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From: "stilor at att dot net" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/65673] New: Compound literal with initializer for zero-sized array drops other initializers
Date: Sun, 05 Apr 2015 06:42:00 -0000
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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.