[Bug c/108127] New: FP due to combination of -Warray-bounds and UBSAN

["frolov.da at phystech dot edu" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108127

            Bug ID: 108127
           Summary: FP due to combination of -Warray-bounds and UBSAN
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: frolov.da at phystech dot edu
  Target Milestone: ---

I've tried to build the next example (which is a reduced example of one of the
functions from firefox):

static const unsigned DECPOWERS[10] = { 1, 10, 100, 1000, 10000, 100000,
1000000, 10000000, 100000000, 1000000000};

unsigned foo(int discard, unsigned char *lsu, unsigned char *up) {
  int count;
  unsigned cut, quot;

  count = discard;

  cut = discard - (count - 1) - 1;

  if (cut == 1 - 1) {
    return 8;
  } else {
    quot = *up >> cut;
    cut++;
    return DECPOWERS[1 - cut];
  }
}

$ gcc -O2 -fsanitize-undefined-trap-on-error  -Werror=array-bounds 
-fsanitize=shift,signed-integer-overflow

and got an error:

<source>: In function 'foo':
<source>:16:21: error: array subscript 4294967265 is above array bounds of
'const unsigned int[10]' [-Werror=array-bounds=]
   16 |     return DECPOWERS[1 - cut];
      |            ~~~~~~~~~^~~~~~~~~
<source>:1:23: note: while referencing 'DECPOWERS'
    1 | static const unsigned DECPOWERS[10] = { 1, 10, 100, 1000, 10000,
100000, 1000000, 10000000, 100000000, 1000000000}

Looks like the builtin functions of the ubsan (such as UBSAN_CHECK_SUB) does
not allow optimizations and cause these false positives. If I remove the
sanitizer flags then there will be no warning.