finalize_type_size): Undo DECL_PACKED when possible.

Index: stor-layout.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/stor-layout.c,v
retrieving revision 1.230
diff -c -3 -p -r1.230 stor-layout.c
*** stor-layout.c	27 Apr 2005 16:02:43 -0000	1.230
--- stor-layout.c	9 May 2005 13:16:34 -0000
*************** finalize_type_size (tree type)
*** 1404,1409 ****
--- 1404,1411 ----
  void
  finish_record_layout (record_layout_info rli, int free_p)
  {
+   tree field;
+   
    /* Compute the final size.  */
    finalize_record_size (rli);
  
*************** finish_record_layout (record_layout_info
*** 1413,1418 ****
--- 1415,1429 ----
    /* Perform any last tweaks to the TYPE_SIZE, etc.  */
    finalize_type_size (rli->t);
  
+   /* We might be able to clear DECL_PACKED on any members that happen
+      to be suitably aligned (not forgetting the alignment of the type
+      itself).  */
+   for (field = TYPE_FIELDS (rli->t); field; field = TREE_CHAIN (field))
+     if (TREE_CODE (field) == FIELD_DECL && DECL_PACKED (field)
+ 	&& DECL_OFFSET_ALIGN (field) >= TYPE_ALIGN (TREE_TYPE (field))
+ 	&& TYPE_ALIGN (rli->t) >= TYPE_ALIGN (TREE_TYPE (field)))
+       DECL_PACKED (field) = 0;
+   
    /* Lay out any static members.  This is done now because their type
       may use the record's type.  */
    while (rli->pending_statics)

// Copyright (C) 2005 Free Software Foundation, Inc.
// Contributed by Nathan Sidwell 26 Apr 2005 <nathan@codesourcery.com>

// DR21166. unnecessary error on packed char

struct s1 {
        char    c1;
} __attribute__((packed));

char&
f(struct s1 *s)
{
        return s->c1;
}

char *
g(struct s1 *s)
{
        return &s->c1;
}