https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68971
Bug ID: 68971
Summary: -Woverflow false alarm in code unreachable after
__builtin_mul_overflow
Product: gcc
Version: 5.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: eggert at gnu dot org
Target Milestone: ---
Compile the following code with 'gcc -O2 -S u.c' (x86-64, GCC 5.3.1 20151207
(Red Hat 5.3.1-2)):
int
main (void)
{
int result;
return (__builtin_mul_overflow (0x7fffffff, 0x7fffffff, &result)
? 2
: result == 0x7fffffff * 0x7fffffff);
}
GCC complains:
u.c: In function ‘main’:
u.c:7:29: warning: integer overflow in expression [-Woverflow]
: result == 0x7fffffff * 0x7fffffff);
^
But line 7 is unreachable, and GCC determined this (as can be seen in the
assembly-language output: 'main' simply returns 2 unconditionally). GCC should
not warn about integer overflows in unreachable code.
This bug appears to be independent of Bug#4210, as the problem occurs in both
if statements and if expressions.
This bug is causing problems with gnulib; see:
http://lists.gnu.org/archive/html/bug-gnulib/2015-12/msg00011.html
I guess we'll work around it in the meantime by compiling the program with
-Wno-overflow, but GCC should get fixed.
