When I compile the following code with -O1 -ftree-vrp or -O2, it prints
-2147483647, instead of 1 as expected. (The intent of the code was to search
for integers which, when squared and truncated, give small positive values.)
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
int main() {
int min = INT_MAX, i;
for(i = 1; i > 0; ++i) {
int candidate = i * i;
if((candidate < min) && (candidate > 0))
min = candidate;
}
printf("min was %d\n", min);
return 0;
}
If you put a printf each time a new min is found, you can see it behaves as
though the candidate > 0 check is not occurring. Manually replacing candidate
with i*i does not fix the problem. Initializing candidate to (i * i) &
0xffffffff does fix it.
I'm running Gentoo on amd64, and the problem occurs with gcc-4.1.0-beta20060210
and gcc-beta20060127. It occurs with both -m64 and -m32. I can't trigger it
with gcc-3.4.3. I haven't tested it on any other versions.
$ gcc -v
Using built-in specs.
Target: x86_64-pc-linux-gnu
Configured with:
/var/tmp/portage/gcc-4.1.0_beta20060210/work/gcc-4.1-20060210/configure
--prefix=/usr --bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/4.1.0-beta20060210
--includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.1.0-beta20060210/include
--datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.1.0-beta20060210
--mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.1.0-beta20060210/man
--infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.1.0-beta20060210/info
--with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.1.0-beta20060210/include/g++-v4
--host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --disable-altivec
--enable-nls --without-included-gettext --with-system-zlib --disable-checking
--disable-werror --disable-libunwind-exceptions --enable-multilib
--disable-libmudflap --disable-libgcj --enable-languages=c,c++,fortran
--enable-shared --enable-threads=posix --enable-__cxa_atexit
--enable-clocale=gnu
Thread model: posix
gcc version 4.1.0-beta20060210 (prerelease)
--
Summary: -ftree-vrp generates invalid code with this loop that
overflows integer multiplication
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: tomfelker at gmail dot com
GCC build triplet: 4.1.0-beta20060210
GCC host triplet: x86_64-pc-linux-gnu
GCC target triplet: x86_64-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26358
