Re: [PATCH] Use complete_array_type on flexible array member initializers

[Jason Merrill <jason@redhat.com>]

On Sun, Aug 26, 2018 at 4:52 AM, Bernd Edlinger
<bernd.edlinger@hotmail.de> wrote:
> On 08/26/18 07:36, Jeff Law wrote:
>> On 08/24/2018 07:13 AM, Bernd Edlinger wrote:
>>> Hi!
>>>
>>>
>>> This patch prevents init values of STRING_CST and braced
>>> array initializers to reach the middle-end with incomplete
>>> type.
>>>
>>> This will allow further simplifications in the middle-end,
>>> and address existing issues with STRING_CST in a correct
>>> way.
>>>
>>>
>>>
>>> Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
>>> Is it OK for trunk?
>>>
>>>
>>> Thanks
>>> Bernd.
>>>
>>>
>>> patch-flexarray.diff
>>>
>>>
>>> gcc:
>>> 2018-08-24  Bernd Edlinger  <bernd.edlinger@hotmail.de>
>>>
>>>      * varasm.c (output_constructor_regular_field): Check TYPE_SIZE_UNIT of
>>>      the init value.
>>>
>>> c-family:
>>> 2018-08-24  Bernd Edlinger  <bernd.edlinger@hotmail.de>
>>>
>>>      * c-common.c (complete_flexible_array_elts): New helper function.
>>>      * c-common.h (complete_flexible_array_elts): Declare.
>>>
>>> c:
>>> 2018-08-24  Bernd Edlinger  <bernd.edlinger@hotmail.de>
>>>
>>>      * c-decl.c (finish_decl): Call complete_flexible_array_elts.
>>>
>>> cp:
>>> 2018-08-24  Bernd Edlinger  <bernd.edlinger@hotmail.de>
>>>
>>>      * decl.c (check_initializer): Call complete_flexible_array_elts.
>>>
>>>
>>> diff -Npur gcc/c/c-decl.c gcc/c/c-decl.c
>>> --- gcc/c/c-decl.c   2018-08-21 08:17:41.000000000 +0200
>>> +++ gcc/c/c-decl.c   2018-08-24 12:06:21.374892294 +0200
>>> @@ -5035,6 +5035,8 @@ finish_decl (tree decl, location_t init_
>>>         if (init && TREE_CODE (init) == CONSTRUCTOR)
>>>      add_flexible_array_elts_to_size (decl, init);
>>>
>>> +      complete_flexible_array_elts (DECL_INITIAL (decl));
>>> +
>>>         if (DECL_SIZE (decl) == NULL_TREE && TREE_TYPE (decl) != error_mark_node
>>>        && COMPLETE_TYPE_P (TREE_TYPE (decl)))
>>>      layout_decl (decl, 0);
>>> diff -Npur gcc/c-family/c-common.c gcc/c-family/c-common.c
>>> --- gcc/c-family/c-common.c  2018-08-17 05:02:11.000000000 +0200
>>> +++ gcc/c-family/c-common.c  2018-08-24 12:45:56.559011703 +0200
>>> @@ -6427,6 +6427,28 @@ complete_array_type (tree *ptype, tree i
>>>     return failure;
>>>   }
>>>
>>> +/* INIT is an constructor of a structure with a flexible array member.
>>> +   Complete the flexible array member with a domain based on it's value.  */
>>> +void
>>> +complete_flexible_array_elts (tree init)
>>> +{
>>> +  tree elt, type;
>>> +
>>> +  if (init == NULL_TREE || TREE_CODE (init) != CONSTRUCTOR)
>>> +    return;
>>> +
>>> +  if (vec_safe_is_empty (CONSTRUCTOR_ELTS (init)))
>>> +    return;
>>> +
>>> +  elt = CONSTRUCTOR_ELTS (init)->last ().value;
>>> +  type = TREE_TYPE (elt);
>>> +  if (TREE_CODE (type) == ARRAY_TYPE
>>> +      && TYPE_SIZE (type) == NULL_TREE)
>>> +    complete_array_type (&TREE_TYPE (elt), elt, false);
>>> +  else
>>> +    complete_flexible_array_elts (elt);
>>> +}
>> Shouldn't this be handled in c-decl.c by the call to
>> add_flexible_array_elts_to_size?    Why the recursion when the
>> CONSTRUCTOR_ELT isn't an array type?
>>
>
> There are tests in the test suite that use something like that:
> struct {
>    union {
>      struct {
