[LLVMdev] Register pressure mechanism in PRE or Smarter rematerialization/split/spiller/coalescing ?

[Daniel Berlin]

On Wed, Jul 15, 2015 at 1:10 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
> IMHO, This doesn't make a lot of sense to turn off this part on it's own.
> I would just use the enable-pre flag to turn off scalar PRE, as it
> will cause the same issue in other cases as well.
> Is there some reason you aren't just doing that?
> I suspect if this is a performance win, that would be as well.
>

Ugh, actually, it should be a win with the following change:


diff --git a/lib/Transforms/Scalar/GVN.cpp b/lib/Transforms/Scalar/GVN.cpp
index 2c47a8a..a3387e3 100644
--- a/lib/Transforms/Scalar/GVN.cpp
+++ b/lib/Transforms/Scalar/GVN.cpp
@@ -1767,7 +1767,7 @@ bool GVN::processNonLocalLoad(LoadInst *LI) {
   }

   // Step 4: Eliminate partial redundancy.
-  if (!EnablePRE || !EnableLoadPRE)
+  if (!EnableLoadPRE)
     return false;

   return PerformLoadPRE(LI, ValuesPerBlock, UnavailableBlocks);




This will disable Scalar PRE without disabling load PRE.


(note, again, however, that load PRE can create exactly the same GEP
situation you are referring to)