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From: "ebotcazou at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug ada/98228] [11 Regression] ICE: Assert_Failure atree.adb:931:
 Error detected at s-gearop.adb:382:34 [a-ngrear.adb:313:7
 [a-nllrar.ads:18:1]] on s390x-linux-gnu
Date: Fri, 22 Jan 2021 23:25:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98228

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |ebotcazou at gcc do=
t gnu.org
             Status|NEW                         |ASSIGNED
                 CC|ebotcazou at gcc dot gnu.org       |

--- Comment #16 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> # High-level flow of the resulting crashes:
>=20
> - the procedure sem_type__get_next_interp has an output parameter of
>   record type sem_type__interp and overwrites that completely.
> - in locations that call sem_type__get_next_interp, local variables of
>   type sem_type__interp get wrapped by maybe_pad_type in an outer
>   padding record for proper alignment. the padding record has a single
>   field "F" for the inner record.
> - on s390x, that field gets falsely flagged as nonaddressable (see
>   zoom-in below).
> - as a consequence of that flag, type based alias analysis does not
>   relate the padded record to the alias set of the inner record.
> - modref_may_conflict disambiguates references to the local variables
>   (padded record) from sem_type__get_next_interp actually overwriting
>   the (inner) record -- "correct" decision based on the data, but
>   clearly the wrong result.
> - as a result, loops that iterate via sem_type__get_next_interp are
>   "optimized" into endless loops, because their abort condition is
>   never checked against the updated data.
> - these loops overrun All_Interp.Table and trigger assertions or
>   segfault (i've seen both).

Thanks for the detailed investigation.  The effect of nonaddressability on
alias sets is as expected, but it is invalid to take the address of a
nonaddressable field of course.=