diff --git a/compiler-rt/lib/asan/asan_emscripten.cpp b/compiler-rt/lib/asan/asan_emscripten.cpp
index 90b3c5bbae8ee..59457db3a8252 100644
--- a/compiler-rt/lib/asan/asan_emscripten.cpp
+++ b/compiler-rt/lib/asan/asan_emscripten.cpp
@@ -109,8 +109,6 @@ void GetAllocatorCacheRange(uptr *begin, uptr *end) {
 }
 #endif
 
-u32 GetCurrentThread() { return __asan::GetCurrentThread()->tid(); }
-
 } // namespace __lsan
 
 #endif // SANITIZER_EMSCRIPTEN
diff --git a/compiler-rt/lib/asan/asan_interceptors.cpp b/compiler-rt/lib/asan/asan_interceptors.cpp
index 12f5fe918d95c..bd87c64d78e96 100644
--- a/compiler-rt/lib/asan/asan_interceptors.cpp
+++ b/compiler-rt/lib/asan/asan_interceptors.cpp
@@ -286,9 +286,9 @@ INTERCEPTOR(int, pthread_detach, void *thread) {
   return result;
 }
 
-INTERCEPTOR(int, pthread_exit, void *retval) {
+INTERCEPTOR(void, pthread_exit, void *retval) {
   asanThreadArgRetval().Finish(GetThreadSelf(), retval);
-  return REAL(pthread_exit)(retval);
+  REAL(pthread_exit)(retval);
 }
 
 #    if ASAN_INTERCEPT_TRYJOIN
diff --git a/compiler-rt/lib/asan/asan_rtl.cpp b/compiler-rt/lib/asan/asan_rtl.cpp
index d77b27bc336e3..fb9dab47eb6b3 100644
--- a/compiler-rt/lib/asan/asan_rtl.cpp
+++ b/compiler-rt/lib/asan/asan_rtl.cpp
@@ -447,7 +447,9 @@ static void AsanInitInternal() {
 
   ReplaceSystemMalloc();
 
+#if !SANITIZER_EMSCRIPTEN
   DisableCoreDumperIfNecessary();
+#endif
 
   InitializeShadowMemory();
 
diff --git a/compiler-rt/lib/lsan/lsan_common.cpp b/compiler-rt/lib/lsan/lsan_common.cpp
index 22259e30288fc..d3dfec5ede1b5 100644
--- a/compiler-rt/lib/lsan/lsan_common.cpp
+++ b/compiler-rt/lib/lsan/lsan_common.cpp
@@ -595,17 +595,17 @@ void ScanRootRegions(Frontier *frontier,
 static void ProcessRootRegions(Frontier *frontier) {
   if (!flags()->use_root_regions || !HasRootRegions())
     return;
+  InternalMmapVector<Region> mapped_regions;
 #if SANITIZER_EMSCRIPTEN
-  ScanRootRegion(frontier, root_region, 0, emscripten_get_heap_size(), true);
+  mapped_regions.push_back({0, emscripten_get_heap_size()});
 #else
   MemoryMappingLayout proc_maps(/*cache_enabled*/ true);
   MemoryMappedSegment segment;
-  InternalMmapVector<Region> mapped_regions;
   while (proc_maps.Next(&segment))
     if (segment.IsReadable())
       mapped_regions.push_back({segment.start, segment.end});
-  ScanRootRegions(frontier, mapped_regions);
 #endif // SANITIZER_EMSCRIPTEN
+  ScanRootRegions(frontier, mapped_regions);
 }
 
 static void FloodFillTag(Frontier *frontier, ChunkTag tag) {
diff --git a/compiler-rt/lib/lsan/lsan_common_emscripten.cpp b/compiler-rt/lib/lsan/lsan_common_emscripten.cpp
index f2b1301577238..46bfedb9b519b 100644
--- a/compiler-rt/lib/lsan/lsan_common_emscripten.cpp
+++ b/compiler-rt/lib/lsan/lsan_common_emscripten.cpp
@@ -105,15 +105,13 @@ void LockStuffAndStopTheWorld(StopTheWorldCallback callback,
                               CheckForLeaksParam *argument) {
   // Currently, on Emscripten this does nothing and just calls the callback.
   // This works fine on a single-threaded environment.
-  LockThreadRegistry();
+  LockThreads();
   LockAllocator();
   StopTheWorld(callback, argument);
   UnlockAllocator();
-  UnlockThreadRegistry();
+  UnlockThreads();
 }
 
-u32 GetCurrentThread();
-
 // This is based on ProcessThreads in lsan_common.cc.
 // We changed this to be a callback that gets called per thread by
 // ThreadRegistry::RunCallbackForEachThreadLocked.
@@ -142,7 +140,7 @@ static void ProcessThreadsCallback(ThreadContextBase *tctx, void *arg) {
 
