[llvm] r309355 - [JumpThreading] Stop falsely preserving LazyValueInfo.

[Davide Italiano via llvm-commits]

On Fri, Jul 28, 2017 at 2:05 PM, Hans Wennborg <hans at chromium.org> wrote:
> On Thu, Jul 27, 2017 at 8:12 PM, Davide Italiano <davide at freebsd.org> wrote:
>> On Thu, Jul 27, 2017 at 8:10 PM, Davide Italiano via llvm-commits
>> <llvm-commits at lists.llvm.org> wrote:
>>> Author: davide
>>> Date: Thu Jul 27 20:10:43 2017
>>> New Revision: 309355
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=309355&view=rev
>>> Log:
>>> [JumpThreading] Stop falsely preserving LazyValueInfo.
>>>
>>> JumpThreading claims to preserve LVI, but it doesn't preserve
>>> the analyses which LVI holds a reference to (e.g. the Dominator).
>>> In the current pass manager infrastructure, after JT runs, the
>>> PM frees these analyses (including DominatorTree) but preserves
>>> LVI.
>>>
>>> CorrelatedValuePropagation runs immediately after and queries
>>> a corrupted domtree, causing weird miscompiles.
>>>
>>> This commit disables the preservation of LVI for the time being.
>>> Eventually, we should either move LVI to a proper dependency
>>> tracking mechanism (i.e. an analyses shouldn't hold references
>>> to other analyses and compute them on demand if needed), or
>>> we should teach all the passes preserving LVI to preserve the
>>> analyses LVI depends on.
>>>
>>> The new pass manager has a mechanism to invalidate LVI in case
>>> one of the analyses it depends on becomes invalid, so this problem
>>> shouldn't exist (at least not in this immediate form), but handling
>>> of analyses holding references is still a very delicate subject.
>>>
>>> Fixes PR33917 (and rustc).
>>>
>>
>> Hans, we might consider backporting this (which also depends on r309353) to 5.0.
>
> How does it depend on r309353? It looks to me like that just adds a
> debugging option?

We don't preserve LVI anymore so when the printer runs, the cache is
empty and the test lvi-after-jumpthreading.ll fails.
The alternative to not backport r309353 is that of removing the test altogether.

Thanks,

-- 
Davide

"There are no solved problems; there are only problems that are more
or less solved" -- Henri Poincare