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

Jason Merrill jason@redhat.com
Fri Feb 19 18:30:00 GMT 2016

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?  I would think that what we want is to use emit_block_move in 
the bit-field path whenever we're dealing with whole bytes in memory.


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