Index: lib/asan/asan_stack.h
===================================================================
--- lib/asan/asan_stack.h
+++ lib/asan/asan_stack.h
@@ -33,7 +33,7 @@
 void GetStackTrace(BufferedStackTrace *stack, uptr max_depth, uptr pc, uptr bp,
                    void *context, bool fast) {
 #if SANITIZER_WINDOWS
-  stack->Unwind(max_depth, pc, bp, context, 0, 0, fast);
+  stack->Unwind(max_depth, pc, 0, context, 0, 0, false);
 #else
   AsanThread *t;
   stack->size = 0;
@@ -43,8 +43,11 @@
       uptr stack_bottom = t->stack_bottom();
       ScopedUnwinding unwind_scope(t);
       if (!SANITIZER_MIPS || IsValidFrame(bp, stack_top, stack_bottom)) {
-        stack->Unwind(max_depth, pc, bp, context, stack_top, stack_bottom,
-                      fast);
+        if (StackTrace::WillUseFastUnwind(fast))
+          stack->Unwind(max_depth, pc, bp, nullptr, stack_top, stack_bottom,
+                      true);
+        else
+          stack->Unwind(max_depth, pc, 0, context, 0, 0, false);
       }
     } else if (!t && !fast) {
       /* If GetCurrentThread() has failed, try to do slow unwind anyways. */
Index: lib/hwasan/hwasan.cpp
===================================================================
--- lib/hwasan/hwasan.cpp
+++ lib/hwasan/hwasan.cpp
@@ -157,8 +157,11 @@
     SymbolizerScope sym_scope;
     return stack->Unwind(max_s, pc, bp, context, 0, 0, request_fast_unwind);
   }
-  stack->Unwind(max_s, pc, bp, context, t->stack_top(), t->stack_bottom(),
-                request_fast_unwind);
+  if (StackTrace::WillUseFastUnwind(request_fast_unwind))
+    stack->Unwind(max_s, pc, bp, nullptr, t->stack_top(), t->stack_bottom(),
+                  true);
+  else
+    stack->Unwind(max_s, pc, 0, context, 0, 0, false);
 }
 
 static void HWAsanCheckFailed(const char *file, int line, const char *cond,
Index: lib/lsan/lsan.h
===================================================================
--- lib/lsan/lsan.h
+++ lib/lsan/lsan.h
@@ -55,7 +55,10 @@
     stack_bottom = t->stack_begin();
   }
   if (!SANITIZER_MIPS || IsValidFrame(bp, stack_top, stack_bottom)) {
-    stack->Unwind(max_depth, pc, bp, context, stack_top, stack_bottom, fast);
+    if (StackTrace::WillUseFastUnwind(fast))
+      stack->Unwind(max_depth, pc, bp, nullptr, stack_top, stack_bottom, true);
+    else
+      stack->Unwind(max_depth, pc, 0, context, 0, 0, false);
   }
 }
 
Index: lib/msan/msan.cc
===================================================================
--- lib/msan/msan.cc
+++ lib/msan/msan.cc
@@ -227,10 +227,13 @@
   if (!t || !StackTrace::WillUseFastUnwind(request_fast_unwind)) {
     // Block reports from our interceptors during _Unwind_Backtrace.
     SymbolizerScope sym_scope;
-    return stack->Unwind(max_s, pc, bp, context, 0, 0, request_fast_unwind);
+    return stack->Unwind(max_s, pc, bp, context, 0, 0, false);
   }
-  stack->Unwind(max_s, pc, bp, context, t->stack_top(), t->stack_bottom(),
-                request_fast_unwind);
+  if (StackTrace::WillUseFastUnwind(request_fast_unwind))
+    stack->Unwind(max_s, pc, bp, nullptr, t->stack_top(), t->stack_bottom(),
+                  true);
+  else
+    stack->Unwind(max_s, pc, 0, context, 0, 0, false);
 }
 
 void PrintWarning(uptr pc, uptr bp) {
Index: lib/sanitizer_common/sanitizer_stacktrace_libcdep.cc
===================================================================
--- lib/sanitizer_common/sanitizer_stacktrace_libcdep.cc
+++ lib/sanitizer_common/sanitizer_stacktrace_libcdep.cc
@@ -57,6 +57,8 @@
 void BufferedStackTrace::Unwind(u32 max_depth, uptr pc, uptr bp, void *context,
                                 uptr stack_top, uptr stack_bottom,
                                 bool request_fast_unwind) {
+  // Ensures all call sites get what they requested.
+  CHECK_EQ(request_fast_unwind, WillUseFastUnwind(request_fast_unwind));
   top_frame_bp = (max_depth > 0) ? bp : 0;
   // Avoid doing any work for small max_depth.
   if (max_depth == 0) {
Index: lib/sanitizer_common/sanitizer_symbolizer_report.cc
===================================================================
--- lib/sanitizer_common/sanitizer_symbolizer_report.cc
+++ lib/sanitizer_common/sanitizer_symbolizer_report.cc
@@ -103,9 +103,11 @@
   GET_CALLER_PC_BP_SP;
   (void)sp;
   bool fast = common_flags()->fast_unwind_on_fatal;
-  if (StackTrace::WillUseFastUnwind(fast))
+  if (StackTrace::WillUseFastUnwind(fast)) {
     GetThreadStackTopAndBottom(false, &top, &bottom);
-  stack->Unwind(kStackTraceMax, pc, bp, nullptr, top, bottom, fast);
+    stack->Unwind(kStackTraceMax, pc, bp, nullptr, top, bottom, true);
+  } else
+    stack->Unwind(kStackTraceMax, pc, 0, nullptr, 0, 0, false);
 
