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

[Philip Reames via llvm-dev]

On 02/24/2016 08:10 PM, Duncan P. N. Exon Smith via llvm-dev wrote:
>> On 2016-Feb-24, at 19:46, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
>>
>> On Wed, Feb 24, 2016 at 7:38 PM, Chandler Carruth <chandlerc at google.com> wrote:
>>> On Wed, Feb 24, 2016 at 7:34 PM Duncan P. N. Exon Smith
>>> <dexonsmith at apple.com> wrote:
>>>>
>>>>> On 2016-Feb-24, at 19:17, Chandler Carruth <chandlerc at google.com> wrote:
>>>>>
>>>>> On Wed, Feb 24, 2016 at 7:10 PM Sanjoy Das via llvm-dev
>>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>> On Wed, Feb 24, 2016 at 6:51 PM, Duncan P. N. Exon Smith
>>>>> <dexonsmith at apple.com> wrote:
>>>>>>> If we do not inline @foo(), and instead re-link the call site in
>>>>>>> @main
>>>>>>> to some non-optimized copy (or differently optimized copy) of @foo,
>>>>>>> then it is possible for the program to have the behavior {print("Y");
>>>>>>> print ("X")}, which was disallowed in the earlier program.
>>>>>>>
>>>>>>> In other words, opt refined the semantics of @foo() (i.e. reduced the
>>>>>>> set of behaviors it may have) in ways that would make later
>>>>>>> optimizations invalid if we de-refine the implementation of @foo().
>>>>>> I'm probably missing something obvious here.  How could the result of
>>>>>> `%t0 != %t1` be different at optimization time in one file than from
>>>>>> runtime in the "real" implementation?  Doesn't this make the CSE
>>>>>> invalid?
>>>>> `%t0` and `%t1` are "allowed" to "always be the same", i.e. an
>>>>> implementation of @foo that always feeds in the same
>>>>> value for `%t0` and `%t1` is a valid implementation (which is why the
>>>>> CSE was valid); but it is not the *only* valid implementation.  If I
>>>>> don't CSE the two load instructions (also a valid thing to do), and
>>>>> this is a second thread writing to `%par`, then the two values loaded
>>>>> can be different, and you could end up printing `"X"` in `@foo`.
>>>>>
>>>>> Did that make sense?
>>>> Yes.  To be sure I understand the scope: this is only a problem for
>>>> atomics, correct?  (Because multi-threaded behaviour with other globals
>>>> is UB?)
>>>>
>>>>>> Does linkonce_odr linkage have the same problem?
>>>>>> - If so, do you want to change it too?
>>>>>> - Else, why not?
>>>>> Going by the specification in the LangRef, I'd say it depends on how
>>>>> you define "definitive".  If you're allowed to replace the body of a
>>>>> function with a differently optimized body, then the above problem
>>>>> exists.
>>>>>
>>>>> I believe that is the case, and I strongly believe the problem you
>>>>> outline exists for linkonce_odr exactly as it does for available_externally.
>>>>>
>>>>> Which is what makes this scary: every C++ inline function today can
>>>>> trigger this.
>>>> Every C/C++ inline or template function.  But only the ones that use
>>>> atomics, right?
>>>
>>> Well, with *this* example...
>> Atomic are one source of non-determinism that compilers can reason
>> about.  I don't know if the following snippet is well defined or not,
>> but you could have similar issues with
>>
>>
>>   void foo() {
>>     int *p = malloc(sizeof(int));
>>     if (*p < 10) print("X");
>>   }
>>
>> or (again, I don't know if this is actually well defined)
>>
>>   void foo() {
>>     int t;  // it is probably reasonable to fold compares with
>> ptrtoint(alloca) to undef
>>     if ((intptr_t)(&t) < 10) print("X");
>>   }
>>
> The first one at least is UB, but as Richard pointed out the scope
> is certainly broader than atomics (it's not even just well-defined
> non-deterministism).
>
> I'm kind of terrified by the implications.
Me too.  :(
>
>> -- Sanjoy
>>
>>>>
>>>> Not that I'm sure that will end up being a helpful distinction.
>>>
>>> Right. See Richard's comment. I think that sums up the real issue here. =/
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