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

llvm-svn: 248022

2 files changed, 15 insertions, 2 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.
 ///