Index: lib/Transforms/Instrumentation/AddressSanitizer.cpp
===================================================================
--- lib/Transforms/Instrumentation/AddressSanitizer.cpp
+++ lib/Transforms/Instrumentation/AddressSanitizer.cpp
@@ -202,10 +202,13 @@
 
 // These flags allow to change the shadow mapping.
 // The shadow mapping looks like
-//    Shadow = (Mem >> scale) + (1 << offset_log)
+//    Shadow = (Mem >> scale) + offset
 static cl::opt<int> ClMappingScale("asan-mapping-scale",
                                    cl::desc("scale of asan shadow mapping"),
                                    cl::Hidden, cl::init(0));
+static cl::opt<uint64_t> ClMappingOffset("asan-mapping-offset",
+                                   cl::desc("offset of asan shadow mapping"),
+                                   cl::Hidden, cl::init(0));
 
 // Optimization flags. Not user visible, used mostly for testing
 // and benchmarking the tool.
@@ -404,10 +407,14 @@
   }
 
   Mapping.Scale = kDefaultShadowScale;
-  if (ClMappingScale) {
+  if (ClMappingScale.getNumOccurrences() > 0) {
     Mapping.Scale = ClMappingScale;
   }
 
+  if (ClMappingOffset.getNumOccurrences() > 0) {
+    Mapping.Offset = ClMappingOffset;
+  }
+
   // OR-ing shadow offset if more efficient (at least on x86) if the offset
   // is a power of two, but on ppc64 we have to use add since the shadow
   // offset is not necessary 1/8-th of the address space.