Index: include/sanitizer/tsan_interface.h
===================================================================
--- include/sanitizer/tsan_interface.h
+++ include/sanitizer/tsan_interface.h
@@ -137,6 +137,24 @@
 void __tsan_external_read(void *addr, void *caller_pc, void *tag);
 void __tsan_external_write(void *addr, void *caller_pc, void *tag);
 
+// Fiber switching API.
+//   - TSAN context for fiber can be created by __tsan_create_fiber
+//     and freed by __tsan_destroy_fiber.
+//   - TSAN context of current fiber or thread can be obtained
+//     by calling __tsan_get_current_fiber.
+//   - __tsan_switch_to_fiber should be called immediatly before switch
+//     to fiber, such as call of swapcontext.
+//   - Fiber name can be set by __tsan_set_fiber_name.
+void *__tsan_get_current_fiber(void);
+void *__tsan_create_fiber(unsigned flags);
+void __tsan_destroy_fiber(void *fiber);
+void __tsan_switch_to_fiber(void *fiber, unsigned flags);
+void __tsan_set_fiber_name(void *fiber, const char *name);
+
+// Flags for __tsan_switch_to_fiber:
+// Do not establish a happens-before relation between fibers
+const unsigned __tsan_switch_to_fiber_no_sync = 1 << 0;
+
 #ifdef __cplusplus
 }  // extern "C"
 #endif
Index: lib/tsan/rtl/tsan_interface.cc
===================================================================
--- lib/tsan/rtl/tsan_interface.cc
+++ lib/tsan/rtl/tsan_interface.cc
@@ -124,6 +124,33 @@
   __tsan_unaligned_write8(addr);
   *addr = v;
 }
+
+#if !SANITIZER_MAC && !SANITIZER_ANDROID
+SANITIZER_INTERFACE_ATTRIBUTE
+void *__tsan_get_current_fiber() {
+  return cur_thread();
+}
+
+SANITIZER_INTERFACE_ATTRIBUTE
+void *__tsan_create_fiber(unsigned flags) {
+  return FiberCreate(flags, CALLERPC);
+}
+
+SANITIZER_INTERFACE_ATTRIBUTE
+void __tsan_destroy_fiber(void *fiber) {
+  FiberDestroy(static_cast<ThreadState *>(fiber), CALLERPC);
+}
+
+SANITIZER_INTERFACE_ATTRIBUTE
+void __tsan_switch_to_fiber(void *fiber, unsigned flags) {
+  FiberSwitch(static_cast<ThreadState *>(fiber), flags, CALLERPC);
+}
+
+SANITIZER_INTERFACE_ATTRIBUTE
+void __tsan_set_fiber_name(void *fiber, const char *name) {
+  ThreadSetName(static_cast<ThreadState *>(fiber), name);
+}
+#endif // !SANITIZER_MAC && !SANITIZER_ANDROID
 }  // extern "C"
 
 void __tsan_acquire(void *addr) {
Index: lib/tsan/rtl/tsan_interface_atomic.cc
===================================================================
--- lib/tsan/rtl/tsan_interface_atomic.cc
+++ lib/tsan/rtl/tsan_interface_atomic.cc
@@ -474,8 +474,8 @@
 }
 
 #define SCOPED_ATOMIC(func, ...) \
-    ThreadState *const thr = cur_thread(); \
-    if (thr->ignore_sync || thr->ignore_interceptors) { \
+    ThreadState *const thr = cur_thread_fast(); \
+    if (UNLIKELY(thr->ignore_sync || thr->ignore_interceptors)) { \
       ProcessPendingSignals(thr); \
       return NoTsanAtomic##func(__VA_ARGS__); \
     } \
Index: lib/tsan/rtl/tsan_interface_inl.h
===================================================================
--- lib/tsan/rtl/tsan_interface_inl.h
+++ lib/tsan/rtl/tsan_interface_inl.h
@@ -19,73 +19,73 @@
 using namespace __tsan;  // NOLINT
 
 void __tsan_read1(void *addr) {
-  MemoryRead(cur_thread(), CALLERPC, (uptr)addr, kSizeLog1);
+  MemoryRead(cur_thread_fast(), CALLERPC, (uptr)addr, kSizeLog1);
 }
 
