[Bug sanitizer/91707] New: spurious stringop-overflow warning with -fsanitize=undefined

[uecker at eecs dot berkeley.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91707

            Bug ID: 91707
           Summary: spurious stringop-overflow warning with
                    -fsanitize=undefined
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: uecker at eecs dot berkeley.edu
                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: ---

Created attachment 46854
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46854&action=edit
test case

With recent GCC versions from git there is a spurious warning
when the undefined behavior sanitizer is used.


$ ../gcc_build_clean/gcc/xgcc -B ../gcc_build_clean/gcc -Wall -O2 
-fsanitize=undefined -c strop.c 
In function ‘real_from_complex_dims’,
    inlined from ‘md_test’ at strop.c:14:2:
strop.c:7:11: warning: writing 8 bytes into a region of size 0
[-Wstringop-overflow=]
    7 |  odims[0] = 2;
      |  ~~~~~~~~~^~~
$ ../gcc_build_clean/gcc/xgcc --version
xgcc (GCC) 10.0.0 20190907 (experimental)
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat strop.c 

extern void md_copy_dims(unsigned int D, long odims[D], const long idims[D]);
extern void md_test(unsigned int D, const long idims[D]);

static void real_from_complex_dims(unsigned int D, long odims[D + 1], const
long idims[D])
{
   odims[0] = 2;
   md_copy_dims(D, odims + 1, idims);
}

void md_test(unsigned int D, const long idims[D])
{
   long dimsR[D + 1];
   real_from_complex_dims(D, dimsR, idims);

   long strsR[D + 1];
   (void)strsR;

   return;
}