[backport r171347 from gcc-4.7/trunk, part 1/2 of ARM PR51442 fix <mikpe@it.uu.se>]

Date: Wed, 23 Mar 2011 12:00:34 +0000
From: Julian Brown <julian at codesourcery dot com>
Subject: Re: [PATCH] Volatile bitfields vs. inline asm memory constraints
List-Archive: <http://gcc.gnu.org/ml/gcc-patches/>

On Wed, 9 Feb 2011 09:37:41 -0500
Diego Novillo <dnovillo@google.com> wrote:

> On Mon, Nov 22, 2010 at 08:28, Julian Brown <julian@codesourcery.com>
> wrote:
> > Hi,
> >
> > This patch fixes the issue in the (Launchpad, not GCC) bug tracker:
> >
> > https://bugs.launchpad.net/gcc-linaro/+bug/675347
> >
> > The problem was introduced by the patch from DJ to honour volatile
> > bitfield types:
> >
> > http://gcc.gnu.org/ml/gcc-patches/2010-06/msg01167.html
> >
> > but not exposed (on ARM) until the option was made the default (on
> > the Linaro branch) -- it's not yet the default on mainline.
> >
> > The issue is as follows: after DJ's patch and with
> > -fstrict-volatile-bitfields, in expr.c:expand_expr_real_1, the if
> > condition with the comment "In cases where an aligned union has an
> > unaligned object as a field, we might be extracting a BLKmode value
> > from an integer-mode (e.g., SImode) object [...]" triggers for a
> > normal (non-bitfield) volatile field of a struct/class.
> >
> 
> Could you add a comment before the test to describe why you are
> excluding non-natural alignments?
> 
> OK with that change, though I think you'll have to stage this for 4.7.

I've committed this version now we're in stage 1. (I also re-tested it
for good measure, no regressions with cross to ARM Linux.)

Thanks,

Julian

gcc/

2011-03-23  Julian Brown  <julian@codesourcery.com>

	* expr.c (expand_expr_real_1): Only use BLKmode for volatile
	accesses which are not naturally aligned.

--- gcc-4.6.2/gcc/expr.c.~1~	2011-08-10 11:17:32.000000000 +0200
+++ gcc-4.6.2/gcc/expr.c	2011-12-10 14:20:50.000000000 +0100
@@ -9189,8 +9189,11 @@ expand_expr_real_1 (tree exp, rtx target
 		&& modifier != EXPAND_CONST_ADDRESS
 		&& modifier != EXPAND_INITIALIZER)
 	    /* If the field is volatile, we always want an aligned
-	       access.  */
-	    || (volatilep && flag_strict_volatile_bitfields > 0)
+	       access.  Only do this if the access is not already naturally
+	       aligned, otherwise "normal" (non-bitfield) volatile fields
+	       become non-addressable.  */
+	    || (volatilep && flag_strict_volatile_bitfields > 0
+		&& (bitpos % GET_MODE_ALIGNMENT (mode) != 0))
 	    /* If the field isn't aligned enough to fetch as a memref,
 	       fetch it as a bit field.  */
 	    || (mode1 != BLKmode