   Printf("%s", d.Warning());
   Report("WARNING: %s: writable-executable page usage\n", SanitizerToolName);
Index: lib/tsan/rtl/tsan_rtl.cc
===================================================================
--- lib/tsan/rtl/tsan_rtl.cc
+++ lib/tsan/rtl/tsan_rtl.cc
@@ -331,9 +331,11 @@
   uptr top = 0;
   uptr bottom = 0;
   bool fast = common_flags()->fast_unwind_on_fatal;
-  if (StackTrace::WillUseFastUnwind(fast))
+  if (StackTrace::WillUseFastUnwind(fast)) {
     GetThreadStackTopAndBottom(false, &top, &bottom);
-  stack->Unwind(kStackTraceMax, sig.pc, sig.bp, sig.context, top, bottom, fast);
+    stack->Unwind(kStackTraceMax, sig.pc, sig.bp, nullptr, top, bottom, true);
+  } else
+    stack->Unwind(kStackTraceMax, sig.pc, 0, sig.context, 0, 0, false);
 }
 
 static void TsanOnDeadlySignal(int signo, void *siginfo, void *context) {
Index: lib/tsan/rtl/tsan_rtl_report.cc
===================================================================
--- lib/tsan/rtl/tsan_rtl_report.cc
+++ lib/tsan/rtl/tsan_rtl_report.cc
@@ -732,14 +732,15 @@
   uptr bp = 0;
   uptr top = 0;
   uptr bottom = 0;
-  if (__sanitizer::StackTrace::WillUseFastUnwind(false)) {
-    bp = GET_CURRENT_FRAME();
-    __sanitizer::GetThreadStackTopAndBottom(false, &top, &bottom);
-  }
   BufferedStackTrace *ptrace =
       new(internal_alloc(MBlockStackTrace, sizeof(BufferedStackTrace)))
           BufferedStackTrace();
-  ptrace->Unwind(kStackTraceMax, pc, bp, nullptr, top, bottom, false);
+  if (__sanitizer::StackTrace::WillUseFastUnwind(false)) {
+    bp = GET_CURRENT_FRAME();
+    __sanitizer::GetThreadStackTopAndBottom(false, &top, &bottom);
+    ptrace->Unwind(kStackTraceMax, pc, bp, nullptr, top, bottom, true);
+  } else
+    ptrace->Unwind(kStackTraceMax, pc, 0, nullptr, 0, 0, false);
   for (uptr i = 0; i < ptrace->size / 2; i++) {
     uptr tmp = ptrace->trace_buffer[i];
     ptrace->trace_buffer[i] = ptrace->trace_buffer[ptrace->size - i - 1];
Index: lib/ubsan/ubsan_diag.cc
===================================================================
--- lib/ubsan/ubsan_diag.cc
+++ lib/ubsan/ubsan_diag.cc
@@ -30,9 +30,11 @@
                             uptr bp, void *context, bool fast) {
   uptr top = 0;
   uptr bottom = 0;
-  if (StackTrace::WillUseFastUnwind(fast))
+  if (StackTrace::WillUseFastUnwind(fast)) {
     GetThreadStackTopAndBottom(false, &top, &bottom);
-  stack->Unwind(max_depth, pc, bp, context, top, bottom, fast);
+    stack->Unwind(max_depth, pc, bp, nullptr, top, bottom, true);
+  } else
+    stack->Unwind(max_depth, pc, bp, context, 0, 0, false);
 }
 
 static void MaybePrintStackTrace(uptr pc, uptr bp) {
Index: lib/ubsan/ubsan_diag_standalone.cc
===================================================================
--- lib/ubsan/ubsan_diag_standalone.cc
+++ lib/ubsan/ubsan_diag_standalone.cc
@@ -22,14 +22,15 @@
   uptr top = 0;
   uptr bottom = 0;
   bool request_fast_unwind = common_flags()->fast_unwind_on_fatal;
-  if (__sanitizer::StackTrace::WillUseFastUnwind(request_fast_unwind))
-    __sanitizer::GetThreadStackTopAndBottom(false, &top, &bottom);
-
   GET_CURRENT_PC_BP_SP;
   (void)sp;
   BufferedStackTrace stack;
-  stack.Unwind(kStackTraceMax, pc, bp, nullptr, top, bottom,
-               request_fast_unwind);
+  if (__sanitizer::StackTrace::WillUseFastUnwind(request_fast_unwind)) {
+    __sanitizer::GetThreadStackTopAndBottom(false, &top, &bottom);
+    stack.Unwind(kStackTraceMax, pc, bp, nullptr, top, bottom, true);
+  } else {
+    stack.Unwind(kStackTraceMax, pc, 0, nullptr, 0, 0, false);
+  }
   stack.Print();
 }
 } // extern "C"