[Bug tree-optimization/101062] [10/11/12 Regression] wrong code with "-O2 -fno-toplevel-reorder -frename-registers"

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101062

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #7)
> Now, it looks to me this is rather an issue that the access is larger than
> the object and thus a general bug - at least I don't see how it should only
> manifest with bitfields in unions?
> 
> Note we do
> 
>       if (TREE_CODE (to) == COMPONENT_REF
>           && DECL_BIT_FIELD_TYPE (TREE_OPERAND (to, 1)))
>         get_bit_range (&bitregion_start, &bitregion_end, to, &bitpos,
> &offset);
>       /* The C++ memory model naturally applies to byte-aligned fields.
>          However, if we do not have a DECL_BIT_FIELD_TYPE but BITPOS or
>          BITSIZE are not byte-aligned, there is no need to limit the range
>          we can access.  This can occur with packed structures in Ada.  */
>       else if (maybe_gt (bitsize, 0)
>                && multiple_p (bitsize, BITS_PER_UNIT)
>                && multiple_p (bitpos, BITS_PER_UNIT))
>         {
>           bitregion_start = bitpos;
>           bitregion_end = bitpos + bitsize - 1;
>         }
> 
> but if we assume that for DECL_BIT_FIELD_TYPE there's a representative
> then we miss the else if, so - maybe get_bit_range should return whether
> it handled things or the else if part should be done unconditionally
> in case bitregion_start/end is not {0,0}?

This wouldn't help us, bitsize is > 0, but not a multiple of BITS_PER_UNIT in
this case.  Furthermore, even if we add there bitregion_start/end for the base
variable if any as further fallthrough, I think most C/C++ programmers will
expect that with
union U { int a; int b : 5; } u[64];
u[4].b = 1; can be done safely in one thread and
u[5].a = 2; in another one.

My patch fixes that (or another possibility would be to compute the
representative even in UNION_TYPE (no idea about QUAL_UNION_TYPE) - could be as
simple as removing the early out and instead of doing prev = field; in the loop
do if (TREE_CODE (t) != RECORD_TYPE) { finish_bitfield_representative (repr,
field); repr = NULL_TREE; } else prev = field; and in
finish_bitfield_representative override nextf to NULL_TREE).

Improving expand_assignment can be done too, sure, but independently to this.