[PATCH] D61160: [SimplifyCFG] Implement the suggested ctlz transform

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: lib/Transforms/Utils/SimplifyCFG.cpp:5515
+    return false;
+  // TODO a single instruction can be saved on x86 by using cttz over ctlz
+  // when only ctlz results in a subtraction instruction being added
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shawnl wrote:
> jmolloy wrote:
> > This would be a pattern for instcombine to pick up. We shouldn't have to worry about this here.
> Yes we do, because InstCombine does not know how to rearrange case statements.
Sounds like a missed optimization on instcombine side then?
Trying to handle every single unrelated micro-transform in every optimization is a path to failure.


Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D61160/new/

https://reviews.llvm.org/D61160