[LLVMdev] Jump Theading/GVN bug - moving discussion to llvm-dev

Tue Feb 24 11:06:46 PST 2015

On Tue, Feb 24, 2015 at 10:45 AM, Philip Reames <listmail at philipreames.com>
wrote:

>  Whatever we end up deciding, I think we do need to preserve the ability
> of a language frontend to generate complete garbage in the form of
> unreachable code.  If we decide to move in the direction of disallowing the
> production of unreachable code, we need to write a canonicalization pass
> that runs first thing in the pipeline.  A frontend should be able to
> produce unreachable code.  This enables important simplifications in the
> frontend.
>

Sure.
Note that we essentially already do this in the very first actual
optimization pass that gets run both in IPO mode and not IPO mode.

>
>
I happen to agree with Chandler, but I don't have a strong opinion either
> way.  His testing point in particular is one that resonates with me.  I'll,
> in practice, take testability over verification any day.  :)
>

So, in the current world of "unreachable code can contain  anything", it's
not simply testable, contrary to chandler's assertion (while the reverse,
"do not produce unreachable code" is actually pretty easily testable)
If we change that, yes, it becomes a much more tractable problem.

>
> Sanjoy's suggestion of having an "unreachable code analysis" might be a
> way to split the difference.  Every pass could depend on it.  If it doesn't
> report "no unreachable code", the pass calls a cleanup transform utility.
> Any pass that wanted to spend time to "preserve" the analysis (by making
> sure there was no unreachable code), could do so.  Everything would "just
> work".
>
> Do we have a set of unreachable, but otherwise valid, code examples?  It
> doesn't seem to hard to write extra run lines for each pass.  The corpus of
> valid but problematic examples should be common to all passes.
>
> Philip
>
>
> On 02/23/2015 08:32 PM, Chandler Carruth wrote:
>
> So, first things first.
>
>  Handling unreachable code is annoying. I agree. However, its important
> to characterize how annoying. Most code is not unreachable, and we're
> (obviously) fine just bailing on such crazy code paths. So in practice the
> common cost is keeping a local set to detect cycles in the graph where we
> aren't expecting them. We are essentially always traversing a linked list
> representing this graph. Because these linked list nodes don't have any
> particular reason to have high locality, we have a cache-hostile traversal
> where we have to do primarily local and cache friendly book-keeping at each
> step to catch duplication. I suspect that this has a very small (but
> non-zero) impact on compile time.
>
>  Handling unreachable code is also error prone. We have had a long
> history of bugs here. So if it were free to get rid of unreachable code,
> that would be super awesome.
>
>
>  The first problem to realize is that for a large chunk of LLVM, we can't
> actually get rid of handling unreachable code. Passes are quite likely to
> create unreachable code while running, and that will mean that all of the
> utilities will end up needing to be conservatively correct and handle
> unreachable code even when they don't need to. =/
>
>
>  The second problem is that cleaning up unreachable code isn't free. The
> code which is most likely to create unreachable code is similarly likely to
> destroy the analysis information we would use to remove it cheaply. And
> even then, just walking lists of basic blocks as we go out of the function
> is going to dirty cache lines that we might not have any other reason to
> look at. I can genuinely imagine cases where batching this will be
> beneficial. Will it outstrip the cost of handling it? I don't know. But I
> think it will mitigate the gains, especially if the gains aren't as
> significant as we might naively think.
>
>
>  The third problem I have is that this makes it much harder to
> constructively produce a correct IR transformation pass. When transforming
> the IR we must never forget about regions we have made unreachable. A
> single mistake here will cascade to a verification failure. This is the
> reverse of the problem we have today where your pass must *accept*
> unreachable IR. But I think the constructive reasoning is much easier. It
> makes it harder to have a bug through omission of a case.
>
>
>  The fourth problem I have is related to the third problem. How do we
> gain confidence in the correctness of any IR transformation? We must
> construct test cases that we believe will exercise the system. It seems
> *incredibly* easier to construct test cases with interesting unreachable IR
> patterns that should all be handled, than to construct test cases where the
> pass will happen to form unreachable IR patterns in each way and ensure
> that none escape the pass. One is essentially covering input diversity, the
> other has to cover *output* diversity, and must prove the negative. We
> fundamentally must gain confidence that the pass *never* produces
> unreachable IR. This seems much harder than demonstrating that it does
> handle unreachable IR.
>
>
>  The fifth problem I have is also related to the third and fourth
> problems. Precluding unreachable code seems likely to make tools like delta
> test case reduction dramatically less effective. Now, when cutting edges in
> the CFG to minimize a test case they must actually build the graph and
> prune all unreachable code.
>
>
>  All of these problems seem surmountable, but in aggregate, they make me
> strongly doubt that the benefit is worth the cost.
>
>  -Chandler
>
>
>
>
> On Mon, Feb 23, 2015 at 1:04 PM, Romanova, Katya <
> Katya_Romanova at playstation.sony.com> wrote:
>
>>  Hello,
>>
>> I encountered a problem triggered by Jump-Threading optimization.  This
>> pass is creating an unreachable block with an instruction that is not well
>> formed, which then causes the subsequent GVN pass to enter an infinite loop.
>>
>>
>>
>> I have submitted a bug report and proposed fix to llvm-commits. This bug
>> opened a can of worms. I was asked to move the discussion to llvm-dev to
>> reach for a wider audience.
>>
>>
>>
>> >> Can we move the general discussion to llvm-dev?  This probably
>> warrants a wider audience.
>>
>>
>>
>> Here is the original post and a couple of replies from last week:
>>
>>
>> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150216/261330.html
>>
>>
>>
>> Here is a thread of replies from today:
>>
>>
>> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150223/261463.html
>>
>>
>>
>> Katya.
>>
>>
>>
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150224/ad4f164c/attachment.html>