 void __tsan_read2(void *addr) {
-  MemoryRead(cur_thread(), CALLERPC, (uptr)addr, kSizeLog2);
+  MemoryRead(cur_thread_fast(), CALLERPC, (uptr)addr, kSizeLog2);
 }
 
 void __tsan_read4(void *addr) {
-  MemoryRead(cur_thread(), CALLERPC, (uptr)addr, kSizeLog4);
+  MemoryRead(cur_thread_fast(), CALLERPC, (uptr)addr, kSizeLog4);
 }
 
 void __tsan_read8(void *addr) {
-  MemoryRead(cur_thread(), CALLERPC, (uptr)addr, kSizeLog8);
+  MemoryRead(cur_thread_fast(), CALLERPC, (uptr)addr, kSizeLog8);
 }
 
 void __tsan_write1(void *addr) {
-  MemoryWrite(cur_thread(), CALLERPC, (uptr)addr, kSizeLog1);
+  MemoryWrite(cur_thread_fast(), CALLERPC, (uptr)addr, kSizeLog1);
 }
 
 void __tsan_write2(void *addr) {
-  MemoryWrite(cur_thread(), CALLERPC, (uptr)addr, kSizeLog2);
+  MemoryWrite(cur_thread_fast(), CALLERPC, (uptr)addr, kSizeLog2);
 }
 
 void __tsan_write4(void *addr) {
-  MemoryWrite(cur_thread(), CALLERPC, (uptr)addr, kSizeLog4);
+  MemoryWrite(cur_thread_fast(), CALLERPC, (uptr)addr, kSizeLog4);
 }
 
 void __tsan_write8(void *addr) {
-  MemoryWrite(cur_thread(), CALLERPC, (uptr)addr, kSizeLog8);
+  MemoryWrite(cur_thread_fast(), CALLERPC, (uptr)addr, kSizeLog8);
 }
 
 void __tsan_read1_pc(void *addr, void *pc) {
-  MemoryRead(cur_thread(), (uptr)pc, (uptr)addr, kSizeLog1);
+  MemoryRead(cur_thread_fast(), (uptr)pc, (uptr)addr, kSizeLog1);
 }
 
 void __tsan_read2_pc(void *addr, void *pc) {
-  MemoryRead(cur_thread(), (uptr)pc, (uptr)addr, kSizeLog2);
+  MemoryRead(cur_thread_fast(), (uptr)pc, (uptr)addr, kSizeLog2);
 }
 
 void __tsan_read4_pc(void *addr, void *pc) {
-  MemoryRead(cur_thread(), (uptr)pc, (uptr)addr, kSizeLog4);
+  MemoryRead(cur_thread_fast(), (uptr)pc, (uptr)addr, kSizeLog4);
 }
 
 void __tsan_read8_pc(void *addr, void *pc) {
-  MemoryRead(cur_thread(), (uptr)pc, (uptr)addr, kSizeLog8);
+  MemoryRead(cur_thread_fast(), (uptr)pc, (uptr)addr, kSizeLog8);
 }
 
 void __tsan_write1_pc(void *addr, void *pc) {
-  MemoryWrite(cur_thread(), (uptr)pc, (uptr)addr, kSizeLog1);
+  MemoryWrite(cur_thread_fast(), (uptr)pc, (uptr)addr, kSizeLog1);
 }
 
 void __tsan_write2_pc(void *addr, void *pc) {
-  MemoryWrite(cur_thread(), (uptr)pc, (uptr)addr, kSizeLog2);
+  MemoryWrite(cur_thread_fast(), (uptr)pc, (uptr)addr, kSizeLog2);
 }
 
 void __tsan_write4_pc(void *addr, void *pc) {
-  MemoryWrite(cur_thread(), (uptr)pc, (uptr)addr, kSizeLog4);
+  MemoryWrite(cur_thread_fast(), (uptr)pc, (uptr)addr, kSizeLog4);
 }
 
 void __tsan_write8_pc(void *addr, void *pc) {
-  MemoryWrite(cur_thread(), (uptr)pc, (uptr)addr, kSizeLog8);
+  MemoryWrite(cur_thread_fast(), (uptr)pc, (uptr)addr, kSizeLog8);
 }
 
