Index: include/sanitizer/common_interface_defs.h
===================================================================
--- include/sanitizer/common_interface_defs.h
+++ include/sanitizer/common_interface_defs.h
@@ -176,7 +176,9 @@
   // use-after-return detection.
   void __sanitizer_start_switch_fiber(void **fake_stack_save,
                                       const void *bottom, size_t size);
-  void __sanitizer_finish_switch_fiber(void *fake_stack_save);
+  void __sanitizer_finish_switch_fiber(void *fake_stack_save,
+                                       const void **bottom_old,
+                                       size_t *size_old);
 #ifdef __cplusplus
 }  // extern "C"
 #endif
Index: lib/asan/asan_thread.h
===================================================================
--- lib/asan/asan_thread.h
+++ lib/asan/asan_thread.h
@@ -94,7 +94,8 @@
   }
 
   void StartSwitchFiber(FakeStack **fake_stack_save, uptr bottom, uptr size);
-  void FinishSwitchFiber(FakeStack *fake_stack_save);
+  void FinishSwitchFiber(FakeStack *fake_stack_save, uptr *bottom_old,
+                         uptr *size_old);
 
   bool has_fake_stack() {
     return !atomic_load(&stack_switching_, memory_order_relaxed) &&
Index: lib/asan/asan_thread.cc
===================================================================
--- lib/asan/asan_thread.cc
+++ lib/asan/asan_thread.cc
@@ -141,7 +141,9 @@
     current_fake_stack->Destroy(this->tid());
 }
 
-void AsanThread::FinishSwitchFiber(FakeStack *fake_stack_save) {
+void AsanThread::FinishSwitchFiber(FakeStack *fake_stack_save,
+                                   uptr *bottom_old,
+                                   uptr *size_old) {
   if (!atomic_load(&stack_switching_, memory_order_relaxed)) {
     Report("ERROR: finishing a fiber switch that has not started\n");
     Die();
@@ -152,6 +154,12 @@
     fake_stack_ = fake_stack_save;
   }
 
+  if (bottom_old) {
+    *bottom_old = stack_bottom_;
+  }
+  if (size_old) {
+    *size_old = stack_top_ - stack_bottom_;
+  }
   stack_bottom_ = next_stack_bottom_;
   stack_top_ = next_stack_top_;
   atomic_store(&stack_switching_, 0, memory_order_release);
@@ -447,12 +455,16 @@
 }
 
 SANITIZER_INTERFACE_ATTRIBUTE
-void __sanitizer_finish_switch_fiber(void* fakestack) {
+void __sanitizer_finish_switch_fiber(void* fakestack,
+                                     const void **bottom_old,
+                                     uptr *size_old) {
   AsanThread *t = GetCurrentThread();
   if (!t) {
     VReport(1, "__asan_finish_switch_fiber called from unknown thread\n");
     return;
   }
-  t->FinishSwitchFiber((FakeStack*)fakestack);
+  t->FinishSwitchFiber((FakeStack*)fakestack,
+                       (uptr*)bottom_old,
+                       (uptr*)size_old);
 }
 }
Index: test/asan/TestCases/Linux/swapcontext_annotation.cc
===================================================================
--- test/asan/TestCases/Linux/swapcontext_annotation.cc
+++ test/asan/TestCases/Linux/swapcontext_annotation.cc
@@ -6,7 +6,7 @@
 // RUN: %clangxx_asan -lpthread -O3 %s -o %t && %run %t 2>&1 | FileCheck %s
 //
 // This test is too subtle to try on non-x86 arch for now.
-// REQUIRES: x86_64-supported-target,i386-supported-target
+// REQUIRES: x86-target-arch
 
 #include <pthread.h>
 #include <setjmp.h>
@@ -44,14 +44,16 @@
 
 void NextChild() {
   CallNoReturn();
-  __sanitizer_finish_switch_fiber();
+  __sanitizer_finish_switch_fiber(NULL, NULL, NULL);
 
   char x[32] = {0};  // Stack gets poisoned.
   printf("NextChild: %p\n", x);
 
   CallNoReturn();
 
-  __sanitizer_start_switch_fiber(main_thread_stack, main_thread_stacksize);
+  __sanitizer_start_switch_fiber(NULL,
+                                 main_thread_stack,
+                                 main_thread_stacksize);
   CallNoReturn();
   if (swapcontext(&next_child_context, &orig_context) < 0) {
     perror("swapcontext");
@@ -61,7 +63,7 @@
 
 void Child(int mode) {
   CallNoReturn();
-  __sanitizer_finish_switch_fiber();
+  __sanitizer_finish_switch_fiber(NULL, NULL, NULL);
   char x[32] = {0};  // Stack gets poisoned.
   printf("Child: %p\n", x);
   CallNoReturn();
@@ -70,10 +72,14 @@
   //     something.
   // (c) Jump to another function which will then jump back to the main function
   if (mode == 0) {
-    __sanitizer_start_switch_fiber(main_thread_stack, main_thread_stacksize);
+    __sanitizer_start_switch_fiber(NULL,
+                                   main_thread_stack,
+                                   main_thread_stacksize);
     CallNoReturn();
   } else if (mode == 1) {
-    __sanitizer_start_switch_fiber(main_thread_stack, main_thread_stacksize);
+    __sanitizer_start_switch_fiber(NULL,
+                                   main_thread_stack,
+                                   main_thread_stacksize);
     CallNoReturn();
     if (swapcontext(&child_context, &orig_context) < 0) {
       perror("swapcontext");
@@ -84,7 +90,8 @@
     next_child_context.uc_stack.ss_sp = next_child_stack;
     next_child_context.uc_stack.ss_size = kStackSize / 2;
     makecontext(&next_child_context, (void (*)())NextChild, 0);
-    __sanitizer_start_switch_fiber(next_child_context.uc_stack.ss_sp,
+    __sanitizer_start_switch_fiber(NULL,
+                                   next_child_context.uc_stack.ss_sp,
                                    next_child_context.uc_stack.ss_size);
     CallNoReturn();
     if (swapcontext(&child_context, &next_child_context) < 0) {
@@ -105,7 +112,8 @@
   }
   makecontext(&child_context, (void (*)())Child, 1, mode);
   CallNoReturn();
-  __sanitizer_start_switch_fiber(child_context.uc_stack.ss_sp,
+  __sanitizer_start_switch_fiber(NULL,
+                                 child_context.uc_stack.ss_sp,
                                  child_context.uc_stack.ss_size);
   CallNoReturn();
   if (swapcontext(&orig_context, &child_context) < 0) {
@@ -113,7 +121,7 @@
     _exit(1);
   }
   CallNoReturn();
-  __sanitizer_finish_switch_fiber();
+  __sanitizer_finish_switch_fiber(NULL, NULL, NULL);
   CallNoReturn();
 
   // Touch childs's stack to make sure it's unpoisoned.