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

[Sanjoy Das via llvm-dev]

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");
  }

-- 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. =/