Reverted r152620 - DSE: Shorten memset when a later store overwrites the start of it. There were all sorts of buildbot issues

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151621 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 2 insertions, 66 deletions

diff --git a/lib/Transforms/Scalar/DeadStoreElimination.cpp b/lib/Transforms/Scalar/DeadStoreElimination.cpp
index 9f6a953..c8c5360 100644
--- a/lib/Transforms/Scalar/DeadStoreElimination.cpp
+++ b/lib/Transforms/Scalar/DeadStoreElimination.cpp
@@ -259,13 +259,6 @@ static bool isShortenable(Instruction *I) {
   }
 }
 
-
-/// isMemset - Returns true if this instruction is an intrinsic memset
-static bool isMemset(Instruction *I) {
-  IntrinsicInst *II = dyn_cast<IntrinsicInst>(I);
-  return II && II->getIntrinsicID() == Intrinsic::memset;
-}
-
 /// getStoredPointerOperand - Return the pointer that is being written to.
 static Value *getStoredPointerOperand(Instruction *I) {
   if (StoreInst *SI = dyn_cast<StoreInst>(I))
@@ -317,17 +310,14 @@ namespace {
   {
     OverwriteComplete,
     OverwriteEnd,
-    OverwriteStart,
     OverwriteUnknown
   };
 }
 
 /// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
 /// completely overwrites a store to the 'Earlier' location.
-/// 'OverwriteEnd' if the end of the 'Earlier' location is completely
-/// overwritten by 'Later', 'OverWriteStart' if the start of 'Earlier'
-/// is completely overwritten by 'Later' or 'OverwriteUnknown' if nothing
-/// can be determined
+/// 'OverwriteEnd' if the end of the 'Earlier' location is completely 
+/// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
 static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
                                    const AliasAnalysis::Location &Earlier,
                                    AliasAnalysis &AA,
@@ -428,21 +418,6 @@ static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
       LaterOff < int64_t(EarlierOff + Earlier.Size) &&
       int64_t(LaterOff + Later.Size) >= int64_t(EarlierOff + Earlier.Size))
     return OverwriteEnd;
-  
-  // The other interesting case is if the later store overwrites the end of
-  // the earlier store
-  //
-  //                    |--earlier--|
-  //      |--   later   --|
-  //
-  // In this case we may want to trim the size of earlier to avoid generating
-  // writes to addresses which will definitely be overwritten later
-  if (EarlierOff >= LaterOff &&
-      EarlierOff < int64_t(LaterOff + Later.Size) &&
-      int64_t(EarlierOff + Earlier.Size) >= int64_t(LaterOff + Later.Size)) {
-    LaterOff = LaterOff + Later.Size;
-    return OverwriteStart;
-  }
 
   // Otherwise, they don't completely overlap.
   return OverwriteUnknown;
@@ -614,45 +589,6 @@ bool DSE::runOnBasicBlock(BasicBlock &BB) {
             DepIntrinsic->setLength(TrimmedLength);
             MadeChange = true;
           }
-        } else if (OR == OverwriteStart && isMemset(DepWrite)) {
-          // TODO: base this on the target vector size so that if the earlier
-          // store was too small to get vector writes anyway then its likely
-          // a good idea to shorten it
-          // Power of 2 vector writes are probably always a bad idea to optimize
-          // as any store/memset/memcpy is likely using vector instructions so
-          // shortening it to not vector size is likely to be slower
-          // TODO: shorten memcpy and memmove by offsetting the source address.
-          MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
-          unsigned DepWriteAlign = DepIntrinsic->getAlignment();
-          if (llvm::isPowerOf2_64(InstWriteOffset) ||
-              ((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
-            
-            DEBUG(dbgs() << "DSE: Remove Dead Store:\n  OW START: "
-                  << *DepWrite << "\n  KILLER (offset " 
-                  << InstWriteOffset << ", " 
-                  << DepWriteOffset << ", " 
-                  << DepLoc.Size << ")"
-                  << *Inst << '\n');
-            
-            Value* DepWriteLength = DepIntrinsic->getLength();
-            Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
-                                                    DepLoc.Size -
-                                                    (InstWriteOffset - 
-                                                    DepWriteOffset));
-            DepIntrinsic->setLength(TrimmedLength);
-            const TargetData *TD = AA->getTargetData();
-            Type *IntPtrTy = TD->getIntPtrType(BB.getContext());
-            Value* Offset = ConstantInt::get(IntPtrTy,
-                                             InstWriteOffset - DepWriteOffset);
-            // Offset the start of the memset with a GEP.  As the memset type is
-            // i8* a GEP will do this without needing to use ptrtoint, etc.
-            Value *Dest = GetElementPtrInst::Create(DepIntrinsic->getRawDest(),
-                                                    Offset,
-                                                    "",
-                                                    DepWrite);
-            DepIntrinsic->setDest(Dest);
-            MadeChange = true;
-          }
         }
       }