[llvm] r261430 - When MemoryDependenceAnalysis hits a CFG with many transparent blocks,

[Joerg Sonnenberger via llvm-commits]

On Sat, Feb 20, 2016 at 11:24:44AM -0000, Joerg Sonnenberger via llvm-commits wrote:
> Author: joerg
> Date: Sat Feb 20 05:24:44 2016
> New Revision: 261430
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=261430&view=rev
> Log:
> When MemoryDependenceAnalysis hits a CFG with many transparent blocks,
> the algorithm easily degrades into quadratic memory and time complexity.
> The easiest example is a long chain of BBs that don't otherwise use a
> location. The caching will add an entry for every intermediate block and
> limiting the number of results doesn't help as no results are produced
> until a definition is found.
> 
> Introduce a limit similar to the existing instructions-per-block limit.
> This limit counts the total number of blocks checked. If the limit is
> reached, entries are considered unknown. The initial value is 1000,
> which avoids regressions for normal sized functions while still
> limiting edge cases to reasnable memory consumption and execution time.
> 
> Differential Revision: http://reviews.llvm.org/D16123

Hello Hans,
as mentioned on IRC the other day, I would like to see this merged into
the 3.8 branch after the 3.8.0 release is cut. It fixes some long
standing issues, but not regressions from earlier releases.

Joerg