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From: "eggert at gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug preprocessor/55715] New: bogus overflow warning for #if A-B when A<0 & B==minimum integer
Date: Mon, 17 Dec 2012 07:23:00 -0000	[thread overview]
Message-ID: <bug-55715-4@http.gcc.gnu.org/bugzilla/> (raw)


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55715

             Bug #: 55715
           Summary: bogus overflow warning for #if A-B when A<0 &
                    B==minimum integer
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: eggert@gnu.org


Consider this C program:

#if 9223372036854775807 < 0
#endif
#if -1 - 9223372036854775807 < 0
#endif
#if -1 - (-1 - 9223372036854775807) < 0
#endif

When compiled with GCC (x86-64), GCC 4.7.2 warns:

t.c:5:37: warning: integer overflow in preprocessor expression [enabled by
default]

The warning is incorrect, since there is no overflow in this program.

Evidently GCC's preprocessor computes A-B by first negating B, and then adding
the result to A.  This causes GCC to incorrectly report overflow when B is the
minimum possible integer and A is negative, because in this case -B overflows
even though A-B does not overflow.


             reply	other threads:[~2012-12-17  7:23 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-12-17  7:23 eggert at gnu dot org [this message]
2013-12-10  1:23 ` [Bug preprocessor/55715] " jsm28 at gcc dot gnu.org
2013-12-10  1:25 ` jsm28 at gcc dot gnu.org

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