 void __tsan_vptr_update(void **vptr_p, void *new_val) {
   CHECK_EQ(sizeof(vptr_p), 8);
   if (*vptr_p != new_val) {
-    ThreadState *thr = cur_thread();
+    ThreadState *thr = cur_thread_fast();
     thr->is_vptr_access = true;
     MemoryWrite(thr, CALLERPC, (uptr)vptr_p, kSizeLog8);
     thr->is_vptr_access = false;
@@ -94,7 +94,7 @@
 
 void __tsan_vptr_read(void **vptr_p) {
   CHECK_EQ(sizeof(vptr_p), 8);
-  ThreadState *thr = cur_thread();
+  ThreadState *thr = cur_thread_fast();
   thr->is_vptr_access = true;
   MemoryRead(thr, CALLERPC, (uptr)vptr_p, kSizeLog8);
   thr->is_vptr_access = false;
@@ -105,21 +105,21 @@
 }
 
 void __tsan_func_exit() {
-  FuncExit(cur_thread());
+  FuncExit(cur_thread_fast());
 }
 
 void __tsan_ignore_thread_begin() {
-  ThreadIgnoreBegin(cur_thread(), CALLERPC);
+  ThreadIgnoreBegin(cur_thread_fast(), CALLERPC);
 }
 
 void __tsan_ignore_thread_end() {
-  ThreadIgnoreEnd(cur_thread(), CALLERPC);
+  ThreadIgnoreEnd(cur_thread_fast(), CALLERPC);
 }
 
 void __tsan_read_range(void *addr, uptr size) {
-  MemoryAccessRange(cur_thread(), CALLERPC, (uptr)addr, size, false);
+  MemoryAccessRange(cur_thread_fast(), CALLERPC, (uptr)addr, size, false);
 }
 
 void __tsan_write_range(void *addr, uptr size) {
-  MemoryAccessRange(cur_thread(), CALLERPC, (uptr)addr, size, true);
+  MemoryAccessRange(cur_thread_fast(), CALLERPC, (uptr)addr, size, true);
 }
Index: lib/tsan/rtl/tsan_rtl.h
===================================================================
--- lib/tsan/rtl/tsan_rtl.h
+++ lib/tsan/rtl/tsan_rtl.h
@@ -462,11 +462,24 @@
 #if SANITIZER_MAC || SANITIZER_ANDROID
 ThreadState *cur_thread();
 void cur_thread_finalize();
+INLINE ThreadState *cur_thread_fast() {
+  return cur_thread();
+}
 #else
 __attribute__((tls_model("initial-exec")))
+extern THREADLOCAL ThreadState *cur_thread1;
+__attribute__((tls_model("initial-exec")))
 extern THREADLOCAL char cur_thread_placeholder[];
+INLINE ThreadState *cur_thread_fast() {
+  return cur_thread1;
+}
 INLINE ThreadState *cur_thread() {
-  return reinterpret_cast<ThreadState *>(&cur_thread_placeholder);
+  if (UNLIKELY(!cur_thread1))
+    cur_thread1 = reinterpret_cast<ThreadState *>(cur_thread_placeholder);
+  return cur_thread1;
+}
+INLINE void set_cur_thread(ThreadState *thr) {
+  cur_thread1 = thr;
 }
 INLINE void cur_thread_finalize() { }
 #endif  // SANITIZER_MAC || SANITIZER_ANDROID
@@ -867,6 +880,16 @@
 }
 #endif
 
+ThreadState *FiberCreate(unsigned flags, uptr pc);
+void FiberDestroy(ThreadState *fiber, uptr pc);
+void FiberSwitch(ThreadState *fiber, unsigned flags, uptr pc);
+
+// These need to match __tsan_swtich_to_fiber_* flags defined in
+// tsan_interface.h. See documentation there as well.
+enum SwtchFiberFlags {
+  FiberSwitchFlagNoSync = 1 << 0, // __tsan_switch_to_fiber_no_sync
+};
+
 }  // namespace __tsan
 
 #endif  // TSAN_RTL_H
Index: lib/tsan/rtl/tsan_rtl.cc
===================================================================
--- lib/tsan/rtl/tsan_rtl.cc
+++ lib/tsan/rtl/tsan_rtl.cc
@@ -47,6 +47,8 @@
 
