Re: [PATCH] Fix expansion of TREE_ADDRESSABLE bitwise copies (PR c++/69851)

[Jakub Jelinek <jakub at redhat dot com>]

On Fri, Feb 19, 2016 at 02:00:07PM -0500, Jason Merrill wrote:
> On 02/19/2016 01:41 PM, Jakub Jelinek wrote:
> >On Fri, Feb 19, 2016 at 01:30:52PM -0500, Jason Merrill wrote:
> >>On 02/19/2016 09:03 AM, Jakub Jelinek wrote:
> >>>As described in the PR, in C++ we can have assignments
> >>>where both the lhs and rhs are COMPONENT_REFs with TREE_ADDRESSABLE types,
> >>>including padding, but the FIELD_DECLs are artificial fields that have
> >>>narrower bit sizes.
> >>>store_field in this case takes the path of bit-field handling (even when
> >>>it has bitpos and bitsize multiples of BITS_PER_UNIT (I think that is
> >>>necessarily true for the TREE_ADDRESSABLE types), which is incorrect,
> >>>because the rhs is expanded in that case through expand_normal, which
> >>>for a result type wider than the FIELD_DECL with forces it into a temporary.
> >>>In older GCCs that just generated inefficient code (copy the rhs into a
> >>>stack temporary, then copy that to lhs), but GCC trunk ICEs on that.
> >>>Fixed by not taking the bit-field path in that case after verifying
> >>>we'll be able to expand it properly using the normal store_expr.
> >>
> >>Won't store_expr clobber tail padding because it doesn't know about bitsize?
> >
> >It doesn't clobber it, because it uses get_inner_reference, expands the
> >inner reference (which is necessarily for something TREE_ADDRESSABLE either
> >a MEM_REF or some decl that lives in memory), and get_inner_reference in
> >that case gives it the bitsize/bitpos from the FIELD_DECL.
> >Which is why in the patch I've posted there is the comparison of DECL_SIZE
> >of the FIELD_DECL against the bitsize that is passed to store_field.
> 
> Ah, that makes sense.  Please mention that in your added comment.
> 
> For GCC 7, can we drop the TREE_ADDRESSABLE check?

I think we can't drop it, but we could replace it with a check that
get_inner_reference is something that must live in memory
(MEM_REF/TARGET_MEM_REF of SSA_NAME, or of decl that lives in memory,
or decl itself that lives in memory).

	Jakub