[PATCH] D28934: Write a new SSAUpdater

[Mehdi AMINI via Phabricator via llvm-commits]

mehdi_amini added a comment.

In https://reviews.llvm.org/D28934#651323, @sanjoy wrote:

> I've never really heard people complain about our current `SSAUpdater` (about it being too slow or difficult to use) -- well except in this review :).  Is there a specific problem you're trying to address with this?


One data point : in https://llvm.org/bugs/show_bug.cgi?id=16756 we have a test case where JumpThreading is taking 30% of the total time of a clang -O3 compilation. 
One large issue was that we were spending a lot of time in SSAUpdater (Duncan mentions in this PR that the bottleneck is SSAUpdater).

When I looked at it, it seemed to me that we were constantly rebuilding the DominatorTree inside the SSAUpdater over and over for each and every replacement, while it seemed possible to build one up front and reusing it.


https://reviews.llvm.org/D28934