[PATCH] D23559: [ADCE] Add control dependence computation

[Mehdi AMINI via llvm-commits]

mehdi_amini added inline comments.

================
Comment at: lib/Transforms/Scalar/ADCE.cpp:459
@@ -395,2 +458,3 @@
   void getAnalysisUsage(AnalysisUsage &AU) const override {
-    AU.setPreservesCFG();
+    AU.addRequired<PostDominatorTreeWrapperPass>();
+    AU.setPreservesCFG(); // TODO -- will remove when we start removing branches
----------------
david2050 wrote:
> mehdi_amini wrote:
> > How expensive is this?
> In email, Daniel Berlin discusses this as well.
> 
> I measured ~400 cpp source files internally and impact is about ~0.6% (that is end to end starting with C++ source)
Thanks!

0.6% is not negligible: some LLVM users are starting from bitcode (i.e. they don't pay the frontend cost) so the % could be considerably higher, and have low-latency requirements.

It seems there is a tradeoff where the "existing" "fast" ADCE may still be desirable. Do you remember if this was addressed in a previous email thread? (I kind of remember there was a fairly extensive discussion on your first submissions)


https://reviews.llvm.org/D23559