[Bug c++/83283] New: Casting from boolean to unsigned char to enum returns incorrect results

[lukas.lorimer at snowflake dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83283

            Bug ID: 83283
           Summary: Casting from boolean to unsigned char to enum returns
                    incorrect results
           Product: gcc
           Version: 7.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lukas.lorimer at snowflake dot net
  Target Milestone: ---

Hello, it appears casting from boolean to unsigned char then to an enum returns
incorrect results when the operation is vectorized. Below is a minimized
example.

* Removing the typedef and including `cstdint` also triggers the bug.
* Running it with `-fsanitize=undefined` does not produce any warnings (but
does stop the bug from occurring).
* This appears to be a regression since g++ 6.2.1.

// START main.ii
# 1 "main.cpp"
# 1 "/tmp/a//"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "main.cpp"
typedef unsigned char uint8_t;

enum EN : uint8_t {
  X = 0,
  Y = 1
};

void __attribute__((noinline)) fn(EN *v, int size) {
    for (int i = 0; i < size; ++i) {

        const bool b = (v[i] == EN::Y);
        v[i] = static_cast<EN>(static_cast<uint8_t>(b));
    }
}

int main() {
  constexpr int items = 32;
  EN vals[items] = {X};
  vals[3] = Y;

  fn(vals, items);
  return vals[3];
}
// END main.ii

$ g++72 -g -O1 -ftree-loop-vectorize -Wall -Wextra main.cpp
# No output
$ ./a.out; echo $?
# Expected output: 1
255
$ g++72 -v
Using built-in specs.
COLLECT_GCC=/usr/local/bin/g++72
COLLECT_LTO_WRAPPER=/opt/rh/devtoolset-7/root/usr/libexec/gcc/x86_64-redhat-linux/7/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,fortran,lto --prefix=/opt/rh/devtoolset-7/root/usr
--mandir=/opt/rh/devtoolset-7/root/usr/share/man
--infodir=/opt/rh/devtoolset-7/root/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --enable-plugin --with-linker-hash-style=gnu
--enable-initfini-array --with-default-libstdcxx-abi=gcc4-compatible
--with-isl=/builddir/build/BUILD/gcc-7.2.1-20170829/obj-x86_64-redhat-linux/isl-install
--enable-libmpx
--with-mpc=/builddir/build/BUILD/gcc-7.2.1-20170829/obj-x86_64-redhat-linux/mpc-install
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
Thread model: posix
gcc version 7.2.1 20170829 (Red Hat 7.2.1-1) (GCC) 


The behaviour is correct if one uses:
const uint8_t b = (v[i] == EN::Y);

It appears the difference between the two is a single pand instruction which
applies a mask to the result of some vectorized operations.