[LLVMdev] GlobalsModRef (and thus LTO) is completely broken

[Philip Reames]

Chandler,

Given you say explicitly that this only effects the LTO pipeline, I was 
curious if you thought this is an issue that we could skip past for the 
new pass manager work.  Getting the normal optimization pass manager - 
which doesn't have this issue - working seems like a very reasonable 
first step.  Even if we had to add some hack to the old pass manager - 
like say, separating out the problematic interface and making the few 
passes that use it go through hoops to get to GlobalsModRef specifically 
as opposed to any AA pass with the interface - that seems like a 
worthwhile tradeoff.  Would this type of approach work?  Or am I missing 
something?

Philip

On 07/13/2015 08:19 PM, Chandler Carruth wrote:
> Ok folks,
>
> I wrote up the general high-level thoughts I have about stateful AA in 
> a separate thread. But we need to sort out the completely and horribly 
> broken aspects of GlobalsModRef today, and the practical steps 
> forward. This email is totally about the practical stuff.
>
> Now, as to why I emailed this group of people and with this subject, 
> the only pass pipeline that includes GlobalsModRef, is the LTO 
> pipeline. So we have significantly less testing here than we do for 
> stuff in the main pipeline. Also, I don't have any benchmarks I can 
> effectively run to tell me if my changes impacted performance. =/ So I 
> may need your help to evaluate some of this. Now, onto the challenges....
>
> First, GlobalsModRef as currently implemented completely abuses a 
> loophole in the current pass manager to incorrectly stick around even 
> while it is being "invalidated". I don't know of any way to fix this 
> in the current pass manager without completely defeating the purpose 
> of the analysis pass. The consequence is that whether passes claim to 
> preserve AA or not is irrelevant, GlobalsModRef will be preserved 
> anyways! =[[[[ So the only way to make things work correctly is to 
> make GlobalsModRef survive *any* per-function changes to the IR. We 
> cannot rely on AA updates at all.
>
> Most of the updates that GlobalsModRef needs can be provided by a 
> ValueHandle now that we have them. This will prevent ABA-style issues 
> in its caches, etc. I plan to send out a patch soon that switches it 
> over to this strategy.
>
> It is also relying on a precomputed set of global variables whose 
> address is never used by an instruction other than some very small set 
> (gep, bitcast) as "non-address-taken". It then runs 
> GetUnderlyingObject on the two pointers in alias queries, and if that 
> finds one of these "non-address-taken" globals for one of the memory 
> locations but not the other, it concludes no-alias! This is broken for 
> a number of reasons.
>
> a) If the two locations merely have a different *depth* of instruction 
> stack, because GetUnderlyingObject has a recursion cap, one side can 
> fail while the other succeeds, and we erroneously produce no-alias.
>
> b) If instcombine or any other pass for any reason introduces on one 
> path an instruction that GetUnderlyingObject can't look through 
> (select, phi, load, ....), we incorrectly conclude no-alias. This is 
> what addEscapingUse was intended to solve, but we would literally have 
> to call it from every pass because we can't rely on analysis invalidation!
>
> c) If any pass actually escapes a pointer from one function into 
> another, we invalidate the underlying assumption of 
> 'non-address-taken' that it relies upon.
>
> Now, as I argued in my general AA thread, I think we might be able to 
> assume that (c) doesn't happen today. But both (a) and (b) seem like 
> active nightmares to try to fix. I can see hacky ways to avoid (a) 
> where we detect *why* GetUnderlyingObject fails, but I don't see how 
> to fix both (a) and (b) (or to fix (a) well) without just disabling 
> this specific aspect of GloblasModRef.
>
> So that's what I'd like to do. It shouldn't impact the mod/ref 
> information provided by the analysis, just the alias sets.
>
> However, even this may not be necessary. We may just not in practice 
> see these issues, and I don't really want to perturb the LTO generated 
> code quality for a hypothetical issue until we actually have the tools 
> in place to handle things reasonably.
>
> So my plan is:
>
> 1) Fix obvious issues with GloblasModRef and switch it to ValueHandles
> 2) Mail out a patch to disable this part of GlobalsModRef. I can put 
> it behind a flag or however folks would like it to work.
> 3) Remove addEscapingUse() update API, which without #2 may regress 
> some LTO test case I don't have (because I don't have any other than 
> bootstrap)
>
> Thoughts?
> -Chandler
>
>
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