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

[Philip Reames]

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.

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.  :)

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 
> <mailto: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.
>
>

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