[XRay][compiler-rt] Allow for building the XRay runtime without PREINIT initialization.

Summary:
Define a build-time configuration option for the XRay runtime to
determine whether the archive will add an entry to the `.preinit_array`
section of the binary. We also allow for initializing the XRay data
structures with an explicit call to __xray_init(). This allows us to
give users the capability to initialize the XRay data structures on
demand.

This can allow us to start porting XRay to platforms where
`.preinit_array` isn't a supported section. It also allows us to limit
the effects of XRay in the initialization sequence for applications that
are sensitive to this kind of interference (i.e. large binaries) or
those that want to package XRay control in libraries.

Future changes should allow us to build two different library archives
for the XRay runtime, and allow clang users to determine which version
to link.

Reviewers: dblaikie, kpw, pelikan

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D36080

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@309909 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 25 insertions, 1 deletions

diff --git a/lib/xray/CMakeLists.txt b/lib/xray/CMakeLists.txt
index 72caa9fac..6d24ba8bf 100644
--- a/lib/xray/CMakeLists.txt
+++ b/lib/xray/CMakeLists.txt
@@ -62,6 +62,8 @@ set(XRAY_CFLAGS ${SANITIZER_COMMON_CFLAGS})
 set(XRAY_COMMON_DEFINITIONS XRAY_HAS_EXCEPTIONS=1)
 append_list_if(
   COMPILER_RT_HAS_XRAY_COMPILER_FLAG XRAY_SUPPORTED=1 XRAY_COMMON_DEFINITIONS)
+append_list_if(
+  COMPILER_RT_BUILD_XRAY_NO_PREINIT XRAY_NO_PREINIT XRAY_COMMON_DEFINITIONS)
 
 add_compiler_rt_object_libraries(RTXray
   ARCHS ${XRAY_SUPPORTED_ARCH}
diff --git a/lib/xray/xray_init.cc b/lib/xray/xray_init.cc
index b46fa9880..07f692431 100644
--- a/lib/xray/xray_init.cc
+++ b/lib/xray/xray_init.cc
@@ -44,10 +44,28 @@ __sanitizer::atomic_uint8_t XRayInitialized{0};
 __sanitizer::SpinMutex XRayInstrMapMutex;
 XRaySledMap XRayInstrMap;
 
+// Global flag to determine whether the flags have been initialized.
+__sanitizer::atomic_uint8_t XRayFlagsInitialized{0};
+
+// A mutex to allow only one thread to initialize the XRay data structures.
+__sanitizer::SpinMutex XRayInitMutex;
+
 // __xray_init() will do the actual loading of the current process' memory map
 // and then proceed to look for the .xray_instr_map section/segment.
 void __xray_init() XRAY_NEVER_INSTRUMENT {
-  initializeFlags();
+  __sanitizer::SpinMutexLock Guard(&XRayInitMutex);
+  // Short-circuit if we've already initialized XRay before.
+  if (__sanitizer::atomic_load(&XRayInitialized,
+                               __sanitizer::memory_order_acquire))
+    return;
+
+  if (!__sanitizer::atomic_load(&XRayFlagsInitialized,
+                                __sanitizer::memory_order_acquire)) {
+    initializeFlags();
+    __sanitizer::atomic_store(&XRayFlagsInitialized, true,
+                              __sanitizer::memory_order_release);
+  }
+
   if (__start_xray_instr_map == nullptr) {
     if (Verbosity())
       Report("XRay instrumentation map missing. Not initializing XRay.\n");
@@ -64,9 +82,13 @@ void __xray_init() XRAY_NEVER_INSTRUMENT {
   __sanitizer::atomic_store(&XRayInitialized, true,
                             __sanitizer::memory_order_release);
 
+#ifndef XRAY_NO_PREINIT
   if (flags()->patch_premain)
     __xray_patch();
+#endif
 }
 
+#ifndef XRAY_NO_PREINIT
 __attribute__((section(".preinit_array"),
                used)) void (*__local_xray_preinit)(void) = __xray_init;
+#endif