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

[Hans Wennborg via llvm-commits]

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?