>         int a;
>         char b[];
>      };
>      struct {
>         char x[32];
>      };
>    };
> } u = { { { 1, "test" } } };
>
> So it fails to go through add_flexible_array_elts_to_size.
>
> I am not sure what happens if the string is larger than 32 byte.
> The test suite does not do that.
> Well I just tried, while writing those lines:
>
> struct {
>   union {
>    struct {
>     int a;
>     char b[];
>    };
>    struct {
>     char x[4];
>    };
>   };
> } u = { { { 1, "test" } } };
>
> =>
>
>         .file   "t.c"
>         .text
>         .globl  u
>         .data
>         .align 4
>         .type   u, @object
>         .size   u, 4
> u:
>         .long   1
>         .string "test"
>         .ident  "GCC: (GNU) 9.0.0 20180825 (experimental)"
>         .section        .note.GNU-stack,"",@progbits
>
> So the .size is too small but that is probably only a fauxpas.
>
>
>>
>>> diff -Npur gcc/cp/decl.c gcc/cp/decl.c
>>> --- gcc/cp/decl.c    2018-08-22 22:35:38.000000000 +0200
>>> +++ gcc/cp/decl.c    2018-08-24 12:06:21.377892252 +0200
>>> @@ -6528,6 +6528,8 @@ check_initializer (tree decl, tree init,
>>>
>>>        init_code = store_init_value (decl, init, cleanups, flags);
>>>
>>> +      complete_flexible_array_elts (DECL_INITIAL (decl));
>>> +
>>>        if (pedantic && TREE_CODE (type) == ARRAY_TYPE
>>>            && DECL_INITIAL (decl)
>>>            && TREE_CODE (DECL_INITIAL (decl)) == STRING_CST
>> Should the C++ front-end be going through cp_complete_array_type instead?
>>
>
> No I don't think so, because at that time BRACE_ENCLOSED_INITIALIZER_P
> property does no longer work, as I explained in the previous mail.
>
> And cp_complete_array_type does use that property:
>
> int
> cp_complete_array_type (tree *ptype, tree initial_value, bool do_default)
> {
>    int failure;
>    tree type, elt_type;
>
>    /* Don't get confused by a CONSTRUCTOR for some other type.  */
>    if (initial_value && TREE_CODE (initial_value) == CONSTRUCTOR
>        && !BRACE_ENCLOSED_INITIALIZER_P (initial_value)
>        && TREE_CODE (TREE_TYPE (initial_value)) != ARRAY_TYPE)
>      return 1;

> But I need to do that completion for STRING_CST and CONSTRUCTORS
> initializing a flexible array, of a structure of a union
> within a structure.

That check only returns early on CONSTRUCTORs for a non-array type,
which I assume you don't care about.

> I tried to come up with a test case where the cp_complete_array_type
> might make a difference (looking into string constant with extra braces),
> but it worked:
>
>
> $ cat test.cc
> struct {
>    int a;
>    char b[];
> } xx = { 1,  { "test" } };
>
> $ cat test.cc
> struct {
>   union {
>    struct {
>     int a;
>     char b[];
>    };
>    struct {
>      char c[32];
>    };
>   };
> } xx = { 1,  "test" };
> $ gcc test.cc
> test.cc:11:21: error: initialization of flexible array member in a nested context
> 11 | } xx = { 1,  "test" };
>     |                     ^

The extra braces that code handles are like

struct A
{
  int i;
  char a[];
};

struct A a = { 1, { "test" } };

...but by the time you're calling complete_flexible_array_elts those
extra braces have already been removed by reshape_init.

The flag copying at the end of cp_complete_array_type is likely to be
important, though, so let's use it.

Jason