[PATCH] Sink store based on alias analysis

[Elena Demikhovsky]

================
Comment at: lib/Transforms/Scalar/MergedLoadStoreMotion.cpp:421
@@ -426,11 +420,3 @@
 
-    // Only move loads if they are used in the block.
-    if (isStoreSinkBarrier(Inst))
-      break;
-    if (isa<StoreInst>(Inst)) {
-      AliasAnalysis::Location LocSI = AA->getLocation(SI);
-      AliasAnalysis::Location LocInst = AA->getLocation((StoreInst *)Inst);
-      if (AA->isMustAlias(LocSI, LocInst)) {
-        I = (StoreInst *)Inst;
-        break;
-      }
+    if (!isa<StoreInst>(Inst))
+       continue;
----------------
reames wrote:
> Rather than stopping on the first readwrite call, this function will now rescan the end of the basic block for each store in the basic block.  That seems mildly expensive: O(N^2) in the number of instructions in the BB.  Have you considered the implications for compile time?
Hi Philip,

We already discussed about this when I implemented load hoisting. When you go through the first basic block, you don't know what to check. Because not every store/load is a barrier. 

We are looking at pointer location while going through the second block. If the both strore instructions are indentical we start to look for a barrier - we do less checks in comparison to the previous implementation.
I do not expect the implications for compile time in this case.
- Elena

http://reviews.llvm.org/D6420