 #if !SANITIZER_GO && !SANITIZER_MAC
 __attribute__((tls_model("initial-exec")))
+THREADLOCAL ThreadState *cur_thread1;
+__attribute__((tls_model("initial-exec")))
 THREADLOCAL char cur_thread_placeholder[sizeof(ThreadState)] ALIGNED(64);
 #endif
 static char ctx_placeholder[sizeof(Context)] ALIGNED(64);
@@ -826,7 +828,7 @@
   }
 
   FastState fast_state = thr->fast_state;
-  if (fast_state.GetIgnoreBit()) {
+  if (UNLIKELY(fast_state.GetIgnoreBit())) {
     StatInc(thr, StatMop);
     StatInc(thr, kAccessIsWrite ? StatMopWrite : StatMopRead);
     StatInc(thr, (StatType)(StatMop1 + kAccessSizeLog));
Index: lib/tsan/rtl/tsan_rtl_thread.cc
===================================================================
--- lib/tsan/rtl/tsan_rtl_thread.cc
+++ lib/tsan/rtl/tsan_rtl_thread.cc
@@ -246,7 +246,8 @@
   uptr tls_addr = 0;
   uptr tls_size = 0;
 #if !SANITIZER_GO
-  GetThreadStackAndTls(tid == 0, &stk_addr, &stk_size, &tls_addr, &tls_size);
+  if (os_id)
+    GetThreadStackAndTls(tid == 0, &stk_addr, &stk_size, &tls_addr, &tls_size);
 
