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Volatile bitfields vs. inline asm memory constraints

Message ID 20110323120034.2471dcad@rex.config
State New
Headers show

Commit Message

Julian Brown March 23, 2011, noon UTC 
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

ChangeLog

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

Index: gcc/expr.c
===================================================================
--- gcc/expr.c	(revision 171288)
+++ gcc/expr.c	(working copy)
@@ -9202,8 +9202,11 @@  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