Index: lib/Fuzzer/CMakeLists.txt
===================================================================
--- lib/Fuzzer/CMakeLists.txt
+++ lib/Fuzzer/CMakeLists.txt
@@ -1,7 +1,16 @@
-set(LIBFUZZER_FLAGS_BASE "${CMAKE_CXX_FLAGS_RELEASE}")
-# Disable the coverage and sanitizer instrumentation for the fuzzer itself.
-set(CMAKE_CXX_FLAGS_RELEASE "${LIBFUZZER_FLAGS_BASE} -O2 -fno-sanitize=all -fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters -Werror")
 if( LLVM_USE_SANITIZE_COVERAGE )
+  # Disable the coverage and sanitizer instrumentation for the fuzzer itself.
+  # If sanitization of LLVM itself is enabled we rely on the compiler's
+  # left to right evaluation of compiler flags so that for something like
+  # ``clang++ -fsanitize=address -fno-sanitize=all`` sanitization will be off.
+  # FIXME: Stripping out the flags we don't want would probably be a better approach.
+  set(disable_sanitizer_flags "")
+  list(APPEND disable_sanitizer_flags
+    "-fno-sanitize=all"
+    # FIXME: gcc doesn't know this flag
+    "-fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters"
+    "-Werror")
+  # FIXME: Test the compiler supports those flags
   add_library(LLVMFuzzerNoMainObjects OBJECT
     FuzzerCrossOver.cpp
     FuzzerTraceState.cpp
@@ -13,14 +22,17 @@
     FuzzerTracePC.cpp
     FuzzerUtil.cpp
     )
+  target_compile_options(LLVMFuzzerNoMainObjects PRIVATE ${disable_sanitizer_flags})
   add_library(LLVMFuzzerNoMain STATIC
     $<TARGET_OBJECTS:LLVMFuzzerNoMainObjects>
     )
+  target_compile_options(LLVMFuzzerNoMain PRIVATE ${disable_sanitizer_flags})
   target_link_libraries(LLVMFuzzerNoMain ${PTHREAD_LIB})
   add_library(LLVMFuzzer STATIC
     FuzzerMain.cpp
     $<TARGET_OBJECTS:LLVMFuzzerNoMainObjects>
     )
+  target_compile_options(LLVMFuzzer PRIVATE ${disable_sanitizer_flags})
   target_link_libraries(LLVMFuzzer ${PTHREAD_LIB})
 
   if( LLVM_INCLUDE_TESTS )
Index: lib/Fuzzer/test/CMakeLists.txt
===================================================================
--- lib/Fuzzer/test/CMakeLists.txt
+++ lib/Fuzzer/test/CMakeLists.txt
@@ -1,8 +1,47 @@
+if ("${LLVM_USE_SANITIZER}" STREQUAL "")
+  message(FATAL_ERROR
+          "LibFuzzer tests cannot be built without an enabled sanitizer")
+endif()
+message(STATUS "LibFuzzer test default sanitization: ${LLVM_USE_SANITIZER}")
+
+function(llvm_append_linker_flag target)
+  if (NOT (TARGET "${target}"))
+    message(FATAL_ERROR "Specified target \"${target}\" is not a target")
+  endif()
+  foreach(flag ${ARGN})
+    set_property(TARGET ${target} APPEND_STRING PROPERTY LINK_FLAGS " ${flag}")
+  endforeach()
+endfunction()
+
 # Build all these tests with -O0, otherwise optimizations may merge some
 # basic blocks and we'll fail to discover the targets.
-# Also enable the coverage instrumentation back (it is disabled
-# for the Fuzzer lib)
-set(CMAKE_CXX_FLAGS_RELEASE "${LIBFUZZER_FLAGS_BASE} -O0 -fsanitize-coverage=edge,indirect-calls")
+# We change the flags for every build type because we might be doing
+# a multi-configuration build (e.g. Xcode) where CMAKE_BUILD_TYPE doesn't
+# mean anything.
+set(variables_to_filter
+      CMAKE_CXX_FLAGS_RELEASE
+      CMAKE_CXX_FLAGS_DEBUG
+      CMAKE_CXX_FLAGS_RELWITHDEBINFO
+      CMAKE_CXX_FLAGS_MINSIZEREL
+      # FIXME: Optimisation level flags aren't supposed to appear here
+      # but they do so we have to strip them out
+      CMAKE_CXX_FLAGS
+)
+foreach (VARNAME ${variables_to_filter})
+  string(REPLACE " " ";" BUILD_FLAGS_AS_LIST "${${VARNAME}}")
+  set(new_flags "")
+  foreach (flag ${BUILD_FLAGS_AS_LIST})
+    # FIXME: Use of XX here is to avoid CMP0054
+    if (NOT ("XX${flag}" MATCHES "XX-O[0123s]"))
+      set(new_flags "${new_flags} ${flag}")
+    else()
+      set(new_flags "${new_flags} -O0")
+    endif()
+  endforeach()
+  set(${VARNAME} "${new_flags}")
+endforeach()
+
+set(SANITIZE_COVERAGE_FLAGS "-fsanitize-coverage=edge,indirect-calls")
 