   if (tid) {
     if (stk_addr && stk_size)
@@ -404,4 +405,42 @@
   }
 }
 
+#if !SANITIZER_MAC && !SANITIZER_ANDROID && !SANITIZER_GO
+void FiberSwitchImpl(ThreadState *from, ThreadState *to) {
+  Processor *proc = from->proc();
+  ProcUnwire(proc, from);
+  ProcWire(proc, to);
+  set_cur_thread(to);
+}
+
+ThreadState *FiberCreate(unsigned flags, uptr pc) {
+  ThreadState *thr = cur_thread();
+  void *mem = internal_alloc(MBlockThreadContex, sizeof(ThreadState));
+  ThreadState *fiber = static_cast<ThreadState *>(mem);
+  internal_memset(fiber, 0, sizeof(*fiber));
+  int tid = ThreadCreate(thr, pc, 0, true);
+  FiberSwitchImpl(thr, fiber);
+  ThreadStart(fiber, tid, 0, false);
+  FiberSwitchImpl(fiber, thr);
+  return fiber;
+}
+
+void FiberDestroy(ThreadState *fiber, uptr pc) {
+  ThreadState *thr = cur_thread();
+  FiberSwitchImpl(thr, fiber);
+  ThreadFinish(fiber);
+  FiberSwitchImpl(fiber, thr);
+  internal_free(fiber);
+}
+
+void FiberSwitch(ThreadState *fiber, unsigned flags, uptr pc) {
+  ThreadState *thr = cur_thread();
+  if (!(flags & FiberSwitchFlagNoSync))
+    Release(thr, pc, (uptr)fiber);
+  FiberSwitchImpl(thr, fiber);
+  if (!(flags & FiberSwitchFlagNoSync))
+    Acquire(fiber, pc, (uptr)fiber);
+}
+#endif
+
 }  // namespace __tsan
Index: lib/tsan/rtl/tsan_update_shadow_word_inl.h
===================================================================
--- lib/tsan/rtl/tsan_update_shadow_word_inl.h
+++ lib/tsan/rtl/tsan_update_shadow_word_inl.h
@@ -42,7 +42,7 @@
         StoreIfNotYetStored(sp, &store_word);
       break;
     }
-    if (old.IsBothReadsOrAtomic(kAccessIsWrite, kIsAtomic))
+    if (LIKELY(old.IsBothReadsOrAtomic(kAccessIsWrite, kIsAtomic)))
       break;
     goto RACE;
   }
@@ -56,7 +56,7 @@
     StatInc(thr, StatShadowAnotherThread);
     if (old.IsBothReadsOrAtomic(kAccessIsWrite, kIsAtomic))
       break;
-    if (HappensBefore(old, thr))
+    if (LIKELY(HappensBefore(old, thr)))
       break;
     goto RACE;
   }
Index: test/tsan/fiber_asm.cc
===================================================================
--- /dev/null
+++ test/tsan/fiber_asm.cc
@@ -0,0 +1,85 @@
+// RUN: %clang_tsan -O1 %s -o %t && %run %t 2>&1 | FileCheck %s
+// UNSUPPORTED: darwin
+#include "test.h"
+
+struct ucontext {
+  void *sp;
+  void *fiber;
+};
+
+extern "C" {
+  void ucontext_do_switch(void **save, void **load);
+  void ucontext_trampoline();
+}
+
+__asm__(".global ucontext_do_switch\n"
+        "ucontext_do_switch:\n\t"
+        "pushq %rbp\n\t"
+        "pushq %r15\n\t"
+        "pushq %r14\n\t"
+        "pushq %r13\n\t"
+        "pushq %r12\n\t"
+        "pushq %rbx\n\t"
+        "movq %rsp, (%rdi)\n\t"
+        "movq (%rsi), %rsp\n\t"
+        "popq %rbx\n\t"
+        "popq %r12\n\t"
+        "popq %r13\n\t"
+        "popq %r14\n\t"
+        "popq %r15\n\t"
+        "popq %rbp\n\t"
+        "retq");
+
+__asm__(".global ucontext_trampoline\n"
+        "ucontext_trampoline:\n\t"
+        ".cfi_startproc\n\t"
+        ".cfi_undefined rip\n\t"
+        "movq %r12, %rdi\n\t"
+        "jmpq *%rbx\n\t"
+        ".cfi_endproc");
+
+void ucontext_init(ucontext *context, void *stack, unsigned stack_sz,
+                   void (*func)(void*), void *arg) {
+  void **sp = reinterpret_cast<void **>(static_cast<char *>(stack) + stack_sz);
+  *(--sp) = 0;
+  *(--sp) = reinterpret_cast<void *>(ucontext_trampoline);
+  *(--sp) = 0;   // rbp
+  *(--sp) = 0;   // r15
+  *(--sp) = 0;   // r14
+  *(--sp) = 0;   // r13
+  *(--sp) = arg; // r12
+  *(--sp) = reinterpret_cast<void *>(func); // rbx
+  context->sp = sp;
+  context->fiber = __tsan_create_fiber(0);
+}
+
+void ucontext_free(ucontext *context) {
+  __tsan_destroy_fiber(context->fiber);
+}
+
+__attribute__((no_sanitize_thread))
+void ucontext_switch(ucontext *save, ucontext *load) {
+  save->fiber = __tsan_get_current_fiber();
+  __tsan_switch_to_fiber(load->fiber, 0);
+  ucontext_do_switch(&save->sp, &load->sp);
+}
+
+char stack[64 * 1024] __attribute__((aligned(16)));
+
+ucontext uc, orig_uc;
