Clean up: Refactoring the hardcoded value of 6 for FindAvailableLoadedValue()'s parameter MaxInstsToScan. (Complete version of r247497. See D12886)

llvm-svn: 248022

4 files changed, 23 insertions, 8 deletions

diff --git a/llvm/lib/Analysis/Lint.cpp b/llvm/lib/Analysis/Lint.cpp
index 0f5599e8204..15acc783f07 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, DefMaxInstsToScan, AA))
         return findValueImpl(U, OffsetOk, Visited);
       if (BBI != BB->begin()) break;
       BB = BB->getUniquePredecessor();
diff --git a/llvm/lib/Analysis/Loads.cpp b/llvm/lib/Analysis/Loads.cpp
index 4d6d960f159..a544ac0c504 100644
--- a/llvm/lib/Analysis/Loads.cpp
+++ b/llvm/lib/Analysis/Loads.cpp
@@ -161,6 +161,18 @@ bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
   return false;
 }
 
+/// DefMaxInstsToScan - 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.
+cl::opt<unsigned>
+llvm::DefMaxInstsToScan("available-load-scan-limit", cl::init(6), cl::Hidden,
+  cl::desc("Use this to specify the default maximum number of instructions "
+           "to scan backward from a given instruction, when searching for "
+           "available loaded value"));
+
 /// \brief Scan the ScanBB block backwards to see if we have the value at the
 /// memory address *Ptr locally available within a small number of instructions.
 ///
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index 3c70f442647..30478f4e7d0 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,
+                               DefMaxInstsToScan, 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 534b47eccd4..9c48a1ddd50 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, DefMaxInstsToScan)) {
     // If the value of 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,
+                                                    DefMaxInstsToScan,
                                                     nullptr, &ThisAATags);
     if (!PredAvailable) {
       OneUnavailablePred = PredBB;