[llvm-dev] The undef story

[Mehdi AMINI via llvm-dev]

2017-06-29 17:26 GMT-07:00 Peter Lawrence <peterl95124 at sbcglobal.net>:

> Hal,
>       Mehdi points out I mis-quoted you, I apologize sincerely.
>
> Mehdi,
>            Thank you for forcing me to go back and re-read what Hal wrote,
> I could have sworn Hal and I were in agreement at the time I wrote you,
> Must have been asleep at the wheel, not enough sleep last night
> However my request for a more concrete example stands
>
> Here’s what I said
>
> 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”.
>
>
> Here’s what Hal said in response
> > 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.
>
>
> The problem is I don’t know how to interpret what Hal said here,
> I can’t construct the “real” example from the “representative” example
> given his advise.
>
> If by “that code” he means the “if(cond)” code then the only way I see that
> the user can make it clear that “that code” is never execute is if “cond”
> is
> always false. But if “cond” is always false then the if-statement is just
> plain
> old dead code, has nothing to do with undefined behavior.
>

The compiler does not know it is dead code based on knowing the condition
is false.
The compiler knows this is dead code because if it wasn't dead code there
would be undefined behavior.

This is how we optimize a perfectly valid program based on the assumption
that the program does not exhibits undefined behavior.

To give your more to inspect, look at the output of this slightly modified
example: https://godbolt.org/g/89rB8M

Here we instantiate the template using char and long long, the code in the
branch is not dead at the source level: it is valid to take this path with
the long long (assuming 64bits here...) but not the char. If you look at
the generated code, it is better optimized for the char than for the long
long instantiation, and this is only thanks to the UB property of the
shift: we know the condition has to be false in the char instantiation
because it would be UB otherwise.




>
> So I am puzzled,
>
> That’s why I’m still asking for a more concrete example showing how
> We can optimize based on undefined behavior.
>

We don't optimized based on undefined behavior, this is the opposite: we
optimize based on knowing that there can't be undefined behavior ;)


-- 
Mehdi



>
>
> Peter Lawrence.
>
>
> For reference here’s your second version with the plain “else” rather than
> “else if"
>
>
> void do_something_1(int);
> void do_something_2();
>
>
> template <typename T>
> int do_something(T mask, bool cond) {
>   if (mask & 2)
>     return 42;
>
>   if (cond) {
>     T high_mask = mask >> 48;
>     if (high_mask > 5)
>       do_something_1(high_mask);
>     else
>       do_something_2();
>   }
>
>   return 0;
> }
>
> char foo();
> bool alwaysfalse();
> char f = do_something<char>(foo(), alwaysfalse());
>
>
>
>
>
>
> On Jun 29, 2017, at 2:49 PM, Mehdi AMINI <joker.eph at gmail.com> wrote:
>
>
>
> 2017-06-29 14:32 GMT-07:00 Peter Lawrence <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.
>
>
>
>> 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
>>
>> --
>> Mehdi
>>
>>
>>
>>
>>
>
>
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