[compiler-rt] Update implementation of sanitizer coverage to work when guards are initialized to 0xffffffff.

[mpividori]

Summary

Update the default implementation of sanitizer coverage to work when the guards are initialized to 0xffffffff , as mentioned in: https://reviews.llvm.org/D28436

[kcc]

I'd like to keep the default zero.

lib/sanitizer_common/sanitizer_coverage_libcdep_new.cc

void Initialize() {
     CHECK(!initialized);
 
     initialized = true;
     pc_vector.Initialize(0);
   }
 
   void InitTracePcGuard(u32* start, u32* end) {
     if (!initialized) Initialize();
-    CHECK(!*start);
     CHECK_NE(start, end);
 
     u32 i = pc_vector.size();
-    for (u32* p = start; p < end; p++) *p = ++i;
+    for (u32* p = start; p < end; p++)
+      // Ignore zero padding.
+      if (*p) {
+        if (*p != 0xffffffff) // This module was initialized before.
+          return;
+        *p = ++i;
+      }
     pc_vector.resize(i);
   }
 
   void TracePcGuard(u32* guard, uptr pc) {
     atomic_uint32_t* guard_ptr = reinterpret_cast<atomic_uint32_t*>(guard);
     u32 idx = atomic_exchange(guard_ptr, 0, memory_order_relaxed);
     if (!idx) return;
     // we start indices from 1.

 SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE void
 __sanitizer_cov_trace_pc_guard(u32* guard) {
   if (!*guard) return;
   pc_guard_controller.TracePcGuard(guard, GET_CALLER_PC() - 1);
 }
 
 SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE void
 __sanitizer_cov_trace_pc_guard_init(u32* start, u32* end) {
-  if (start == end || *start) return;
+  if (start == end) return;
   pc_guard_controller.InitTracePcGuard(start, end);
 }
 
 SANITIZER_INTERFACE_ATTRIBUTE void __sanitizer_dump_trace_pc_guard_coverage() {
   pc_guard_controller.Dump();
 }
 }  // extern "C"