[Bug middle-end/39954] [4.5 Regression] Revision 146817 caused unaligned access in gcc.dg/torture/pr26565.c

[rguenther at suse dot de]

------- Comment #17 from rguenther at suse dot de  2009-05-06 11:45 -------
Subject: Re:  [4.5 Regression] Revision 146817 caused
 unaligned access in gcc.dg/torture/pr26565.c

On Wed, 6 May 2009, matz at gcc dot gnu dot org wrote:

> ------- Comment #16 from matz at gcc dot gnu dot org  2009-05-06 11:20 -------
> Option (2) lies somewhere in between, although it should be nearly equivalent
> to option 1, as there aren't many other contexts than COMPONENT_REF where
> a pure FIELD_DECL could be used.  But option (2) has the advantage that the
> structure type is completely laid out already, not so for option (1).  That
> might be important for performance sometimes, e.g. when the user declares
> a field as packed (or aligned(1)), but it so happens (due to other members)
> that it got it's natural alignment in the end.  In that case we don't _have_
> to construct unaligned types.

I'm all for option (1), but concering (2) - a packed structure does
have alignment 1, so how can we derive anything but alignment 1 from
(the sizes of, supposedly) other members?  That is, we would have
to build an explicitly aligned _decl_ of the packed type to be able
to derive more than 1 alignment.  No?

Richard.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39954