[llvm-dev] Function Inlining and undef / poison question

mats petersson via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 15 16:29:42 PDT 2017

On 15 June 2017 at 23:12, John Regehr via llvm-dev <llvm-dev at lists.llvm.org>

> Hi Peter, I don't see what you're driving at here. You can perhaps move
> the discussion forward with a complete, compilable example.
> John

I don't KNOW for sure, but I think I understand, see below:

> On 6/15/17 11:27 AM, Peter Lawrence via llvm-dev wrote:
>> Nuno,
>>           One of your recent emails got me thinking about inlining.
>> I say this function is always well defined,
>> And it always executes statement S
>> F(a)
>> {
>>    If (a == a) S;
>> }
I think the problem can be seen as:
void F(int a)
    if (a == a) do_stuff();

    int x;

If the compiler doesn't inline F, it will definitely call F, where the
compiler doesn't KNOW that x is undef, and as a result, do_stuff is
guaranteed to execute. If F is inlned, x is known as undef, so the compiler
decides that the comparison is false (despite x always has to be equal to
itself, no matter whether it's value is undef or not), and thus do_stuff
wlll not be called.

Of course, it would be easy to fix this by initializing x, but with some
more complex code, it may not be so easy to follow what is going on and
what the effects will be.

I don't have a good answer as to what to do - I can sort of see both sides
on this one.


>> And that if this function is inlined it must still be well defined,
>> And still always execute statement S
>> But if I read your example correctly you feel that the result
>> of inlining is undefined behavior because “if (undef == undef)”
>> and “if (poison == poison)” is undefined behavior  ?
>> I say that if the compiler makes it so that the programmer cannot
>> correctly reason about how their program executes then there is
>> something wrong with the compiler.
>> My own opinion about this is that it is yet another example of why
>> "Since each use of ‘undef’ can yield a different result…”
>> is such a bad definition, and should never have been accepted into
>> the IR in the first place.
>> IIUC, both “undef” and “poison” share this undesirable definition
>> for which “freeze” is proposed as a fix, but if I understand your
>> logic correctly there is no way to fix this example with “freeze” ?
>> IMHO "freeze” is a bandaid over a bad definition, that doesn’t
>> seem to solve the inlining problem, and we should fix the definition
>> rather than add a bandaid.
>> Peter Lawrence.
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