[Bug c/85158] New: ubsan and -O3 finds a problem

[dcb314 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85158

            Bug ID: 85158
           Summary: ubsan and -O3 finds a problem
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dcb314 at hotmail dot com
  Target Milestone: ---

For this C code:

short *a;
long b;
d() {
  char *c;
  for (; b; b++)
    if (a[b + 1])
      c[1] = a[b];
  e();
}

and recent ubsan trunk gcc

$ ~/gcc/results/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/home/dcb/gcc/results/bin/gcc
COLLECT_LTO_WRAPPER=/home/dcb/gcc/results.259004.ubsan/libexec/gcc/x86_64-pc-linux-gnu/8.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../trunk/configure --prefix=/home/dcb/gcc/results.259004.ubsan
--with-build-config=bootstrap-ubsan --disable-multilib --disable-werror
--enable-checking=df,extra,fold,rtl,yes --enable-languages=c,c++,fortran
Thread model: posix
gcc version 8.0.1 20180402 (experimental) (GCC) 
$

Then I get:

$ ~/gcc/results/bin/gcc -c -w -O3 bug424.c
../../trunk/gcc/tree-data-ref.c:3758:26: runtime error: signed integer
overflow: 9223372036854775807 - -1 cannot be represented in type 'long int'
../../trunk/gcc/tree-data-ref.c:3760:40: runtime error: signed integer
overflow: -9223372036854775808 - 1 cannot be represented in type 'long int'
$