Index: lib/Transforms/Instrumentation/EfficiencySanitizer.cpp
===================================================================
--- lib/Transforms/Instrumentation/EfficiencySanitizer.cpp
+++ lib/Transforms/Instrumentation/EfficiencySanitizer.cpp
@@ -57,6 +57,14 @@
     "esan-instrument-memintrinsics", cl::init(true),
     cl::desc("Instrument memintrinsics (memset/memcpy/memmove)"), cl::Hidden);
 
+// Experiments show that the performance difference can be 2x or more,
+// and accuracy loss is typically negligible, so we turn this on by default.
+static cl::opt<bool>  ClAssumeIntraCacheLine(
+    "esan-assume-intra-cache-line", cl::init(true),
+    cl::desc("Assume each memory access touches just one cache line, for "
+             "better performance but with a potential loss of accuracy."),
+    cl::Hidden);
+
 STATISTIC(NumInstrumentedLoads, "Number of instrumented loads");
 STATISTIC(NumInstrumentedStores, "Number of instrumented stores");
 STATISTIC(NumFastpaths, "Number of instrumented fastpaths");
@@ -701,7 +709,8 @@
   // and thus larger than a cache line for platforms this tool targets
   // (and our shadow memory setup assumes 64-byte cache lines).
   assert(TypeSize <= 64);
-  if (!(TypeSize == 8 ||
+  if (!ClAssumeIntraCacheLine &&
+      !(TypeSize == 8 ||
         (Alignment % (TypeSize / 8)) == 0))
     return false;