https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82210
--- Comment #5 from rguenther at suse dot de <rguenther at suse dot de> --- On Wed, 7 Feb 2018, aldyh at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82210 > > Aldy Hernandez <aldyh at gcc dot gnu.org> changed: > > What |Removed |Added > ---------------------------------------------------------------------------- > CC| |jakub at gcc dot gnu.org, > | |rguenth at gcc dot gnu.org > > --- Comment #4 from Aldy Hernandez <aldyh at gcc dot gnu.org> --- > [Looping in the experts: Hi Jakub. Hi Richard.] > > Imagine this: > > struct big_container { > __attribute__((aligned(16))) > struct aa_container { > short aa; > } a[size]; > int b[size]; > } s; > > While laying out the type of big_container, and looking at field "a" in > place_field() we get a DECL_SIZE_UNIT(a) of SAVE_EXPR <size> * 2 (courtesy of > update_alignment_for_field). > > This size*2 is used in calculating the size of the record thus far: > > /* Now add size of this field to the size of the record. If the size is > not constant, treat the field as being a multiple of bytes and just > adjust the offset, resetting the bit position. > ... > else if (TREE_CODE (DECL_SIZE (field)) != INTEGER_CST > || TREE_OVERFLOW (DECL_SIZE (field))) > { > rli->offset > = size_binop (PLUS_EXPR, rli->offset, > fold_convert (sizetype, > size_binop (CEIL_DIV_EXPR, rli->bitpos, > bitsize_unit_node))); > rli->offset > = size_binop (PLUS_EXPR, rli->offset, DECL_SIZE_UNIT (field)); > rli->bitpos = bitsize_zero_node; > rli->offset_align = MIN (rli->offset_align, desired_align); > } > > (gdb) p debug_generic_stmt(rli.offset) > (sizetype) SAVE_EXPR <size> * 2; > > Which means that when we call place_field(b), we think we have to start at > size*2. Surely the start of field b should take into consideration the > alignment of aa_container, no? > > Question: > > 1. Should DECL_SIZE_UNIT(a) include the size of the alignment padding? I'm not sure whether the attribute aligns the array elements or the array. If it aligns the array then no. > 2. Or, should the snippet above add the padding? This is for b, so why? There should be no padding needed. > Or, am I missing something? > > Thanks. > >
