http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56917
Bug #: 56917
Summary: -ftrapv detects a overflow wrongly.
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: [email protected]
ReportedBy: [email protected]
Created attachment 29855
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29855
steps to reproduce the -ftrapv problem
I found that following program aborts with -ftrapv with gcc-4.8.0 on
Debian GNU/Linux (x86_64).
% cat tst.c
unsigned long ul = 0x8000000000000000UL;
int main(int argc, char *argv[])
{
long l;
l = (-(long)(ul-1))-1;
return 0;
}
% /home/src/gcc/bin/gcc -g -ftrapv -Wall tst.c
tst.c: In function 'main':
tst.c:5:8: warning: variable 'l' set but not used [-Wunused-but-set-variable]
long l;
^
% ./a.out
zsh: abort (core dumped) ./a.out
I think it should not abort on x86_64.
Since ul is LONG_MAX+1 (in indefinite precision), ul-1 is LONG_MAX which is
representable in long.
So the cast, (long)(ul-1), doesn't change the value.
Its negation, -(long)(ul-1), is -LONG_MAX which is also representable in long.
Its predecessor, (-(long)(ul-1))-1, is -LONG_MAX-1 = LONG_MIN which is also
representable in long.
gcc-ftrapv-problem-log.txt contains the log with gcc -v.
