[PATCH, Fortran] Fix initialization of UNIONs with -finit-derived

[Fritz Reese <fritzoreese at gmail dot com>]

All,

Currently the '-finit-derived' flag does not handle UNIONs properly.
Specifically, a default initializer may not be appropriately generated
for BT_UNION components, leaving some memory in the middle of the
structure uninitialized even with -finit-derived. This patch ensures
UNION nodes receive a NULL structure constructor with -finit-derived
while simultaneously respecting the first user-given initializers
within the union's MAP nodes, if any. With the NULL structure
constructor the gcc middle-end figures out the full initialization, as
if you were initializing a C union like "u = {0}", and a bit of work
is saved on our end.

The front-end stores an EXPR_NULL node as the UNION's constructor
expression when -finit-derived is given and it has no user-specified
MAP initializers. When the node propagates through the resolution
phase and makes it to trans-expr.c, gfc_trans_subcomponent_assign and
gfc_conv_union_initializer use this to identify whether to generate a
null initializer or use the user-given MAP initializer.

For the attached testcases: dec_init_3.f90 exhibits improper
initialization for unions before this patch, and dec_init_4.f90
verifies that explicit user-given initializers are not regressed with
the implementation.

Since this only touches my own UNION and -finit-derived code I aim to
commit it in a few days if nobody finds anything wrong with the patch.
Bootstraps and regtests on x86_64-redhat-linux.

---
Fritz Reese


2016-10-25  Fritz Reese  <fritzoreese@gmail.com>

Fix initialization of UNIONs with -finit-derived.

        gcc/fortran/
        * expr.c (generate_union_initializer, get_union_initializer): New.
        * expr.c (component_initializer): Consider BT_UNION specially.
        * resolve.c (resolve_structure_cons): Hack for BT_UNION.
        * trans-expr.c (gfc_trans_subcomponent_assign): Ditto.
        * trans-expr.c (gfc_conv_union_initializer): New.
        * trans-expr.c (gfc_conv_structure): Replace UNION handling code with
        new function gfc_conv_union_initializer.

        gcc/testsuite/gfortran.dg/
        * dec_init_1.f90, dec_init_2.f90: Remove -fdump-tree-original.
        * dec_init_3.f90, dec_init_4.f90: New testcases.

Attachment: init_derived_union.diff
Description: Text document