[llvm-dev] The undef story

Hal Finkel via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 29 15:00:15 PDT 2017

On 06/29/2017 04:49 PM, Mehdi AMINI wrote:
> 2017-06-29 14:32 GMT-07:00 Peter Lawrence <peterl95124 at sbcglobal.net 
> <mailto:peterl95124 at sbcglobal.net>>:
>     Mehdi,
>                I think the following was the point of the conversation,
>     That both those examples are illegal C programs.
>     They are both “undefined behavior” because they both
>     use a shift amount that is too large.
> Can you confirm that even if the shift isn't executed the program 
> exhibits undefined behavior?
> That wasn't my understanding, so I don't believe this program exhibits UB.

That's correct. The program does not exhibit UB. The too-large-shift is 
never dynamically reached. Furthermore, that's the point. Using the fact 
that we assume the program does not have UB, and the program did not 
have UB, to prune dead code out of some of the instantiated templates, 
was a good thing.

Regarding whether it should be rejected by the compiler, it shouldn't. 
If you put the source I put in the email through Clang, it will indeed 
warn about the shift amount. A user is free to use -Werror, or similar, 
if they'd like such warnings to be an error. In the real code, the shift 
amount was only available after inlining, so the frontend could not have 
generated a warning or error directly.


>     They both should have been rejected by the compiler
>     even though they weren’t.
>     Hal agrees wth this assessment,
> I'm surprise by the confidence you're exhibiting while speaking for 
> others.
> -- 
> Mehdi
>     That’s why we’re waiting for a more complete example.
>     My belief is that undefined behavior is an optimization hazard,
>     Not an optimization opportunity.  Its just a belief, I could be proved
>     Wrong at any moment, but it feels right to me.
>     I would start looking for a more complete example myself, but my
>     Belief is so strong that "optimizing undefined behavior" seems
>     like a self-contradiction to me, and I don’t know where to
>     Even start looking.
>     I write compiler test programs in my spare time as a hobby,
>     (which someday I’d like to contribute to llvm)
>     So it’s not like I don’t have the knowledge or the inclination,
>     I just don’t know how to approach this problem.
>     You would think that since “optimization of undefined behavior”
>     Has become such a bedrock concept in llvm that by now some
>     Concrete examples would be readily at hand,
>     But this doesn’t seem to be the case.
>     So I’m eagerly awaiting Hal’s (or anyone else's) next email
>     That has a complete example.
>     Peter Lawrence.
>>>>>         I can't comment on SPEC, but this does remind me of code I
>>>>>         was working on recently. To abstract the relevant parts,
>>>>>         it looked something like this:
>>>>>         template <typename T>
>>>>>         int do_something(T mask, bool cond) {
>>>>>         if (mask & 2)
>>>>>         return 1;
>>>>>         if (cond) {
>>>>>         T high_mask = mask >> 48;
>>>>>         if (high_mask > 5)
>>>>>         do_something_1(high_mask);
>>>>>         else if (high_mask > 3)
>>>>>         do_something_2();
>>>>>         }
>>>>>         return 0;
>>>>>         }
>>>>>         This function ended up being instantiated on different
>>>>>         types T (e.g. unsigned char, unsigned int, unsigned long,
>>>>>         etc.) and, dynamically, cond was always false when T was
>>>>>         char. The question is: Can the compiler eliminate all of
>>>>>         the code predicated on cond for the smaller types? In this
>>>>>         case, this code was hot, and moreover, performance
>>>>>         depended on the fact that, for T = unsigned char, the
>>>>>         function was inlined and the branch on cond was
>>>>>         eliminated. In the relevant translation unit, however, the
>>>>>         compiler would never see how cond was set.
>>>>>         Luckily, we do the right thing here currently. In the case
>>>>>         where T = unsigned char, we end up folding both of the
>>>>>         high_mask tests as though they were false. That entire
>>>>>         part of the code is eliminated, the function is inlined,
>>>>>         and everyone is happy.
>>>>>         Why was I looking at this? As it turns out, if the 'else
>>>>>         if' in this example is just 'else', we don't actually
>>>>>         eliminate both sides of the branch. The same is true for
>>>>>         many other variants of the conditionals (i.e. we don't
>>>>>         recognize all of the code as dead).
>>>>         I apologize in advance if I have missed something here and
>>>>         am misreading your example...
>>>>         This doesn’t make sense to me, a shift amount of 48 is
>>>>         “undefined” for unsigned char,
>>>>         How do we know this isn’t a source code bug,
>>>>         What makes us think the the user intended the result to be “0”.
>>>         As I said, this is representation of what the real code did,
>>>         and looked like, after other inlining had taken place, etc.
>>>         In the original form, the user's intent was clear. That code
>>>         is never executed when T is a small integer type.
>>         I will still have a hard time believing this until I see a
>>         real example, can you fill in the details ?
>>     Hal gave you a real example, have you tried? I feel like you're
>>     asking more effort from others than you are ready to put in: it
>>     took me less than 5 minutes to reproduce what Hal was describing
>>     using his snippet:
>>     See the difference between https://godbolt.org/g/YYtsxB and
>>     https://godbolt.org/g/dTBBDq <https://godbolt.org/g/dTBBDq>
>>     -- 
>>     Mehdi

Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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