     // We can't get the SP for other threads to narrow down the range, but we
     // we can for the current thread.
-    if (tctx->tid == GetCurrentThread()) {
+    if (tctx->os_id == GetTid()) {
       uptr sp = (uptr) __builtin_frame_address(0);
       if (sp < stack_begin || sp >= stack_end) {
         // SP is outside the recorded stack range (e.g. the thread is running a
diff --git a/compiler-rt/lib/lsan/lsan_interceptors.cpp b/compiler-rt/lib/lsan/lsan_interceptors.cpp
index 10494cee230eb..c4c6088d3663b 100644
--- a/compiler-rt/lib/lsan/lsan_interceptors.cpp
+++ b/compiler-rt/lib/lsan/lsan_interceptors.cpp
@@ -40,6 +40,7 @@ int emscripten_builtin_pthread_create(void *thread, void *attr,
                                       void *(*callback)(void *), void *arg);
 int emscripten_builtin_pthread_join(void *th, void **ret);
 int emscripten_builtin_pthread_detach(void *th);
+void emscripten_builtin_pthread_exit(void *th);
 }
 #endif
 
@@ -452,13 +453,21 @@ INTERCEPTOR(int, pthread_create, void *th, void *attr,
   ENSURE_LSAN_INITED;
   EnsureMainThreadIDIsCorrect();
 
+#if SANITIZER_EMSCRIPTEN
+  // In Emscripten sanitizer, attr can be nonzero but __ATTRP_C11_THREAD in case
+  // of C11 threads, in which case we need to run pthread_attr_init as well, so
+  // we treat __ATTRP_C11_THREAD like the nullptr in this function.
+  if (attr == __ATTRP_C11_THREAD)
+    attr = nullptr;
+#endif
+
   bool detached = [attr]() {
     int d = 0;
     return attr && !pthread_attr_getdetachstate(attr, &d) && IsStateDetached(d);
   }();
 
   __sanitizer_pthread_attr_t myattr;
-  if (!attr || attr == __ATTRP_C11_THREAD) {
+  if (!attr) {
     pthread_attr_init(&myattr);
     attr = &myattr;
   }
@@ -501,9 +510,9 @@ INTERCEPTOR(int, pthread_detach, void *thread) {
   return result;
 }
 
-INTERCEPTOR(int, pthread_exit, void *retval) {
+INTERCEPTOR(void, pthread_exit, void *retval) {
   GetThreadArgRetval().Finish(GetThreadSelf(), retval);
-  return REAL(pthread_exit)(retval);
+  REAL(pthread_exit)(retval);
 }
 
 #  if SANITIZER_INTERCEPT_TRYJOIN
diff --git a/compiler-rt/lib/lsan/lsan_thread.cpp b/compiler-rt/lib/lsan/lsan_thread.cpp
index f27e36f2e9ca4..41fc7da7c7ebe 100644
--- a/compiler-rt/lib/lsan/lsan_thread.cpp
+++ b/compiler-rt/lib/lsan/lsan_thread.cpp
@@ -45,7 +45,10 @@ void InitializeThreads() {
   thread_arg_retval = new (thread_arg_retval_placeholder) ThreadArgRetval();
 }
 
-ThreadArgRetval &GetThreadArgRetval() { return *thread_arg_retval; }
+ThreadArgRetval &GetThreadArgRetval() {
+  ENSURE_LSAN_INITED;
+  return *thread_arg_retval;
+}
 
 ThreadContextLsanBase::ThreadContextLsanBase(int tid)
     : ThreadContextBase(tid) {}
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.h b/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.h
index 08d1a685208be..9b3d95ef592c3 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_platform_limits_posix.h
@@ -445,8 +445,7 @@ struct __sanitizer_file_handle {
 };
 #endif
 
-// These fields are not actually pointers, and so wasm64 must use unsigned and not uptr for them
-#if SANITIZER_APPLE || SANITIZER_EMSCRIPTEN
+#if SANITIZER_APPLE
 struct __sanitizer_msghdr {
   void *msg_name;
   unsigned msg_namelen;
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp
index 8159bc0b92c10..ae8f8fe07c588 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_posix.cpp
@@ -159,7 +159,11 @@ bool MprotectReadOnly(uptr addr, uptr size) {
 }
 
 bool MprotectReadWrite(uptr addr, uptr size) {
+#if SANITIZER_EMSCRIPTEN
+  return true;
+#else
   return 0 == internal_mprotect((void *)addr, size, PROT_READ | PROT_WRITE);
+#endif
 }
 
 #if !SANITIZER_APPLE
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
index 2c34bcbc8bf0e..b780f57279b7d 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_posix_libcdep.cpp
@@ -83,6 +83,7 @@ bool DontDumpShadowMemory(uptr addr, uptr length) {
 #endif  // MADV_DONTDUMP
 }
 
