diff --git a/llvm/include/llvm/Analysis/MemorySSA.h b/llvm/include/llvm/Analysis/MemorySSA.h
--- a/llvm/include/llvm/Analysis/MemorySSA.h
+++ b/llvm/include/llvm/Analysis/MemorySSA.h
@@ -735,6 +735,15 @@
   void dump() const;
   void print(raw_ostream &) const;
 
+  // Return true if the *approximate* number of accesses seen by MemorySSA for
+  // the current function is prohibitively large.
+  //
+  // This information can be used to limit optimizations that need to update
+  // MemorySSA for IRs with pathological patterns.
+  // FIXME: A better informative number would be the number of Defs, but this
+  // is currently only targetting extreme cases. Revisit if the usecases change.
+  bool prohibitivelyLargeNumberOfAccesses() const;
+
   /// Return true if \p MA represents the live on entry value
   ///
   /// Loads and stores from pointer arguments and other global values may be
diff --git a/llvm/lib/Analysis/MemorySSA.cpp b/llvm/lib/Analysis/MemorySSA.cpp
--- a/llvm/lib/Analysis/MemorySSA.cpp
+++ b/llvm/lib/Analysis/MemorySSA.cpp
@@ -95,6 +95,12 @@
     "enable-mssa-loop-dependency", cl::Hidden, cl::init(true),
     cl::desc("Enable MemorySSA dependency for loop pass manager"));
 
+static cl::opt<unsigned> ProhibitivelyLargeAccessNo(
+    "memssa-prohibitively-large-access-no", cl::Hidden, cl::init(100000),
+    cl::desc("The number of accesses seen in a function by MemorySSA, past"
+             "which optimizations should consider updates to MemorySSA to be"
+             "prohibitively expensive (default = 100000)"));
+
 static cl::opt<bool, true>
     VerifyMemorySSAX("verify-memoryssa", cl::location(VerifyMemorySSA),
                      cl::Hidden, cl::desc("Enable verification of MemorySSA."));
@@ -1864,6 +1870,10 @@
 LLVM_DUMP_METHOD void MemorySSA::dump() const { print(dbgs()); }
 #endif
 
+bool MemorySSA::prohibitivelyLargeNumberOfAccesses() const {
+  return NextID > ProhibitivelyLargeAccessNo;
+}
+
 void MemorySSA::verifyMemorySSA() const {
   verifyOrderingDominationAndDefUses(F);
   verifyDominationNumbers(F);
diff --git a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
--- a/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
+++ b/llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
@@ -1760,6 +1760,10 @@
   if (!TLI->has(LibFunc_memset) || !TLI->has(LibFunc_memcpy))
     return false;
 
+  // If the number of memory accesses is prohibitively large, skip the pass.
+  if (MSSA && MSSA->prohibitivelyLargeNumberOfAccesses())
+    return false;
+
   while (true) {
     if (!iterateOnFunction(F))
       break;