[llvm-dev] Semantics of udef values in PHI instructions

[Johannes Doerfert via llvm-dev]

On 8/16/21 9:04 AM, Roman Lebedev via llvm-dev wrote:
> I would strongly advise against expecting/depending on undef happening
> to be replaced with anything in particular.
> That behaviour is not guaranteed.

+1


>
> Roman
>
> On Mon, Aug 16, 2021 at 5:00 PM Charitha Saumya via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Thanks for the nice explanation.
>>
>> Can I depend on this behaviour? In the example, this %unify.phi is added by one of my custom passes. Can I assume that if a loop is present the real value of %c.2 will still be %c.1 regardless of loop's execution behavior.
>>
>> Thanks
>> Charitha
>>
>>
>> On Mon, Aug 16, 2021 at 7:55 AM Hideto Ueno <uenoku.tokotoko at gmail.com> wrote:
>>> Hi,
>>>
>>> I don't know how lli picks the value for `undef` but I think optimizations eliminate the `undef` value in phi before the execution
>>> because optimization is allowed to change `undef` to an arbitrary value[0], so `undef` can be replaced by %c.1.
>>> For example, early-cse optimizes `%c.2` to `%c.2 = phi i32 [ %c.0, %while.end ], [ %c.1, %while.end12 ]`.
>>> (link: https://godbolt.org/z/e1M7x5z5K )
>>>
>>> [0] https://llvm.org/docs/LangRef.html#undefined-values
>>>
>>> Regards,
>>> Hideto
>>>
>>> On Mon, Aug 16, 2021 at 8:50 PM Hideto Ueno <uenoku.tokotoko at gmail.com> wrote:
>>>> Hi,
>>>>
>>>> I don't know how lli picks the value for `undef` but I think optimizations eliminate the `undef` value in phi before the execution
>>>> because optimization is allowed to change `undef` to an arbitrary value[0], so `undef` can be replaced by %c.1.
>>>> For example, early-cse optimizes `%c.2` to `%c.2 = phi i32 [ %c.0, %while.end ], [ %c.1, %while.end12 ]`.
>>>> (link: https://godbolt.org/z/e1M7x5z5K )
>>>>
>>>> [0] https://llvm.org/docs/LangRef.html#undefined-values
>>>>
>>>> Regards,
>>>> Hideto
>>>>
>>>> On Mon, Aug 16, 2021 at 6:55 PM Charitha Saumya via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>>> Hi,
>>>>>
>>>>> Consider the following LLVM IR snippet. (also attached)
>>>>>
>>>>> the value of %unify.phi is undefined if the loop body (while.body9)  is executed, otherwise it has some specific value (%c.1 is a defined value).
>>>>>
>>>>> However when I execute this IR (using lli), the value of %c.2 is always the value of %c.1, even if the loop is executed multiple times.
>>>>>
>>>>> What is the reason for this behavior?  how undefined values are handled in LLVM and is this behaviour architecture dependent (depending on how undef is handled in Codegen) ?
>>>>>
>>>>> Thanks
>>>>> Charitha
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