On 02/19/2016 02:07 PM, Jakub Jelinek wrote:
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).
Please mention that in the comment, as well. OK with those comment changes.
Jason
