[Bug c/63944] New: [DR#413] Partial overriding of nonconstant struct/union initializers with designated initializers

[Bug c/63944] New: [DR#413] Partial overriding of nonconstant struct/union initializers with designated initializers

[jsm28 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63944

            Bug ID: 63944
           Summary: [DR#413] Partial overriding of nonconstant
                    struct/union initializers with designated initializers
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jsm28 at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org

C11 DR#413 has now moved to closed (see N1892).  This says that if an
initializer (a) initializes a struct or union member of a larger object using
an expression of that struct or union type and (b) then uses a designated
initializer to override just part of that struct or union initialization, then
the overriding does not affect the initialization of the rest of the struct or
union.  Thus, the following should pass (based on the test in the DR):

extern void exit (int);
extern void abort (void);
typedef struct {
int k;
int l;
int a[2];
} T;

typedef struct {
int i;
T t;
} S;

T x = {.l = 43, .k = 42, .a[1] = 19, .a[0] = 18 };

int
main (void)
{
S l = { 1, .t = x, .t.l = 41, .t.a[1] = 17};
if (l.t.k == 42) exit (0); else abort ();
}

Clearly the existing approach for string constants (of splitting them
individual characters) won't work here (but is still needed for initializers of
objects with static storage duration).  This bug is specific to initializers of
objects with automatic storage duration; it can apply both to initialization
with an expression of struct type, and to initialization with one of union type
(in the union case, the override would have to be of just part of a union
member, as setting the whole of a union member sets the whole union).  I
suppose some initializations with such partial overriding need to generate more
complicated code - one possibility might be for such partial overriding to
replace the initializer for .t with (tmp = x, tmp.l = 41, tmp) for a new
temporary variable tmp.

[Bug c/63944] [DR#413] Partial overriding of nonconstant struct/union initializers with designated initializers

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63944

--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
Actually, I suppose the existing approach for strings could be used if 
desired - split the struct into a series of component references to a 
SAVE_EXPR of the nonconstant struct initializer (in the case of a union, 
the partially overriding initializer would say which union element needs 
splitting like that).  Whether that's desirable is another matter (say the 
struct being partially overridden is struct S { int a[1000000]; } - 
splitting into separate initializers would mean 1000000 SAVE_EXPRs, which 
seems unlikely to work well).

[Bug c/63944] [DR#413] Partial overriding of nonconstant struct/union initializers with designated initializers

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63944

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-11-19
     Ever confirmed|0                           |1

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I'm afraid I don't remember anything from my designated initializers changes,
it was too long time ago, so if anybody else (Joseph, Marek, anyone else?)
wants to implement this, go ahead ;).

[Bug c/63944] [DR413] Partial overriding of nonconstant struct/union initializers with designated initializers

[jsm28 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63944

Joseph S. Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dje at gcc dot gnu.org

--- Comment #9 from Joseph S. Myers <jsm28 at gcc dot gnu.org> ---
*** Bug 94225 has been marked as a duplicate of this bug. ***

[Bug c/63944] [DR413] Partial overriding of nonconstant struct/union initializers with designated initializers

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63944

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|mpolacek at gcc dot gnu.org        |unassigned at gcc dot gnu.org
             Status|ASSIGNED                    |NEW