base/android: Fix the orderfile build.

The orderfile build doesn't prefetch the native library, creating
unreached code, which is a compile time error with the flags we
use. Move the unreached code out of the build in this case. See the
linked bug for the detailed error message.

Bug: 1065182
Change-Id: Ic8c5b17e6077c4639a82b9caf574a7ae4018e64f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2124258Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754012}

base/android/library_loader/library_prefetcher.cc

@@ -40,35 +40,9 @@ namespace android {
 
 namespace {
 
-// Android defines the background priority to this value since at least 2009
-// (see Process.java).
-constexpr int kBackgroundPriority = 10;
 // Valid for all Android architectures.
 constexpr size_t kPageSize = 4096;
 
-// Reads a byte per page between |start| and |end| to force it into the page
-// cache.
-// Heap allocations, syscalls and library functions are not allowed in this
-// function.
-// Returns true for success.
-#if defined(ADDRESS_SANITIZER)
-// Disable AddressSanitizer instrumentation for this function. It is touching
-// memory that hasn't been allocated by the app, though the addresses are
-// valid. Furthermore, this takes place in a child process. See crbug.com/653372
-// for the context.
-__attribute__((no_sanitize_address))
-#endif
-void Prefetch(size_t start, size_t end) {
-  unsigned char* start_ptr = reinterpret_cast<unsigned char*>(start);
-  unsigned char* end_ptr = reinterpret_cast<unsigned char*>(end);
-  unsigned char dummy = 0;
-  for (unsigned char* ptr = start_ptr; ptr < end_ptr; ptr += kPageSize) {
-    // Volatile is required to prevent the compiler from eliminating this
-    // loop.
-    dummy ^= *static_cast<volatile unsigned char*>(ptr);
-  }
-}
-
 // Populates the per-page residency between |start| and |end| in |residency|. If
 // successful, |residency| has the size of |end| - |start| in pages.
 // Returns true for success.
@@ -188,6 +162,30 @@ void DumpResidency(size_t start,
   }
 }
 
+#if !BUILDFLAG(ORDERFILE_INSTRUMENTATION)
+// Reads a byte per page between |start| and |end| to force it into the page
+// cache.
+// Heap allocations, syscalls and library functions are not allowed in this
+// function.
+// Returns true for success.
+#if defined(ADDRESS_SANITIZER)
+// Disable AddressSanitizer instrumentation for this function. It is touching
+// memory that hasn't been allocated by the app, though the addresses are
+// valid. Furthermore, this takes place in a child process. See crbug.com/653372
+// for the context.
+__attribute__((no_sanitize_address))
+#endif
+void Prefetch(size_t start, size_t end) {
+  unsigned char* start_ptr = reinterpret_cast<unsigned char*>(start);
+  unsigned char* end_ptr = reinterpret_cast<unsigned char*>(end);
+  unsigned char dummy = 0;
+  for (unsigned char* ptr = start_ptr; ptr < end_ptr; ptr += kPageSize) {
+    // Volatile is required to prevent the compiler from eliminating this
+    // loop.
+    dummy ^= *static_cast<volatile unsigned char*>(ptr);
+  }
+}
+
 // These values were used in the past for recording
 // "LibraryLoader.PrefetchDetailedStatus".
 enum class PrefetchStatus {
@@ -219,6 +217,9 @@ PrefetchStatus ForkAndPrefetch(bool ordered_only) {
 
   pid_t pid = fork();
   if (pid == 0) {
+    // Android defines the background priority to this value since at least 2009
+    // (see Process.java).
+    constexpr int kBackgroundPriority = 10;
     setpriority(PRIO_PROCESS, 0, kBackgroundPriority);
     // _exit() doesn't call the atexit() handlers.
     for (const auto& range : ranges) {
@@ -255,6 +256,7 @@ PrefetchStatus ForkAndPrefetch(bool ordered_only) {
     return PrefetchStatus::kChildProcessKilled;
   }
 }
+#endif  // !BUILDFLAG(ORDERFILE_INSTRUMENTATION)
 
 }  // namespace
 
@@ -264,13 +266,13 @@ void NativeLibraryPrefetcher::ForkAndPrefetchNativeLibrary(bool ordered_only) {
   // Avoid forking with orderfile instrumentation because the child process
   // would create a dump as well.
   return;
-#endif
-
+#else
   PrefetchStatus status = ForkAndPrefetch(ordered_only);
   if (status != PrefetchStatus::kSuccess) {
     LOG(WARNING) << "Cannot prefetch the library. status = "
                  << static_cast<int>(status);
   }
+#endif  // BUILDFLAG(ORDERFILE_INSTRUMENTATION)
 }
 
 // static