[Bug c/100061] New: Comparison guarding undefined behaviour disappears

["t.josefowitz at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100061

            Bug ID: 100061
           Summary: Comparison guarding undefined behaviour disappears
           Product: gcc
           Version: 10.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: t.josefowitz at gmail dot com
  Target Milestone: ---

Created attachment 50580
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50580&action=edit
test case

with int a = 0, b = -1, c = MAX_INT, d = 0,

        if ((a > d) && (a > d + 1)) {
                puts("YES");
                ret = 1;
        } else if ((b > c) && (b > c + 1)) {
                puts("NO NOT GREAT");
                ret = 0;
        } else {
                puts("OK");
                ret = 2;
        }

and optimization enabled, "else if" gets selected. While b > c + 1 is undefined
for c == MAX_INT, b > c should guard against it. The generated amd64 machine
code simply lacks the (b > c) check in the generated code, while curiously the
(a > d) from the first if is present.

with -fwrapv the (b > c + 1) becomes "more defined" and thus the bug then does
not trigger anymore (at least not in the same way), but I think e.g. optimizing
away (b > c) is not actually valid here as it seems to be a valid guard for the
undefined behaviour.

I reproduced this on GCC 10.2.0 and 10.3.0.

The attached cmp.c can be compiled:
gcc -O2 cmp.c -Wall -Wextra

and then executed:
./a.out 0 -1 2147483647 0

to trigger the issue.

Notably, if compiled with -fsanitize=undefined, no undefined behaviour is
reported but the issue goes away.


# gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
10.2.0-13ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-10
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-gnu-unique-object --disable-vtable-verify --enable-plugin
--enable-default-pie --with-system-zlib --enable-libphobos-checking=release
--with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch
--disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-10-JvwpWM/gcc-10-10.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-10-JvwpWM/gcc-10-10.2.0/debian/tmp-gcn/usr,hsa
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.0 (Ubuntu 10.2.0-13ubuntu1)