[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")

[Sanjoy Das via llvm-dev]

On Thu, Feb 25, 2016 at 6:35 PM, Duncan P. N. Exon Smith
<dexonsmith at apple.com> wrote:
>>  // In C
>>  void foo() {
>>    int c;
>>    if (c) print("X");
>>    escape(&c);  // escape is an empty function
>>  }
>>
>> which I think is not UB in C (is it?), but will boil down to the kind
>> of IR above.
>
> I'm pretty sure the `if (c)` is UB because it's branching on an uninitialized
> value, which could have a trap representation.

I am *way* out of my depth here, but what if 'c' was an 'unsigned
char' (and not an 'int')?  Wouldn't that prevent UB, since it is
escaped (cannot be a register variable), and is an 'unsigned char'
(doesn't have a trap representation)?

-- Sanjoy