Clean up: Refactoring the hardcoded value of 6 for FindAvailableLoadedValue()'s parameter MaxInstsToScan.

llvm-svn: 247497

4 files changed, 27 insertions, 11 deletions

diff --git a/llvm/include/llvm/Analysis/Loads.h b/llvm/include/llvm/Analysis/Loads.h
index 698b665a93f..3107a765992 100644
--- a/llvm/include/llvm/Analysis/Loads.h
+++ b/llvm/include/llvm/Analysis/Loads.h
@@ -29,15 +29,28 @@ 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
@@ -48,7 +61,7 @@ bool isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
 /// is found, it is left unmodified.
 Value *FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
                                 BasicBlock::iterator &ScanFrom,
-                                unsigned MaxInstsToScan = 6,
+                                unsigned MaxInstsToScan = DEF_MAX_INSTS_TO_SCAN,
                                 AliasAnalysis *AA = nullptr,
                                 AAMDNodes *AATags = nullptr);
 
diff --git a/llvm/lib/Analysis/Lint.cpp b/llvm/lib/Analysis/Lint.cpp
index 0f5599e8204..4928e45f892 100644
--- a/llvm/lib/Analysis/Lint.cpp
+++ b/llvm/lib/Analysis/Lint.cpp
@@ -829,8 +829,9 @@ Value *Lint::findValueImpl(Value *V, bool OffsetOk,
     for (;;) {
       if (!VisitedBlocks.insert(BB).second)
         break;
-      if (Value *U = FindAvailableLoadedValue(L->getPointerOperand(),
-                                              BB, BBI, 6, AA))
+      if (Value *U =
+          FindAvailableLoadedValue(L->getPointerOperand(),
+                                   BB, BBI, DEF_MAX_INSTS_TO_SCAN, AA))
         return findValueImpl(U, OffsetOk, Visited);
       if (BBI != BB->begin()) break;
       BB = BB->getUniquePredecessor();
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index 3c70f442647..32d3f80d15e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -750,8 +750,9 @@ Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
   // separated by a few arithmetic operations.
   BasicBlock::iterator BBI = &LI;
   AAMDNodes AATags;
-  if (Value *AvailableVal = FindAvailableLoadedValue(Op, LI.getParent(), BBI,
-                                                     6, AA, &AATags)) {
+  if (Value *AvailableVal =
+      FindAvailableLoadedValue(Op, LI.getParent(), BBI,
+                               DEF_MAX_INSTS_TO_SCAN, AA, &AATags)) {
     if (LoadInst *NLI = dyn_cast<LoadInst>(AvailableVal)) {
       unsigned KnownIDs[] = {
         LLVMContext::MD_tbaa,
@@ -822,7 +823,7 @@ 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;
diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index f71376827a4..d3b9a34ba39 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/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, 6)) {
+        FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt)) {
     // 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";
@@ -922,7 +922,8 @@ 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, 6,
+    Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt,
+                                                    DEF_MAX_INSTS_TO_SCAN,
                                                     nullptr, &ThisAATags);
     if (!PredAvailable) {
       OneUnavailablePred = PredBB;