 set(DFSanTests
   MemcmpTest
@@ -67,6 +106,8 @@
   add_executable(LLVMFuzzer-${Test}
     ${Test}.cpp
     )
+  target_compile_options(LLVMFuzzer-${Test} PRIVATE ${SANITIZE_COVERAGE_FLAGS})
+	llvm_append_linker_flag(LLVMFuzzer-${Test} ${SANITIZE_COVERAGE_FLAGS})
   target_link_libraries(LLVMFuzzer-${Test}
     LLVMFuzzer
     )
@@ -77,6 +118,8 @@
   add_executable(LLVMFuzzer-${Test}
     ${Test}.cpp
     )
+  target_compile_options(LLVMFuzzer-${Test} PRIVATE ${SANITIZE_COVERAGE_FLAGS})
+	llvm_append_linker_flag(LLVMFuzzer-${Test} ${SANITIZE_COVERAGE_FLAGS})
   target_link_libraries(LLVMFuzzer-${Test}
     LLVMFuzzerNoMain
     )
@@ -103,6 +146,8 @@
   $<TARGET_OBJECTS:LLVMFuzzerNoMainObjects>
   )
 
+target_compile_options(LLVMFuzzer-Unittest PRIVATE ${SANITIZE_COVERAGE_FLAGS})
+llvm_append_linker_flag(LLVMFuzzer-Unittest ${SANITIZE_COVERAGE_FLAGS})
 target_link_libraries(LLVMFuzzer-Unittest
   gtest
   gtest_main
Index: lib/Fuzzer/test/dfsan/CMakeLists.txt
===================================================================
--- lib/Fuzzer/test/dfsan/CMakeLists.txt
+++ lib/Fuzzer/test/dfsan/CMakeLists.txt
@@ -1,12 +1,14 @@
 # These tests depend on both coverage and dfsan instrumentation.
 
-set(CMAKE_CXX_FLAGS_RELEASE
-  "${LIBFUZZER_FLAGS_BASE} -O0 -fno-sanitize=all -fsanitize=dataflow")
+# NOTE: Additional -fsanitize-coverage= flags come from HandleLLVMOptions.cmake
+set(TEST_SANITIZE_FLAGS "-fno-sanitize=all" "-fsanitize=dataflow")
 
 foreach(Test ${DFSanTests})
   add_executable(LLVMFuzzer-${Test}-DFSan
     ../${Test}.cpp
     )
+  target_compile_options(LLVMFuzzer-${Test}-DFSan PRIVATE ${TEST_SANITIZE_FLAGS})
+  llvm_append_linker_flag(LLVMFuzzer-${Test}-DFSan ${TEST_SANITIZE_FLAGS})
   target_link_libraries(LLVMFuzzer-${Test}-DFSan
     LLVMFuzzer
     )
Index: lib/Fuzzer/test/trace-bb/CMakeLists.txt
===================================================================
--- lib/Fuzzer/test/trace-bb/CMakeLists.txt
+++ lib/Fuzzer/test/trace-bb/CMakeLists.txt
@@ -1,12 +1,13 @@
-# These tests are not instrumented with coverage.
-
-set(CMAKE_CXX_FLAGS_RELEASE
-  "${LIBFUZZER_FLAGS_BASE} -fsanitize-coverage=edge,trace-bb")
+set(TEST_SANITIZE_FLAGS "-fsanitize-coverage=edge,trace-bb")
 
