[Bug sanitizer/67662] New: -fsanitize=undefined cries wolf for X - 1 + X when X is 2**30

[eggert at gnu dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67662

            Bug ID: 67662
           Summary: -fsanitize=undefined cries wolf for X - 1 + X when X
                    is 2**30
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eggert at gnu dot org
                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 36358
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36358&action=edit
Test program illustrating the bug

I uncovered this bug while trying to use -fsanitize=undefined on the tzcode
source.

gcc -fsanitize=undefined (x86-64) reports an error at runtime for the
expression 'X - 1 + X' when X is 2**30, even though that expression does not
overflow. To reproduce the problem, compile and run the attached program u.c
with:

gcc -fsanitize=undefined -static-libubsan -O2 u.c
./a.out

It's a valid program that does not overflow, but the output I get is:

u.c:5:31: runtime error: signed integer overflow: 1073741824 + 1073741824
canno\
t be represented in type 'int'
u.c:5:7: runtime error: signed integer overflow: -2147483648 - 1 cannot be
repr\
esented in type 'int'

There is a similar problem with 2**62 and 'long long'.