https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78215
Bug ID: 78215
Summary: aggressive-loop-optimizations undefined behavior
warning does not trigger
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: paf at cray dot com
Target Milestone: ---
Created attachment 39968
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39968&action=edit
reproducer
On GCC 4.8.5 from Red Hat* and GCC 7.0.0 20161104** (built with only
--disable-multib option), the undefined behavior warning from
aggressive-loop-optimization does not trigger with certain loops.
gcc invocation:
gcc -g -Wall -O2 -Werror loop_error.c
The program (attached in both .c and .i versions) is a simple for loop with
undefined behavior, resulting in an infinite loop (i is an int, 'value' is an
unsigned long long):
for (i = 0; i < 20000; i++) {
/* Remove this break and the compiler warns correctly -
* Note there is no break in the original code. */
/* Overflow of signed int occurs at i = 205 */
if (i == 206)
break;
container->value = i * 1024 * 1024 *10;
printf("i: %d\n",i);
}
The compiler fails to warn on this construct and generates an infinite loop.
The original code (which is much more complex and has dependencies) did not
have a 'break', but had a number of functional and print calls. I was unable
to reproduce the problem without a break statement, but I believe it can be
done.
The compiler compiles the program as above/as attached without any
warnings/errors. If the break is removed, it warns (-Werror is set):
./loop_error.c: In function ‘main’:
./loop_error.c:33:38: error: iteration 205u invokes undefined behavior
[-Werror=aggressive-loop-optimizations]
container->value = i * 1024 * 1024 *10;
^
./loop_error.c:27:2: note: containing loop
for (i = 0; i < 20000; i++) {
So the bug is a failure to warn on undefined behavior in some cases.
*gcc -v output:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-linker-build-id --with-linker-hash-style=gnu
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin
--enable-initfini-array --disable-libgcj
--with-isl=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/isl-install
--with-cloog=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/cloog-install
--enable-gnu-indirect-function --with-tune=generic --with-arch_32=x86-64
--build=x86_64-redhat-linux
Thread model: posix
gcc version 4.8.5 20150623 (Red Hat 4.8.5-4) (GCC)
**gcc -v output:
Using built-in specs.
COLLECT_GCC=./gcc
COLLECT_LTO_WRAPPER=/home/build/gcc_install/usr/local/bin/../libexec/gcc/x86_64-pc-linux-gnu/7.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure --disable-multilib
Thread model: posix
gcc version 7.0.0 20161104 (experimental) (GCC)
