When compiling libstdc++-v3/testsuite/21_strings/basic_string/cons/char/1.cc there are various UBs in cc1plus:
../../gcc/ipa-polymorphic-call.c:926:59: runtime error: signed integer overflow: -9223372036854775808 * 8 cannot be represented in type 'long int'
../../gcc/tree-ssa-alias.c:704:30: runtime error: signed integer overflow: -9223372036854775808 * 8 cannot be represented in type 'long int'
../../gcc/dwarf2out.c:13534:15: runtime error: signed integer overflow: 9223372036854775408 + 400 cannot be represented in type 'long int [4]'
../../gcc/dwarf2out.c:13559:11: runtime error: signed integer overflow: -9223372036854775808 + -400 cannot be represented in type 'long int [4]'

  csz01 = str01.max_size();
  // NB: As strlen(str_lit01) != csz01, this test is undefined. It
  // should not crash, but what gets constructed is a bit arbitrary.
  try {
    std::string str03(str_lit01, csz01 + 1);
    VERIFY( true );
  }              
  catch(std::length_error& fail) {
    VERIFY( true );
  }
  catch(...) {
    VERIFY( false );
  }

csz01 + 1 is 0x8000000000000000UL on x86_64, and both ipa-polymorphic-call.c and tree-ssa-alias.c try to multiply that by BITS_PER_UNIT.

Another similar UB is on:
void
foo (char *p)
{
  p[- __LONG_MAX__ - 1] = 1;
}
at -g -O2 I'm getting
../../gcc/builtins.c:351:54: runtime error: signed integer overflow: -9223372036854775808 * 8 cannot be represented in type 'long int [3]'

For ipa-polymorphic-call.c I wrote untested:
2017-07-28  Jakub Jelinek  <jakub@redhat.com>

	* ipa-polymorphic-call.c
	(ipa_polymorphic_call_context::ipa_polymorphic_call_context): Perform
	offset arithmetic in offset_int, bail out if the resulting bit offset
	doesn't fit into shwi.

--- gcc/ipa-polymorphic-call.c.jj	2017-06-12 12:41:55.000000000 +0200
+++ gcc/ipa-polymorphic-call.c	2017-07-28 15:02:43.747354910 +0200
@@ -921,9 +921,13 @@ ipa_polymorphic_call_context::ipa_polymo
 		 and MEM_REF is meaningless, but we can look futher.  */
 	      if (TREE_CODE (base) == MEM_REF)
 		{
+		  offset_int o = mem_ref_offset (base) * BITS_PER_UNIT;
+		  o += offset;
+		  o += offset2;
+		  if (!wi::fits_shwi_p (o))
+		    break;
 		  base_pointer = TREE_OPERAND (base, 0);
-		  offset
-		    += offset2 + mem_ref_offset (base).to_short_addr () * BITS_PER_UNIT;
+		  offset = o.to_shwi ();
 		  outer_type = NULL;
 		}
 	      /* We found base object.  In this case the outer_type
@@ -961,10 +965,15 @@ ipa_polymorphic_call_context::ipa_polymo
 	    break;
 	}
       else if (TREE_CODE (base_pointer) == POINTER_PLUS_EXPR
-	       && tree_fits_uhwi_p (TREE_OPERAND (base_pointer, 1)))
+	       && TREE_CODE (TREE_OPERAND (base_pointer, 1)) == INTEGER_CST)
 	{
-	  offset += tree_to_shwi (TREE_OPERAND (base_pointer, 1))
-		    * BITS_PER_UNIT;
+	  offset_int o = offset_int::from (TREE_OPERAND (base_pointer, 1),
+					   SIGNED);
+	  o *= BITS_PER_UNIT;
+	  o += offset;
+	  if (!wi::fits_shwi_p (o))
+	    break;
+	  offset = o.to_shwi ();
 	  base_pointer = TREE_OPERAND (base_pointer, 0);
 	}
       else

but given that there are tons of other spots, I wonder if using offset_int for such stuff is the right way to go.