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

[Davide Italiano via llvm-commits]

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.

Thanks,

--
Davide