[Bug c++/64491] warning: loop exit may only be reached after undefined behavior

[joona.kiiski at iki dot fi]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64491

joona.kiiski at iki dot fi changed:

           What    |Removed                     |Added
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                 CC|                            |joona.kiiski at iki dot fi

--- Comment #2 from joona.kiiski at iki dot fi ---
Hi,

My name is Joona Kiiski, I'm one the authors of Stockfish chess software
(https://stockfishchess.org/). This bug was originally reported to us a week
ago. 

After initial analysis it looked obviously like a gcc bug, so we advised the
reporter to raise a bug against GCC. Unfortunately, he wasn't able to give you
nearly enough details. I hope that what I can provide below meets your
expectations:

- Environment: qemu-x86-64 running debian sid
- GCC: Debian snapshot 20141812 5.0

- The bug appears when compiling the Stockfish master branch from
https://github.com/official-stockfish/Stockfish.git with this snapshot.

- For you convenience I've reduced the problem to:

<test.cpp>
int A[8];
int B[8];
int C;

int main() {
    for (int f = 0; f <= 7; ++f)
        A[f] = f;
    for (int f = 0; f <= 7; ++f)
        B[f] = (f > 0 ? A[f - 1] : 0) | (f < 7 ? A[f + 1] : 0);
    C = B[7];
}
</test.cpp>

Compile with

<Makefile>
CXXFLAGS += -O3 -flto
LDFLAGS += $(CXXFLAGS)
test: test.o
    $(CXX) -o $@ test.o $(LDFLAGS)
</Makefile>

Files above are also viewable at
https://github.com/official-stockfish/Stockfis/tree/gcc_bug/src

This produces the bogus warning mentioned in the original bug report.

If you need any further details, please let me know.

Kind Regards,
Joona Kiiski