Index: compiler-rt/trunk/lib/asan/scripts/asan_symbolize.py
===================================================================
--- compiler-rt/trunk/lib/asan/scripts/asan_symbolize.py
+++ compiler-rt/trunk/lib/asan/scripts/asan_symbolize.py
@@ -383,6 +383,7 @@
       self.dsym_hints = set([])
       self.frame_no = 0
       self.process_line = self.process_line_posix
+      self.using_module_map = plugin_proxy.has_plugin(ModuleMapPlugIn.get_name())
 
   def symbolize_address(self, addr, binary, offset, arch):
     # On non-Darwin (i.e. on platforms without .dSYM debug info) always use
@@ -451,14 +452,26 @@
 
   def process_line_posix(self, line):
     self.current_line = line.rstrip()
-    #0 0x7f6e35cf2e45  (/blah/foo.so+0x11fe45)
+    # Unsymbolicated:
+    # #0 0x7f6e35cf2e45  (/blah/foo.so+0x11fe45)
+    # Partially symbolicated:
+    # #0 0x7f6e35cf2e45 in foo (foo.so+0x11fe45)
+    # NOTE: We have to very liberal with symbol
+    # names in the regex because it could be an
+    # Objective-C or C++ demangled name.
     stack_trace_line_format = (
-        '^( *#([0-9]+) *)(0x[0-9a-f]+) *\((.*)\+(0x[0-9a-f]+)\)')
+        '^( *#([0-9]+) *)(0x[0-9a-f]+) *(?:in *.+)? *\((.*)\+(0x[0-9a-f]+)\)')
     match = re.match(stack_trace_line_format, line)
     if not match:
       return [self.current_line]
     logging.debug(line)
     _, frameno_str, addr, binary, offset = match.groups()
+    if not self.using_module_map and not os.path.isabs(binary):
+      # Do not try to symbolicate if the binary is just the module file name
+      # and a module map is unavailable.
+      # FIXME(dliew): This is currently necessary for reports on Darwin that are
+      # partially symbolicated by `atos`.
+      return [self.current_line]
     arch = ""
     # Arch can be embedded in the filename, e.g.: "libabc.dylib:x86_64h"
     colon_pos = binary.rfind(":")
Index: compiler-rt/trunk/test/asan/TestCases/Darwin/asan-symbolize-partial-report-with-module-map.cc
===================================================================
--- compiler-rt/trunk/test/asan/TestCases/Darwin/asan-symbolize-partial-report-with-module-map.cc
+++ compiler-rt/trunk/test/asan/TestCases/Darwin/asan-symbolize-partial-report-with-module-map.cc
@@ -0,0 +1,54 @@
+// When `external_symbolizer_path` is empty on Darwin we fallback on using
+// dladdr as the symbolizer which means we get the symbol name
+// but no source location. The current implementation also doesn't try to
+// change the module name so we end up with the full name so we actually don't
+// need the module map here.
+
+// RUN: %clangxx_asan -O0 -g %s -o %t.executable
+// RUN: %env_asan_opts=symbolize=1,print_module_map=0,external_symbolizer_path= not %run %t.executable > %t2.log 2>&1
+// RUN: FileCheck -input-file=%t2.log -check-prefix=CHECK-PS %s
+// RUN: %asan_symbolize --force-system-symbolizer < %t2.log > %t2.fully_symbolized
+// RUN: FileCheck -input-file=%t2.fully_symbolized -check-prefix=CHECK-FS %s
+
+// Due a quirk in the way atos reports module names we have to use the module
+// map here, otherwise we don't know what the full path to the module is.
+
+// FIXME(dliew): We currently have to use module map for this test due to the atos
+// symbolizer changing the module name from an absolute path to just the file name.
+// rdar://problem/49784442
+//
+// Simulate partial symbolication (can happen with %L specifier) by printing
+// out %L's fallback which will print the module name and offset instead of a
+// source location.
+// RUN: %clangxx_asan -O0 -g %s -o %t2.executable
+// RUN: %env_asan_opts=symbolize=1,print_module_map=1,stack_trace_format='"    #%%n %%p %%F %%M"' not %run %t.executable > %t2.log 2>&1
+// RUN: FileCheck -input-file=%t2.log -check-prefix=CHECK-PS %s
+// Now try to full symbolicate using the module map.
+// RUN: %asan_symbolize --module-map %t2.log --force-system-symbolizer < %t2.log > %t2.fully_symbolized
+// RUN: FileCheck -input-file=%t2.fully_symbolized -check-prefix=CHECK-FS %s
+
+#include <cstdlib>
+
+// Partially symbolicated back-trace where symbol is available but
+// source location is not and instead module name and offset are
+// printed.
+// CHECK-PS: WRITE of size 4
+// CHECK-PS: #0 0x{{.+}} in foo ({{.+}}.executable:{{.+}}+0x{{.+}})
+// CHECK-PS: #1 0x{{.+}} in main ({{.+}}.executable:{{.+}}+0x{{.+}})
+
+// CHECK-FS: WRITE of size 4
+
+extern "C" void foo(int* a) {
+  // CHECK-FS: #0 0x{{.+}} in foo {{.*}}asan-symbolize-partial-report-with-module-map.cc:[[@LINE+1]]
+  *a = 5;
+}
+
+int main() {
+  int* a = (int*) malloc(sizeof(int));
+  if (!a)
+    return 0;
+  free(a);
+  // CHECK-FS: #1 0x{{.+}} in main {{.*}}asan-symbolize-partial-report-with-module-map.cc:[[@LINE+1]]
+  foo(a);
+  return 0;
+}