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

[David Chisnall via cfe-dev]

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