Index: compiler-rt/trunk/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/CMakeLists.txt
+++ compiler-rt/trunk/CMakeLists.txt
@@ -43,6 +43,25 @@
 option(COMPILER_RT_BUILD_XRAY_NO_PREINIT "Build xray with no preinit patching" OFF)
 mark_as_advanced(COMPILER_RT_BUILD_XRAY_NO_PREINIT)
 
+set(COMPILER_RT_ASAN_SHADOW_SCALE ""
+    CACHE STRING "Override the shadow scale to be used in ASan runtime")
+
+if (NOT COMPILER_RT_ASAN_SHADOW_SCALE STREQUAL "")
+  # Check that the shadow scale value is valid.
+  if (NOT (COMPILER_RT_ASAN_SHADOW_SCALE GREATER -1 AND
+           COMPILER_RT_ASAN_SHADOW_SCALE LESS 8))
+    message(FATAL_ERROR "
+      Invalid ASan Shadow Scale '${COMPILER_RT_ASAN_SHADOW_SCALE}'.")
+  endif()
+
+  set(COMPILER_RT_ASAN_SHADOW_SCALE_LLVM_FLAG
+      -mllvm -asan-mapping-scale=${COMPILER_RT_ASAN_SHADOW_SCALE})
+  set(COMPILER_RT_ASAN_SHADOW_SCALE_DEFINITION
+      ASAN_SHADOW_SCALE=${COMPILER_RT_ASAN_SHADOW_SCALE})
+  set(COMPILER_RT_ASAN_SHADOW_SCALE_FLAG
+      -D${COMPILER_RT_ASAN_SHADOW_SCALE_DEFINITION})
+endif()
+
 set(COMPILER_RT_BAREMETAL_BUILD OFF CACHE BOOLEAN
   "Build for a bare-metal target.")
 
Index: compiler-rt/trunk/lib/asan/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/lib/asan/CMakeLists.txt
+++ compiler-rt/trunk/lib/asan/CMakeLists.txt
@@ -40,6 +40,7 @@
 include_directories(..)
 
 set(ASAN_CFLAGS ${SANITIZER_COMMON_CFLAGS})
+set(ASAN_COMMON_DEFINITIONS ${COMPILER_RT_ASAN_SHADOW_SCALE_DEFINITION})
 
 append_rtti_flag(OFF ASAN_CFLAGS)
 
Index: compiler-rt/trunk/lib/asan/asan_mapping.h
===================================================================
--- compiler-rt/trunk/lib/asan/asan_mapping.h
+++ compiler-rt/trunk/lib/asan/asan_mapping.h
@@ -131,7 +131,11 @@
 // || `[0x30000000, 0x35ffffff]` || LowShadow  ||
 // || `[0x00000000, 0x2fffffff]` || LowMem     ||
 
+#if defined(ASAN_SHADOW_SCALE)
+static const u64 kDefaultShadowScale = ASAN_SHADOW_SCALE;
+#else
 static const u64 kDefaultShadowScale = 3;
+#endif
 static const u64 kDefaultShadowSentinel = ~(uptr)0;
 static const u64 kDefaultShadowOffset32 = 1ULL << 29;  // 0x20000000
 static const u64 kDefaultShadowOffset64 = 1ULL << 44;
Index: compiler-rt/trunk/lib/asan/tests/CMakeLists.txt
===================================================================
--- compiler-rt/trunk/lib/asan/tests/CMakeLists.txt
+++ compiler-rt/trunk/lib/asan/tests/CMakeLists.txt
@@ -23,6 +23,7 @@
 set(ASAN_UNITTEST_COMMON_CFLAGS
   ${COMPILER_RT_UNITTEST_CFLAGS}
   ${COMPILER_RT_GTEST_CFLAGS}
+  ${COMPILER_RT_ASAN_SHADOW_SCALE_LLVM_FLAG}
   -I${COMPILER_RT_SOURCE_DIR}/include
   -I${COMPILER_RT_SOURCE_DIR}/lib
   -I${COMPILER_RT_SOURCE_DIR}/lib/asan
@@ -52,6 +53,7 @@
 
 # Use -D instead of definitions to please custom compile command.
 list(APPEND ASAN_UNITTEST_COMMON_CFLAGS
+  ${COMPILER_RT_ASAN_SHADOW_SCALE_FLAG}
   -DASAN_HAS_BLACKLIST=1
   -DASAN_HAS_EXCEPTIONS=1
   -DASAN_UAR=0)
Index: compiler-rt/trunk/test/lit.common.cfg
===================================================================
--- compiler-rt/trunk/test/lit.common.cfg
+++ compiler-rt/trunk/test/lit.common.cfg
@@ -46,6 +46,10 @@
 # Add compiler ID to the list of available features.
 config.available_features.add(compiler_id)
 
+# If needed, add cflag for shadow scale.
+if config.asan_shadow_scale != '':
+  config.target_cflags += " -mllvm -asan-mapping-scale=" + config.asan_shadow_scale
+
 # BFD linker in 64-bit android toolchains fails to find libm.so, which is a
 # transitive shared library dependency (via asan runtime).
 if config.android:
Index: compiler-rt/trunk/test/lit.common.configured.in
===================================================================
--- compiler-rt/trunk/test/lit.common.configured.in
+++ compiler-rt/trunk/test/lit.common.configured.in
@@ -25,6 +25,7 @@
 set_default("compiler_rt_debug", @COMPILER_RT_DEBUG_PYBOOL@)
 set_default("compiler_rt_libdir", "@COMPILER_RT_RESOLVED_LIBRARY_OUTPUT_DIR@")
 set_default("emulator", "@COMPILER_RT_EMULATOR@")
+set_default("asan_shadow_scale", "@COMPILER_RT_ASAN_SHADOW_SCALE@")
 set_default("ios", False)
 set_default("iossim", False)
 set_default("sanitizer_can_use_cxxabi", @SANITIZER_CAN_USE_CXXABI_PYBOOL@)