 foreach(Test ${TraceBBTests})
   add_executable(LLVMFuzzer-${Test}-TraceBB
     ../${Test}.cpp
     )
+  target_compile_options(LLVMFuzzer-${Test}-TraceBB PRIVATE
+    ${TEST_SANITIZE_FLAGS}
+    )
+  llvm_append_linker_flag(LLVMFuzzer-${Test}-TraceBB ${TEST_SANITIZE_FLAGS})
   target_link_libraries(LLVMFuzzer-${Test}-TraceBB
     LLVMFuzzer
     )
Index: lib/Fuzzer/test/trace-pc/CMakeLists.txt
===================================================================
--- lib/Fuzzer/test/trace-pc/CMakeLists.txt
+++ lib/Fuzzer/test/trace-pc/CMakeLists.txt
@@ -1,14 +1,19 @@
-# These tests are not instrumented with coverage.
-
-set(CMAKE_CXX_FLAGS_RELEASE
-  "${LIBFUZZER_FLAGS_BASE} -O0 -fno-sanitize-coverage=8bit-counters -fsanitize-coverage=trace-pc")
+set(TEST_SANITIZE_FLAGS
+      "-fno-sanitize-coverage=8bit-counters"
+      "-fsanitize-coverage=trace-pc"
+)
 
 foreach(Test ${TracePCTests})
   add_executable(LLVMFuzzer-${Test}-TracePC
     ../${Test}.cpp
     )
+  target_compile_options(LLVMFuzzer-${Test}-TracePC PRIVATE
+    ${TEST_SANITIZE_FLAGS}
+    )
+  llvm_append_linker_flag(LLVMFuzzer-${Test}-TracePC
+    ${TEST_SANITIZE_FLAGS}
+    )
   target_link_libraries(LLVMFuzzer-${Test}-TracePC
     LLVMFuzzer
     )
 endforeach()
-
Index: lib/Fuzzer/test/ubsan/CMakeLists.txt
===================================================================
--- lib/Fuzzer/test/ubsan/CMakeLists.txt
+++ lib/Fuzzer/test/ubsan/CMakeLists.txt
@@ -1,12 +1,19 @@
 # These tests are instrumented with ubsan in non-recovery mode.
 
-set(CMAKE_CXX_FLAGS_RELEASE
-  "${LIBFUZZER_FLAGS_BASE} -O0 -fsanitize=undefined -fno-sanitize-recover=all")
+# NOTE: Additional -fsanitize-coverage= flags come from HandleLLVMOptions.cmake
+set(TEST_SANITIZE_FLAGS
+      "-fsanitize=undefined"
+      "-fno-sanitize-recover=all"
+)
 
 foreach(Test ${UbsanTests})
   add_executable(LLVMFuzzer-${Test}-Ubsan
     ../${Test}.cpp
     )
+  target_compile_options(LLVMFuzzer-${Test}-Ubsan PRIVATE
+    ${TEST_SANITIZE_FLAGS}
+    )
+  llvm_append_linker_flag(LLVMFuzzer-${Test}-Ubsan ${TEST_SANITIZE_FLAGS})
   target_link_libraries(LLVMFuzzer-${Test}-Ubsan
     LLVMFuzzer
     )
Index: lib/Fuzzer/test/uninstrumented/CMakeLists.txt
===================================================================
--- lib/Fuzzer/test/uninstrumented/CMakeLists.txt
+++ lib/Fuzzer/test/uninstrumented/CMakeLists.txt
@@ -1,12 +1,20 @@
 # These tests are not instrumented with coverage.
 
-set(CMAKE_CXX_FLAGS_RELEASE
-  "${LIBFUZZER_FLAGS_BASE} -O0 -fno-sanitize=all -fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters")
+set(TEST_SANITIZE_FLAGS
+      "-fno-sanitize=all"
+      "-fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters"
+)
 
 foreach(Test ${UninstrumentedTests})
   add_executable(LLVMFuzzer-${Test}-Uninstrumented
     ../${Test}.cpp
     )
+  target_compile_options(LLVMFuzzer-${Test}-Uninstrumented PRIVATE
+    ${TEST_SANITIZE_FLAGS}
+    )
+  llvm_append_linker_flag(LLVMFuzzer-${Test}-Uninstrumented
+    ${TEST_SANITIZE_FLAGS}
+    )
   target_link_libraries(LLVMFuzzer-${Test}-Uninstrumented
     LLVMFuzzer
     )