Revert "Clean up: Refactoring the hardcoded value of 6 for FindAvailableLoadedValue()'s parameter MaxInstsToScan." for preliminary community discussion (See. D12886)

llvm-svn: 247716

llvm/include/llvm/Analysis/Loads.h

@@ -29,28 +29,15 @@ class MDNode;
 bool isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
                                  unsigned Align);
 
-/// DEF_MAX_INSTS_TO_SCAN - the default number of maximum instructions
-/// to scan in the block, used by FindAvailableLoadedValue().
-/// FindAvailableLoadedValue() was introduced in r60148, to improve jump
-/// threading in part by eliminating partially redundant loads.
-/// At that point, the value of MaxInstsToScan was already set to '6'
-/// without documented explanation.
-/// FIXME: Ask r60148 author for details, and complete this documentation.
-/// NOTE: As of now, every use of FindAvailableLoadedValue() uses this default
-/// value of '6'.
-#ifndef DEF_MAX_INSTS_TO_SCAN
-#define DEF_MAX_INSTS_TO_SCAN 6
-#endif
-
 /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at
 /// the instruction before ScanFrom) checking to see if we have the value at
 /// the memory address *Ptr locally available within a small number of
 ///  instructions. If the value is available, return it.
 ///
-/// If not, return the iterator for the last validated instruction that the
+/// If not, return the iterator for the last validated instruction that the 
 /// value would be live through.  If we scanned the entire block and didn't
 /// find something that invalidates *Ptr or provides it, ScanFrom would be
-/// left at begin() and this returns null.  ScanFrom could also be left
+/// left at begin() and this returns null.  ScanFrom could also be left 
 ///
 /// MaxInstsToScan specifies the maximum instructions to scan in the block.
 /// If it is set to 0, it will scan the whole block. You can also optionally
@@ -61,7 +48,7 @@ bool isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
 /// is found, it is left unmodified.
 Value *FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
                                 BasicBlock::iterator &ScanFrom,
-                                unsigned MaxInstsToScan = DEF_MAX_INSTS_TO_SCAN,
+                                unsigned MaxInstsToScan = 6,
                                 AliasAnalysis *AA = nullptr,
                                 AAMDNodes *AATags = nullptr);
 

llvm/lib/Analysis/Lint.cpp

@@ -829,9 +829,8 @@ Value *Lint::findValueImpl(Value *V, bool OffsetOk,
     for (;;) {
       if (!VisitedBlocks.insert(BB).second)
         break;
-      if (Value *U =
+      if (Value *U = FindAvailableLoadedValue(L->getPointerOperand(),
-          FindAvailableLoadedValue(L->getPointerOperand(),
+                                              BB, BBI, 6, AA))
-                                   BB, BBI, DEF_MAX_INSTS_TO_SCAN, AA))
         return findValueImpl(U, OffsetOk, Visited);
       if (BBI != BB->begin()) break;
       BB = BB->getUniquePredecessor();

llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

@@ -750,9 +750,8 @@ Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
   // separated by a few arithmetic operations.
   BasicBlock::iterator BBI = &LI;
   AAMDNodes AATags;
-  if (Value *AvailableVal =
+  if (Value *AvailableVal = FindAvailableLoadedValue(Op, LI.getParent(), BBI,
-      FindAvailableLoadedValue(Op, LI.getParent(), BBI,
+                                                     6, AA, &AATags)) {
-                               DEF_MAX_INSTS_TO_SCAN, AA, &AATags)) {
     if (LoadInst *NLI = dyn_cast<LoadInst>(AvailableVal)) {
       unsigned KnownIDs[] = {
         LLVMContext::MD_tbaa,
@@ -823,7 +822,7 @@ Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
       }
 
       // load (select (cond, null, P)) -> load P
-      if (isa<ConstantPointerNull>(SI->getOperand(1)) &&
+      if (isa<ConstantPointerNull>(SI->getOperand(1)) && 
           LI.getPointerAddressSpace() == 0) {
         LI.setOperand(0, SI->getOperand(2));
         return &LI;

llvm/lib/Transforms/Scalar/JumpThreading.cpp

@@ -769,7 +769,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
     // If we're branching on a conditional, LVI might be able to determine
     // it's value at the branch instruction.  We only handle comparisons
     // against a constant at this time.
-    // TODO: This should be extended to handle switches as well.
+    // TODO: This should be extended to handle switches as well.  
     BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
     Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
     if (CondBr && CondConst && CondBr->isConditional()) {
@@ -877,7 +877,7 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
   BasicBlock::iterator BBIt = LI;
 
   if (Value *AvailableVal =
-        FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt)) {
+        FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
     // If the value if the load is locally available within the block, just use
     // it.  This frequently occurs for reg2mem'd allocas.
     //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
@@ -922,8 +922,7 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
     // Scan the predecessor to see if the value is available in the pred.
     BBIt = PredBB->end();
     AAMDNodes ThisAATags;
-    Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt,
+    Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6,
-                                                    DEF_MAX_INSTS_TO_SCAN,
                                                     nullptr, &ThisAATags);
     if (!PredAvailable) {
       OneUnavailablePred = PredBB;