[cfe-dev] clang++: Handling of division by zero in array bounds

[James Dennett via cfe-dev]

On Wed, Nov 9, 2016 at 2:06 AM, David Chisnall via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On 9 Nov 2016, at 07:55, Stephan Bergmann via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
> >
> > What I observe with various versions of Clang:
> >
> >> $ cat test.cc
> >> #include <iostream>
> >> int main() {
> >>    char a[1/0];
> >>    std::cout << sizeof a << '\n';
> >> }
> >>
> >> $ clang++ -Weverything test.cc
> >> test.cc:3:11: warning: variable length arrays are a C99 feature
> >>      [-Wvla-extension]
> >>    char a[1/0];
> >>          ^
> >> test.cc:3:11: warning: variable length array used [-Wvla]
> >> 2 warnings generated.
> >>
> >> $ ./a.out
> >> 0
> >
> > Is there a specific reason to not emit a warning/error about the
> undefined behavior in evaluating the constant bounds expression, 1/0?
>
> I believe that the issue here is that 1/0 is *not* a constant expression,
> it is undefined behaviour (typically, run-time trap).  We probably should
> have a special return value for attempting to evaluate something that
> should be an ICE and finding that the result is undefined, which would
> allow this to become a more helpful error along the lines of ‘array length
> is an undefined value, this will abort at run time’.
>
> Currently, I believe that the undefined value is simply marked as
> something that can not be evaluated at compile time and so this is
> equivalent to:
>
> int foo(int d)
> {
>   char a[1/d];
>   std::cout << sizeof a << '\n';
> }
>
> This is valid code when d > 0, but if d == 0 it will likely trap.
>
> David
>

A constant expression that would give undefined behavior is ill-formed, and
the compiler is required to issue a diagnostic.  It does, by default, but
the diagnostic is misleading and doesn't identify why the code is
ill-formed.  So it's not formally a violation of the C++ standard, but it's
not a good error message.  Compiling the code and acting as if 1/0 was 0 is
also fairly dubious (though again, not a violation of the C++ standard,
which only requires a diagnostic and never forbids producing an executable).

-- James
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