Undefined behavior sanitizer reports a failure when running Fortran
testcase gfortran.dg/ieee/large_1.f90 at -O2 and higher:

  /home/mjambor/gcc/mine/src/gcc/ext-dce.cc:600:15: runtime error: shift exponent 64 is too large for 64-bit type 'long unsigned int'
  /home/mjambor/gcc/mine/src/gcc/ext-dce.cc:404:23: runtime error: left shift of negative value -1
  FAIL: gfortran.dg/ieee/large_1.f90   -O2  (test for excess errors)

The failure is present since the introduction of the source file
ext-dce.cc with commit r15-1901-g98914f9eba5f19 (Jeff Law:
[to-be-committed][RISC-V][V3] DCE analysis for extension elimination)

One way to reproduce the issue is to bootstrap GCC with Fortran
enabled and with --with-build-config=bootstrap-ubsan and then run the
test case as usual.

It is however much easier to (on an x86_64-linux at least) simply
apply the following patch and then run
  make -k check-gfortran RUNTESTFLAGS="ieee.exp=large_1.f90"

--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -597,6 +597,7 @@ ext_dce_process_uses (rtx_insn *insn, rtx obj, bitmap live_tmp)
                      bit = subreg_lsb (y).to_constant ();
                      if (dst_mask)
                        {
+                         gcc_assert (bit < 64);
                          dst_mask <<= bit;
                          if (!dst_mask)
                            dst_mask = -0x100000000ULL;