+
+void func(void *arg) {
+  __asm__ __volatile__(".cfi_undefined rip");
+  ucontext_switch(&uc, &orig_uc);
+}
+
+int main() {
+  ucontext_init(&uc, stack, sizeof(stack), func, 0);
+  ucontext_switch(&orig_uc, &uc);
+  ucontext_free(&uc);
+  fprintf(stderr, "PASS\n");
+  return 0;
+}
+
+// CHECK-NOT: WARNING: ThreadSanitizer:
+// CHECK: PASS
Index: test/tsan/fiber_race.cc
===================================================================
--- /dev/null
+++ test/tsan/fiber_race.cc
@@ -0,0 +1,36 @@
+// RUN: %clang_tsan -O1 %s -o %t && %deflake %run %t 2>&1 | FileCheck %s
+// UNSUPPORTED: darwin
+#include "test.h"
+#include <ucontext.h>
+
+char stack[64 * 1024] __attribute__((aligned(16)));
+
+ucontext_t uc, orig_uc;
+void *fiber, *orig_fiber;
+
+int var;
+
+void func() {
+  var = 1;
+  __tsan_switch_to_fiber(orig_fiber, __tsan_switch_to_fiber_no_sync);
+  swapcontext(&uc, &orig_uc);
+}
+
+int main() {
+  orig_fiber = __tsan_get_current_fiber();
+  fiber = __tsan_create_fiber(0);
+  getcontext(&uc);
+  uc.uc_stack.ss_sp = stack;
+  uc.uc_stack.ss_size = sizeof(stack);
+  uc.uc_link = &orig_uc;
+  makecontext(&uc, func, 0);
+  var = 2;
+  __tsan_switch_to_fiber(fiber, __tsan_switch_to_fiber_no_sync);
+  swapcontext(&orig_uc, &uc);
+  __tsan_destroy_fiber(fiber);
+  fprintf(stderr, "PASS\n");
+  return 0;
+}
+
+// CHECK: WARNING: ThreadSanitizer: data race
+// CHECK: PASS
Index: test/tsan/fiber_simple.cc
===================================================================
--- /dev/null
+++ test/tsan/fiber_simple.cc
@@ -0,0 +1,36 @@
+// RUN: %clang_tsan -O1 %s -o %t && %run %t 2>&1 | FileCheck %s
+// UNSUPPORTED: darwin
+#include "test.h"
+#include <ucontext.h>
+
+char stack[64 * 1024] __attribute__((aligned(16)));
+
+ucontext_t uc, orig_uc;
+void *fiber, *orig_fiber;
+
+int var;
+
+void func() {
+  var = 1;
+  __tsan_switch_to_fiber(orig_fiber, 0);
+  swapcontext(&uc, &orig_uc);
+}
+
+int main() {
+  orig_fiber = __tsan_get_current_fiber();
+  fiber = __tsan_create_fiber(0);
+  getcontext(&uc);
+  uc.uc_stack.ss_sp = stack;
+  uc.uc_stack.ss_size = sizeof(stack);
+  uc.uc_link = &orig_uc;
+  makecontext(&uc, func, 0);
+  var = 2;
+  __tsan_switch_to_fiber(fiber, 0);
+  swapcontext(&orig_uc, &uc);
+  __tsan_destroy_fiber(fiber);
+  fprintf(stderr, "PASS\n");
+  return 0;
+}
+
+// CHECK-NOT: WARNING: ThreadSanitizer:
+// CHECK: PASS
Index: test/tsan/fiber_two_threads.cc
===================================================================
--- /dev/null
+++ test/tsan/fiber_two_threads.cc
@@ -0,0 +1,62 @@
+// RUN: %clang_tsan -O1 %s -o %t && %run %t 2>&1 | FileCheck %s
+// UNSUPPORTED: darwin
+#include "test.h"
+#include <ucontext.h>
+
+char stack[64 * 1024] __attribute__((aligned(16)));
+
+ucontext_t uc, orig_uc[2];
+void *fiber, *orig_fiber[2];
+
+const unsigned N = 1000;
+
+__attribute__((noinline))
+void switch0() {
+  __tsan_switch_to_fiber(orig_fiber[0], 0);
+  swapcontext(&uc, &orig_uc[0]);
+}
+
+void func() {
+  for (;;) {
+    switch0();
+    __tsan_switch_to_fiber(orig_fiber[1], 0);
+    swapcontext(&uc, &orig_uc[1]);
+  }
+}
+
+void *Thread(void *x) {
+  orig_fiber[1] = __tsan_get_current_fiber();
+  for (unsigned i = 0; i < N; i++) {
+    barrier_wait(&barrier);
+    __tsan_switch_to_fiber(fiber, 0);
+    swapcontext(&orig_uc[1], &uc);
+    barrier_wait(&barrier);
+  }
+  return 0;
+}
+
+int main() {
+  fiber = __tsan_create_fiber(0);
+  barrier_init(&barrier, 2);
+  pthread_t t;
+  pthread_create(&t, 0, Thread, 0);
+  orig_fiber[0] = __tsan_get_current_fiber();
+  getcontext(&uc);
+  uc.uc_stack.ss_sp = stack;
+  uc.uc_stack.ss_size = sizeof(stack);
+  uc.uc_link = &orig_uc[0];
+  makecontext(&uc, func, 0);
+  for (unsigned i = 0; i < N; i++) {
+    __tsan_switch_to_fiber(fiber, 0);
+    swapcontext(&orig_uc[0], &uc);
+    barrier_wait(&barrier);
+    barrier_wait(&barrier);
+  }
+  pthread_join(t, 0);
+  __tsan_destroy_fiber(fiber);
+  fprintf(stderr, "PASS\n");
+  return 0;
+}
+
+// CHECK-NOT: WARNING: ThreadSanitizer:
+// CHECK: PASS