[profile] Don't publish VMO if there are no counters

[phosek]

Summary

If there are no counters, there's no need to publish the VMO.

[mcgrathr]

lgtm

compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c

void __llvm_profile_initialize(void) {
    * counter relocation is used. */
   if (__llvm_profile_counter_bias == -1) {
     lprofWrite("LLVM Profile: counter relocation at runtime is required\n");
     return;
   }
 
   const __llvm_profile_data *DataBegin = __llvm_profile_begin_data();
   const __llvm_profile_data *DataEnd = __llvm_profile_end_data();
+  const uint64_t *CountersBegin = __llvm_profile_begin_counters();
+  const uint64_t *CountersEnd = __llvm_profile_end_counters();
   const uint64_t DataSize = __llvm_profile_get_data_size(DataBegin, DataEnd);
   const uint64_t CountersOffset =
       sizeof(__llvm_profile_header) + (DataSize * sizeof(__llvm_profile_data));
+  uint64_t CountersSize = CountersEnd - CountersBegin;
+
+  /* Don't publish a VMO if there are no counters. */
+  if (!CounterSize)
+    return;
 
   zx_status_t Status;
 
-  /* Create VMO to hold the profile data. */
+  /* Create a VMO to hold the profile data. */
   zx_handle_t Vmo = ZX_HANDLE_INVALID;
   Status = _zx_vmo_create(0, ZX_VMO_RESIZABLE, &Vmo);
   if (Status != ZX_OK) {
     lprofWrite("LLVM Profile: cannot create VMO: %s\n",
                _zx_status_get_string(Status));
     return;
   }