[Bug sanitizer/96259] New: Regression warning on GCC 10

[freddy77 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96259

            Bug ID: 96259
           Summary: Regression warning on GCC 10
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: freddy77 at gmail dot com
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org, marxin at 
gcc dot gnu.org
  Target Milestone: ---

I found this issue updating a software from Fedora 31 to Fedora 32. I manage to
reproduce the problem with this main.c file:


struct DataIn {
   int type;
   int dummy;
};

struct DataOut {
   struct DataIn in;
};

void func(struct DataOut *data)
{
   if (data->in.type) {
       __builtin_memset(&(data->in), 0, sizeof(struct DataIn));
   }
}


and using this command line:

   gcc -Warray-bounds=2 -Werror -O2 -fsanitize=address -c main.c

this is reporting:


<source>: In function 'void func(DataOut*)':
<source>:15:24: error: 'void* __builtin_memset(void*, int, long unsigned int)'
offset [4, 7] from the object at 'data' is out of the bounds of referenced
subobject 'DataIn::type' with type 'int' at offset 0 [-Werror=array-bounds]
   15 |        __builtin_memset(&(data->in), 0, sizeof(struct DataIn));
      |        ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:4:8: note: subobject 'DataIn::type' declared here
    4 |    int type;
      |        ^~~~
cc1plus: all warnings being treated as errors
Compiler returned: 1


I reproduced the issue using https://godbolt.org/ so I verified that this
happens with GCC 10.1 but not with GCC 9.3. It seems related to:
- nested structure
- having a "if" in the code
- "-Warray-bounds=2"
- "-fsanitize=address"

I know "-Warray-bounds=2" could have false positive but it's still a
regression.