Jump Theading/GVN bug

[Daniel Berlin]

On Mon, Feb 23, 2015 at 9:55 AM, Chandler Carruth <chandlerc at google.com>
wrote:

>
> On Mon, Feb 23, 2015 at 8:21 AM, Daniel Berlin <dberlin at dberlin.org>
> wrote:
>
>> On Mon Feb 23 2015 at 7:38:37 AM Rafael Espíndola <
>> rafael.espindola at gmail.com> wrote:
>>
>>> >> This instruction doesn’t appear to be well formed. However, one could
>>> >> argue that it is – formally %2 def does dominate all %2 uses because
>>> both
>>> >> are located in an unreachable block, so every path to the use is
>>> passing
>>> >> through the def (because there are 0 paths like that).
>>> >>
>>> >> That is likely the reason why –verify doesn’t complain about it.
>>> >
>>> >
>>> > Why is jump-threading creating this funky instruction?
>>>
>>> It is well formed. I agree with Hal: it might be a quality improvement
>>> to generate tidier IR from jump-threading, but GVN has to handle
>>> whatever input passes the verifier.
>>>
>>
>> Then, IMHO, we should start the long process of making
>> forward-unreachable code not acceptable except inside a single pass (IE at
>> the end of each pass, there should be no unreachable code).
>>
>> There are a very very small number of passes that generate such code, and
>> it's not that difficult to fix them to make them not do that.
>>
>
> I will write up a longer reply soon, but I strongly disagree. I think this
> would be a really bad direction for LLVM to move in.
>

That just makes you really really wrong.
:)
Seriously though, i'm pretty wildly curious to here what benefit you think
it adds
It definitely has a real maintenance cost to every pass, and a real compile
time overhead.
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