[cfe-commits] PATCH: Enhance array bounds checking

[John McCall]

On Jul 14, 2011, at 10:28 AM, Ted Kremenek wrote:
> I was reviewing this patch (which I think is a great step), and I had a high-level comment about the following test case:
> 
> +void swallow (const char *x) { (void)x; }
> +void test_pointer_arithmetic() {
> +  const char hello[] = "Hello world!"; // expected-note 2 {{declared here}}
> +  const char *helloptr = hello;
> +
> +  swallow("Hello world!" + 6); // no-warning
> +  swallow("Hello world!" - 6); // expected-warning {{refers before the beginning of the array}}
> +  swallow("Hello world!" + 14); // expected-warning {{refers past the end of the array}}
> 
> Do we really want this to be a warning?  There are plenty of examples where an out-of-bounds pointer is computed for legit reasons.  As long as that address is not dereferenced, there isn't necessarily a problem.  I'm fearful this may generate a fair amount of noise on codebases that do elaborate tricks with pointer offsets.  Indeed this very example doesn't actually exhibit a "bug".

Note that it's undefined behavior to try to construct a pointer to before the beginning or after the end of an array (except *immediately* past the end);  it's not tied to dereferencing that pointer.  LLVM does do some optimizations here; see in 'inbounds' modifier on getelementptr.

John.