[Bug sanitizer/66908] New: Uninitialized variable when compiled with UBsan

[m.guseva at samsung dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66908

            Bug ID: 66908
           Summary: Uninitialized variable when compiled with UBsan
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m.guseva at samsung 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
  Target Milestone: ---

Created attachment 36002
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36002&action=edit
Reproducer

When compiled with "-fsanitize=shift,bounds" the gcc reports that "i" variable
maybe uninitialized:
gcc  -O2  -fsanitize=shift,bounds -std=gnu89 -Werror=maybe-uninitialized -c
testcase.c
testcase.c: In function Б─≤fooБ─≥:
testcase.c:21:36: error: Б─≤i.1Б─≥ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
     res[i] = ((data->scale[i] + tab[i]) << mult);

It's not true in original source code however in produced gimple the "i.1" is
initialized in <D.3009> block but may be used in block <D.3010>:
 23   if (mult.0 > 31) goto <D.3009>; else goto <D.3010>;
 24   <D.3009>:
 25   D.3011 = (unsigned long) mult.0;
 26   D.3012 = data->scale[i];
 27   D.3013 = (int) D.3012;
 28   i.1 = i;
 29   UBSAN_BOUNDS (0B, i.1, 21);
 30   D.3015 = tab[i.1];
 31   D.3016 = (int) D.3015;
 32   D.3017 = D.3013 + D.3016;
 33   D.3018 = (unsigned long) D.3017;
 34   __builtin___ubsan_handle_shift_out_of_bounds (&*.Lubsan_data0, D.3018,
D.3011);
 35   goto <D.3019>;
 36   <D.3010>:
 37   <D.3019>:
 38   D.3020 = (long unsigned int) i;
 39   D.3021 = D.3020 * 4;
 40   D.3022 = res + D.3021;
 41   D.3012 = data->scale[i];
 42   D.3013 = (int) D.3012;
 43   UBSAN_BOUNDS (0B, i.1, 21);

Discovered in gcc 4.9.2, reproduced on trunk with "-std=gnu89".