[This bug was discovered by kantorzs...@yahoo.com]
When compiling the testcase below at -O2, using gcc 4.3.3 instead of an infite
loop, where the variable 'from' takes values from -32768 to 32767,
it goes up way beyond that limit:
$ gcc -O2 tes -Wall -W -Wstrict-overflow=5 && ./a.out
0
1
...
2253491
When using -fwrapv, or -fno-strict-overflow 'from' iterates between -32768 and
32767 in an infinite loop, as expected.
There are 2 bugs here:
- no warning is given with -Wstrict-overflow=5, although -fstrict-overflow
changes the behaviour of the code
- from is of type short, so when sign extended to an int, it should only take
values allowed for a short: -32768 to 32767, not all values allowed for an int
(like 2253491)
I think this is a bug, because although signed overflow is undefined behaviour,
that should affect only the value of 'from', which is still of type 'short',
and converting to int should obey the limits for the 'short' type, i.e. use a
sign-extension.
Here is a diff of the assembly, when using -O2, vs -O2 -fno-strict-overflow:
--- x2.s 2009-04-11 15:28:01.000000000 +0300
+++ x2-no-overflow.s 2009-04-11 15:27:58.000000000 +0300
@@ -10,15 +10,16 @@
.LFB13:
pushq %rbx
.LCFI0:
+ xorl %esi, %esi
xorl %ebx, %ebx
.p2align 4,,10
.p2align 3
.L2:
- movl %ebx, %esi
movl $.LC0, %edi
xorl %eax, %eax
- call printf
addl $1, %ebx
+ call printf
+ movswl %bx,%esi
jmp .L2
.LFE13:
.size main, .-main
Testcase:
#include <stdio.h>
int
main ()
{
int until = 40001;
short from = 0;
for (; from < until; from++)
printf ("%d\n", from);
return 0;
}
$ gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.3.3-5'
--with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3
--program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug
--enable-objc-gc --enable-mpfr --with-tune=generic --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.3.3 (Debian 4.3.3-5)
Also reproduced with trunk:
$ ~/gcc_inst/bin/gcc -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc/configure --disable-multilib --disable-static
--prefix=/home/edwin/gcc_inst/ --enable-languages=c,c++
Thread model: posix
gcc version 4.5.0 20090408 (experimental) [trunk revision 145769] (GCC)
--
Summary: signed overflow in loop induction variable: missing
warning and wrong code
Product: gcc
Version: 4.3.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: edwintorok at gmail dot com
GCC build triplet: x86_64-linux-gnu
GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39736
