Re: -Wlto-type-mismatch with flexible array in struct

[Martin Sebor <msebor at gmail dot com>]

On 07/12/2017 07:39 PM, Dan Halbert wrote:
> I'm getting a -Wlto-type-mismatch warning with a struct that ends with a flexible array, declared in one file and initialized in another. I do NOT get a warning with a simple flexible array in the same situation.
>
> Is this an LTO bug I should report, or is this a limitation of LTO? Thanks.

My Bugzilla searches for "flexible" in the lto component didn't
return any relevant bugs about it.  I would recommend opening
a new bug for it and letting the LTO experts comment on it (they
may not be reading gcc-help).

Martin

> (tested with arm-none-eabi-gcc 6.3.1, but I see the same issue with native gcc 6.3.0)
>
> HAS WARNING:
>
> ab_struct.h
> -----------
> typedef struct {
>   int i;
>   int ints[];
> } struct_t;
>
>
> a_struct.c
> ----------
> #include "ab_struct.h"
>
> extern struct_t my_struct;
>
> int main() {
>   return my_struct.ints[0];
> }
>
>
> b_struct.c
> ----------
> #include "ab_struct.h"
>
> struct_t my_struct = {
>   20,
>   { 1, 2 }
> };
>
>
> $ arm-none-eabi-gcc -flto -Wlto-type-mismatch a_struct.c b_struct.c -o foo
> a_struct.c:3:17: warning: size of 'my_struct' differ from the size of original declaration [-Wlto-type-mismatch]
>  extern struct_t my_struct;
>                  ^
> b_struct.c:3:10: note: 'my_struct' was previously declared here
>  struct_t my_struct = {
>
> -----------------------------------------------------------------------------
> NO WARNING with this plain flexible array, not in a struct:
>
> a_array.c
> ---------
> extern  int ia[];
>
> int main() {
>   return ia[0];
> }
>
>
> b_array.c
> ---------
> int ia[] = { 1, 2 };
>
> $ arm-none-eabi-gcc -flto -Wlto-type-mismatch a_array.c b_array.c -o foo
> [no warning]