Re: [PATCH v2] Fortran: fix reallocation on assignment of polymorphic variables [PR110415]

[Tobias Burnus <tobias@codesourcery.com>]

Hi Andrew,

On 27.11.23 18:35, Andrew Jenner wrote:
> This is the second version of the patch - previous discussion at:
> https://gcc.gnu.org/pipermail/gcc-patches/2023-November/636671.html
>
> This patch adds the testcase from PR110415 and fixes the bug.
>
> The problem is that in a couple of places in trans_class_assignment in
> trans-expr.cc, we need to get the run-time size of the polymorphic
> object from the vtbl, but we are currently getting that vtbl from the
> lhs of the assignment rather than the rhs. This gives us the old value
> of the size but we need to pass the new size to __builtin_malloc and
> __builtin_realloc.
>
> I'm fixing this by adding a parameter to trans_class_vptr_len_assignment
> to retrieve the tree corresponding the vptr from the object on the rhs
> of the assignment, and then passing this where it is needed. In the case
> where trans_class_vptr_len_assignment returns NULL_TREE for the rhs vptr
> we use the lhs vptr as before.
>
> To get this to work I also needed to change the implementation of
> trans_class_vptr_len_assignment to create a temporary for the assignment
> in more circumstances. Currently, the "a = func()" assignment in MAIN__
> doesn't hit the "Create a temporary for complication expressions" case
> on line 9951 because "DECL_P (rse->expr)" is true - the expression has
> already been placed into a temporary. That means we don't hit the "if
> (temp_rhs ..." case on line 10038 and go on to get the vptr_expr from
> "gfc_lval_expr_from_sym (gfc_find_vtab (&re->ts))" on line 10057 which
> is the vtbl of the static type rather than the dynamic one from the rhs.
> So with this fix we create an extra temporary, but that should be
> optimised away in the middle-end so there should be no run-time effect.
>
> I'm not sure if this is the best way to fix this (the Fortran front-end
> is new territory for me) but I've verified that the testcase passes with
> this change, fails without it, and that the change does not introduce
> any FAILs when running the gfortran testcases on x86_64-pc-linux-gnu.
>
> After the previous submission, Tobias Burnus found a closely related
> problem and contributed testcases and a fix for it, which I have
> incorporated into this version of the patch. The problem in this case
> is with the __builtin_realloc call that is executed if one polymorphic
> variable is replaced by another. The return value of this call was
> being ignored rather than used to replace the pointer being reallocated.
>
> Is this OK for mainline, GCC 13 and OG13?

LGTM – Thanks! OK for mainline and after some grace period for GCC 13.

OG13 does not require approval, hence, either cherry pick directly or
wait until it is in GCC 13 and then "git merge" GCC 13 to OG13.

Thanks,

Tobias


> gcc/fortran/
>          PR fortran/110415
>          * trans-expr.cc (trans_class_vptr_len_assignment): Add
>          from_vptrp parameter. Populate it. Don't check for DECL_P
>          when deciding whether to create temporary.
>          (trans_class_pointer_fcn, gfc_trans_pointer_assignment): Add
>          NULL argument to trans_class_vptr_len_assignment calls.
>          (trans_class_assignment): Get rhs_vptr from
>          trans_class_vptr_len_assignment and use it for determining size
>          for allocation/reallocation. Use return value from realloc.
>
> gcc/testsuite/
>          PR fortran/110415
>          * gfortran.dg/pr110415.f90: New test.
>          * gfortran.dg/asan/pr110415-2.f90: New test.
>          * gfortran.dg/asan/pr110415-3.f90: New test.
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