[Bug c/65673] New: Compound literal with initializer for zero-sized array drops other initializers

[Bug c/65673] New: Compound literal with initializer for zero-sized array drops other initializers

[stilor at att dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65673

            Bug ID: 65673
           Summary: Compound literal with initializer for zero-sized array
                    drops other initializers
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stilor at att dot net

I am seeing a strange behavior when a compound initializer is used in a
structure initialization. A test case:


[[[
struct s {
    int y;
    unsigned long *x;
};

struct s foo = {
    .y = 25,
    .x = (unsigned long [SZ]){},
};
]]]

If SZ is defined to non-zero, the expected output is produced:

[[[
/tmp$ gcc -S -o- 1.c -Wall -DSZ=1
    .file    "1.c"
    .local    __compound_literal.0
    .comm    __compound_literal.0,8,8
    .globl    foo
    .data
    .align 16
    .type    foo, @object
    .size    foo, 16
foo:
    .long    25
    .zero    4
    .quad    __compound_literal.0
    .ident    "GCC: (Ubuntu 4.9.1-16ubuntu6) 4.9.1"
    .section    .note.GNU-stack,"",@progbits
]]]

If SZ is zero, the initializer for .y (".y = 25") member is dropped as well:

[[[
/tmp$ gcc -S -o- 1.c -Wall -DSZ=0
    .file    "1.c"
    .globl    foo
    .bss
    .align 16
    .type    foo, @object
    .size    foo, 16
foo:
    .zero    16
    .ident    "GCC: (Ubuntu 4.9.1-16ubuntu6) 4.9.1"
    .section    .note.GNU-stack,"",@progbits
]]]

Tested with GCC 4.6.3 and 4.9.1, both exhibit the same behavior.

With -Wextra, the code rightfully complains that the initializer for .x is
missing - but why the .y initializer is dropped even if there is no initializer
for .x?

In the mailing list, this was some discussion of this issue:
[[[
But in this case, the code attempts to create an unnanmed temporary array of
zero elements which, IMO, makes no sense. At the same time, I wouldn't expect
gcc to simply omit the initialization for foo.x. I suggest to open a gcc bug
for it.
]]]

I'd add that this was a reduced test case from a bigger aggregate type - which
was an array of such structures. When one of the elements became unused and the
size of the bitmap (which was the purpose of the compound literal initializer)
was set to zero, the whole array lost its initializers - i.e., even other
'struct s' members of the array, not just the member with a zero-sized array
compound literal.

[Bug c/65673] Compound literal with initializer for zero-sized array drops other initializers

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65673

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-04-17
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.  It seems weird to drop the initializer.

[Bug c/65673] Compound literal with initializer for zero-sized array drops other initializers

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65673

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
What happens here is that pop_init_level returns error_mark_node because
initializing a zero-length array member with {} is discarded:

 7565       /* Silently discard empty initializations.  The parser will
 7566          already have pedwarned for empty brackets.  */
 7567       if (integer_zerop (constructor_unfilled_index))
 7568         constructor_type = NULL_TREE;

thus ret.value is NULL:

 7718   if (ret.value == 0 && constructor_stack == 0)
 7719     ret.value = error_mark_node;
 7720   return ret;

output_init_element then sees that value == error_mark_node, so it marks the
ctor as erroneous:

 8388   if (type == error_mark_node || value == error_mark_node)
 8389     {
 8390       constructor_erroneous = 1;
 8391       return;
 8392     }

And because the ctor is erroneous, we don't build a CONSTRUCTOR for it:

 7668       if (constructor_erroneous)
 7669         ret.value = error_mark_node;
 7670       else
 7671         {
 7672           ret.value = build_constructor (constructor_type,
 7673                                          constructor_elements);

[Bug c/65673] [9/10/11/12 Regression] Compound literal with initializer for zero-sized array drops other initializers

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65673

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Compound literal with       |[9/10/11/12 Regression]
                   |initializer for zero-sized  |Compound literal with
                   |array drops other           |initializer for zero-sized
                   |initializers                |array drops other
                   |                            |initializers
      Known to work|                            |4.1.2
      Known to fail|                            |4.4.7
   Target Milestone|---                         |9.5
           Keywords|                            |wrong-code

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
GCC 4.1.2 produces:

foo:
        .long   25
        .zero   4
        .quad   __compound_literal.0
        .local  __compound_literal.0
        .comm   __compound_literal.0,0,8

While 4.4.7 (and above) produces:
foo:
        .zero   16

[Bug c/65673] [9/10/11/12 Regression] Compound literal with initializer for zero-sized array drops other initializers

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65673

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug c/65673] [10/11/12/13 Regression] Compound literal with initializer for zero-sized array drops other initializers

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65673

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.5                         |10.4

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9 branch is being closed

[Bug c/65673] [10/11/12/13 Regression] Compound literal with initializer for zero-sized array drops other initializers

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65673

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.4                        |10.5

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 10.4 is being released, retargeting bugs to GCC 10.5.

[Bug c/65673] [10/11/12/13 Regression] Compound literal with initializer for zero-sized array drops other initializers

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65673

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2015-04-17 00:00:00         |2022-12-20
                 CC|                            |mpolacek at gcc dot gnu.org
      Known to fail|                            |13.0

--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> ---
Another option would be to report an error for this?  Anyway, re-confirmed. 
Marek - you had a patch, any updates here?

[Bug c/65673] [10/11/12/13 Regression] Compound literal with initializer for zero-sized array drops other initializers

[luangruo at yahoo dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65673

Po Lu <luangruo at yahoo dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |luangruo at yahoo dot com

--- Comment #9 from Po Lu <luangruo at yahoo dot com> ---
This bug still exists, both in GCC 12.2 and trunk:

struct sfnt_generic_test_args
{
  unsigned int *expected_stack;
  int expected_stack_elements;
  int expected_trap : 1;
  int expected_IP;
};

struct sfnt_generic_test_args stack_underflow_test_args =
  {
    (unsigned int [0]) { },
    0,
    1,
    4,
  };

on:

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--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
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Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.2.1 20220819 (Red Hat 12.2.1-2) (GCC) 

and

Using built-in specs.
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'-v' '-mtune=generic' '-march=x86-64' '-dumpdir' '/app/output.'
Compiler returned: 0

results in the structure being generated as a .zero section of size 24,
while it should really be:

        .quad   <address of compound literal array of size 0>
        .long   0
        .byte   1
        .zero   3
        .long   4
        .zero   4

I hope it can be fixed, because this is causing real problems for Emacs.
Thanks in advance.

[Bug c/65673] [11/12/13/14 Regression] Compound literal with initializer for zero-sized array drops other initializers

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65673

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.5                        |11.5

--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 10 branch is being closed.