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

[Jason Merrill <jason@redhat.com>]

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?

Jason