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

[James Dennett via cfe-dev]

On Wed, Nov 9, 2016 at 2:34 AM, Stephan Bergmann <sbergman at redhat.com>
wrote:

> On 11/09/2016 11:12 AM, James Dennett wrote:
>
>> On Wed, Nov 9, 2016 at 2:06 AM, David Chisnall via cfe-dev
>> <cfe-dev at lists.llvm.org <mailto: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 <mailto: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,
>>
>
> No, 1/0 is not a core constant expression, because evaluating it "would
> have undefined behavior as spedified in Clauses 1 through 16"
> ([expr.const]), so is not a constant expression.


The language requires a constant expression there.  1/0 is indeed not such
a thing, and hence a diagnostic is required.


> So apparently the non-standard C99 vla extension kicks in as David
> explained.  (Question is whether that extension can be disabled?)
>
>
Right, the extension kicks in, and it's only standard-conforming if a
diagnostic is issued (as it is by default).  In this case the extension
can't work (the size isn't defined), so that shouldn't happen -- not
because it violates the standard, but because it's nonsensical.

-- James
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