+#if !SANITIZER_EMSCRIPTEN
 static rlim_t getlim(int res) {
   rlimit rlim;
   CHECK_EQ(0, getrlimit(res, &rlim));
@@ -127,6 +128,7 @@ void SetAddressSpaceUnlimited() {
   setlim(RLIMIT_AS, RLIM_INFINITY);
   CHECK(AddressSpaceIsUnlimited());
 }
+#endif
 
 void Abort() {
 #if !SANITIZER_GO
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_redefine_builtins.h b/compiler-rt/lib/sanitizer_common/sanitizer_redefine_builtins.h
index 6649ff5844f5c..9e4c2028dc430 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_redefine_builtins.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_redefine_builtins.h
@@ -15,7 +15,8 @@
 #define SANITIZER_REDEFINE_BUILTINS_H
 
 // The asm hack only works with GCC and Clang.
-#if !defined(_WIN32)
+// XXX Emscripten This does not work in Wasm.
+#if !defined(_WIN32) && !defined(__wasm__)
 
 asm("memcpy = __sanitizer_internal_memcpy");
 asm("memmove = __sanitizer_internal_memmove");
@@ -46,7 +47,7 @@ using unordered_set = Define_SANITIZER_COMMON_NO_REDEFINE_BUILTINS_in_cpp_file;
 using vector = Define_SANITIZER_COMMON_NO_REDEFINE_BUILTINS_in_cpp_file;
 }  // namespace std
 
-#endif  // !_WIN32
+#endif  // !_WIN32 && !__wasm__
 
 #endif  // SANITIZER_REDEFINE_BUILTINS_H
 #endif  // SANITIZER_COMMON_NO_REDEFINE_BUILTINS
diff --git a/libcxx/include/__locale b/libcxx/include/__locale
index 5c060ab04c768..20c515287ea26 100644
--- a/libcxx/include/__locale
+++ b/libcxx/include/__locale
@@ -406,12 +406,12 @@ public:
     static const mask __regex_word = 0x4000; // 0x8000 and 0x0100 and 0x00ff are used
 # define _LIBCPP_CTYPE_MASK_IS_COMPOSITE_PRINT
 # define _LIBCPP_CTYPE_MASK_IS_COMPOSITE_ALPHA
-#elif defined(__APPLE__) || defined(__FreeBSD__) || defined(__EMSCRIPTEN__) || defined(__NetBSD__)
+#elif defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__)
 # ifdef __APPLE__
     typedef __uint32_t mask;
 # elif defined(__FreeBSD__)
     typedef unsigned long mask;
-# elif defined(__EMSCRIPTEN__) || defined(__NetBSD__)
+# elif defined(__NetBSD__)
     typedef unsigned short mask;
 # endif
     static const mask space  = _CTYPE_S;
diff --git a/libcxx/src/new.cpp b/libcxx/src/new.cpp
index 7653e937f9394..9e51dae446e68 100644
--- a/libcxx/src/new.cpp
+++ b/libcxx/src/new.cpp
@@ -53,7 +53,7 @@ operator new(std::size_t size) _THROW_BAD_ALLOC
     return p;
 }
 
-#if defined(__EMSCRIPTEN__) && defined(_LIBCPP_NO_EXCEPTIONS)
+#if defined(__EMSCRIPTEN__) && defined(_LIBCPP_HAS_NO_EXCEPTIONS)
 void* _new_nothrow(size_t size) noexcept
 {
     /// We cannot call ::operator new(size) here because it would abort
@@ -78,7 +78,7 @@ _LIBCPP_WEAK
 void*
 operator new(size_t size, const std::nothrow_t&) noexcept
 {
-#if defined(__EMSCRIPTEN__) && defined(_LIBCPP_NO_EXCEPTIONS)
+#if defined(__EMSCRIPTEN__) && defined(_LIBCPP_HAS_NO_EXCEPTIONS)
     return _new_nothrow(size);
 #else
     void* p = nullptr;
@@ -94,7 +94,7 @@ operator new(size_t size, const std::nothrow_t&) noexcept
     }
 #endif // _LIBCPP_HAS_NO_EXCEPTIONS
     return p;
-#endif // __EMSCRIPTEN__ && _LIBCPP_NO_EXCEPTIONS
+#endif // __EMSCRIPTEN__ && _LIBCPP_HAS_NO_EXCEPTIONS
 }
 
 _LIBCPP_WEAK
@@ -108,7 +108,7 @@ _LIBCPP_WEAK
 void*
 operator new[](size_t size, const std::nothrow_t&) noexcept
 {
-#if defined(__EMSCRIPTEN__) && defined(_LIBCPP_NO_EXCEPTIONS)
+#if defined(__EMSCRIPTEN__) && defined(_LIBCPP_HAS_NO_EXCEPTIONS)
     return _new_nothrow(size);
 #else
     void* p = nullptr;
@@ -124,7 +124,7 @@ operator new[](size_t size, const std::nothrow_t&) noexcept
     }
 #endif // _LIBCPP_HAS_NO_EXCEPTIONS
     return p;
-#endif // __EMSCRIPTEN__ && _LIBCPP_NO_EXCEPTIONS
+#endif // __EMSCRIPTEN__ && _LIBCPP_HAS_NO_EXCEPTIONS
 }
 
 _LIBCPP_WEAK