Re: Question about generated type for common block in fortran

[Richard Biener <richard dot guenther at gmail dot com>]

On Mon, Nov 13, 2017 at 11:01 AM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Thu, 9 Nov 2017, Bin.Cheng wrote:
>
>> So I have two questions here.
>> A) Is this special kind union type only generated by fortran FE for
>> equivalence+common?
>
> It's not special in that it isn't marked in any way.  For all purposes
> it's a normal union type with surprising field(offset)s.  I don't know if
> it can occur for any situations except equivalence+common.  Not from the
> fortran frontend I believe.
>
>> B) For the special union type that has non-zero offset fields.  Is it
>> safe to assume field A is not the last flex array: if there is field B
>> that offset_B >= (offset_A + length_A)?
>
> No.  You can make equivalences between all kinds of objects,
> including non-arrays.  You can also equivalence (say) an integer variable
> and an individual array element.  Also flex arrays don't enter the
> picture, you can equivalize (tm) also arrays of fixed size.

Just a remark on the optimization you want to achieve.  Rather than
trying to fix things through max_size and friends you can also look
at nonoverlapping_component_refs_of_decl_p and/or
nonoverlapping_component_refs_p both of which currently punt
for UNION_TYPEs.

So I was thinking of a good condition to quickly decide whether UNION_TYPE
fields may overlap or not (and thus to distinguish the Fortran case which then
requires more elaborate processing).
A simple one might be to quickly bail out if DECL_FIELD_[BIT_]OFFSET
are the same.

This might be a more local "fix" for the missed aliasing than trying to
jump into the flex array issues.

Richard.

>
> Ciao,
> Michael.