The following implements Jasons suggestion of using a langhook to
return the size of an aggregate without tail padding that might
be re-used when it is inherited from.
Using this langhook we can fix the size of the representative for the
bitfield properly and thus generate correct code for the testcase.
Previously on x86_64 it was
main:
movl c+4(%rip), %eax
andl $-258049, %eax
orb $16, %ah
movl %eax, c+4(%rip)
movb $2, c+7(%rip)
xorl %eax, %eax
ret
while after the patch we see
main:
movzbl c+5(%rip), %eax
andb $-4, c+6(%rip)
movb $2, c+7(%rip)
andl $15, %eax
orl $16, %eax
movb %al, c+5(%rip)
xorl %eax, %eax
ret
in particular the store to C::B::d is now properly using a byte store.
Bootstrap and regtest in progress on x86_64-unknown-linux-gnu, ok for
trunk?
Thanks,
Richard.
2016-12-14 Richard Biener <rguenther@suse.de>
PR c++/71694
* langhooks-def.h (lhd_unit_size_without_reusable_padding): Declare.
(LANG_HOOKS_UNIT_SIZE_WITHOUT_REUSABLE_PADDING): Define.
(LANG_HOOKS_FOR_TYPES_INITIALIZER): Adjust.
* langhooks.h (struct lang_hooks_for_types): Add
unit_size_without_reusable_padding.
* langhooks.c (lhd_unit_size_without_reusable_padding): New.
* stor-layout.c (finish_bitfield_representative): Use
unit_size_without_reusable_padding langhook to decide on the
last representatives size.
cp/
* cp-objcp-common.h (cp_unit_size_without_reusable_padding): Declare.
(LANG_HOOKS_UNIT_SIZE_WITHOUT_REUSABLE_PADDING): Define.
* cp-objcp-common.c (cp_unit_size_without_reusable_padding): New.
* g++.dg/pr71694.C: New testcase.
Index: gcc/stor-layout.c
===================================================================
*** gcc/stor-layout.c (revision 243632)
--- gcc/stor-layout.c (working copy)
*************** finish_bitfield_representative (tree rep
*** 1864,1876 ****
}
else
{
! /* ??? If you consider that tail-padding of this struct might be
! re-used when deriving from it we cannot really do the following
! and thus need to set maxsize to bitsize? Also we cannot
! generally rely on maxsize to fold to an integer constant, so
! use bitsize as fallback for this case. */
! tree maxsize = size_diffop (TYPE_SIZE_UNIT (DECL_CONTEXT (field)),
! DECL_FIELD_OFFSET (repr));
if (tree_fits_uhwi_p (maxsize))
maxbitsize = (tree_to_uhwi (maxsize) * BITS_PER_UNIT
- tree_to_uhwi (DECL_FIELD_BIT_OFFSET (repr)));
--- 1864,1877 ----
}
else
{
! /* Note that if the C++ FE sets up tail-padding to be re-used it
! creates a as-base variant of the type with TYPE_SIZE adjusted
! accordingly. So it is safe to include tail-padding here. */
! tree aggsize = lang_hooks.types.unit_size_without_reusable_padding
! (DECL_CONTEXT (field));
! tree maxsize = size_diffop (aggsize, DECL_FIELD_OFFSET (repr));
! /* We cannot generally rely on maxsize to fold to an integer constant,
! so use bitsize as fallback for this case. */
if (tree_fits_uhwi_p (maxsize))
maxbitsize = (tree_to_uhwi (maxsize) * BITS_PER_UNIT
- tree_to_uhwi (DECL_FIELD_BIT_OFFSET (repr)));