Overflow hazard in pgbench - Mailing list pgsql-hackers

moonjelly just reported an interesting failure [1].  It seems that
with the latest bleeding-edge gcc, this code is misoptimized:

                /* check random range */
                if (imin > imax)
                {
                    pg_log_error("empty range given to random");
                    return false;
                }
                else if (imax - imin < 0 || (imax - imin) + 1 < 0)
                {
                    /* prevent int overflows in random functions */
                    pg_log_error("random range is too large");
                    return false;
                }

such that the second if-test doesn't fire.  Now, according to the C99
spec this code is broken, because the compiler is allowed to assume
that signed integer overflow doesn't happen, whereupon the second
if-block is provably unreachable.  The failure still represents a gcc
bug, because we're using -fwrapv which should disable that assumption.
However, not all compilers have that switch, so it'd be better to code
this in a spec-compliant way.  I suggest applying the attached in
branches that have the required functions.

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=moonjelly&dt=2021-06-26%2007%3A03%3A17

            regards, tom lane

diff --git a/src/bin/pgbench/pgbench.c b/src/bin/pgbench/pgbench.c
index e61055b6b7..c4023bfa27 100644
--- a/src/bin/pgbench/pgbench.c
+++ b/src/bin/pgbench/pgbench.c
@@ -2450,7 +2450,8 @@ evalStandardFunc(CState *st,
         case PGBENCH_RANDOM_ZIPFIAN:
             {
                 int64        imin,
-                            imax;
+                            imax,
+                            delta;
 
                 Assert(nargs >= 2);
 
@@ -2464,7 +2465,8 @@ evalStandardFunc(CState *st,
                     pg_log_error("empty range given to random");
                     return false;
                 }
-                else if (imax - imin < 0 || (imax - imin) + 1 < 0)
+                else if (pg_sub_s64_overflow(imax, imin, &delta) ||
+                         pg_add_s64_overflow(delta, 1, &delta))
                 {
                     /* prevent int overflows in random functions */
                     pg_log_error("random range is too large");