[DFSan] Support fast16labels mode in dfsan_union.

[morehouse]

Summary

While the instrumentation never calls dfsan_union in fast16labels mode,
the custom wrappers do. We detect fast16labels mode by checking whether
any labels have been created. If not, we must be using fast16labels
mode.

compiler-rt/lib/dfsan/dfsan.cpp

 dfsan_label __dfsan_union(dfsan_label l1, dfsan_label l2) {
   DCHECK_NE(l1, l2);
 
   if (l1 == 0)
     return l2;
   if (l2 == 0)
     return l1;
 
+  // If no labels have been created, yet l1 and l2 are non-zero, we are using
+  // fast16labels mode.
+  if (atomic_load(&__dfsan_last_label, memory_order_relaxed) == 0)
+    return l1 | l2;
+
   if (l1 > l2)
     Swap(l1, l2);
 
   atomic_dfsan_label *table_ent = union_table(l1, l2);
   // We need to deal with the case where two threads concurrently request
   // a union of the same pair of labels.  If the table entry is uninitialized,
   // (i.e. 0) use a compare-exchange to set the entry to kInitializingLabel
   // (i.e. -1) to mark that we are initializing it.

compiler-rt/test/dfsan/custom.cpp

 // RUN: %clang_dfsan %s -o %t && DFSAN_OPTIONS="strict_data_dependencies=0" %run %t
 // RUN: %clang_dfsan -mllvm -dfsan-args-abi %s -o %t && DFSAN_OPTIONS="strict_data_dependencies=0" %run %t
+// RUN: %clang_dfsan -DFAST_16_LABELS -mllvm -dfsan-fast-16-labels %s -o %t && DFSAN_OPTIONS="strict_data_dependencies=0" %run %t
 // RUN: %clang_dfsan -DSTRICT_DATA_DEPENDENCIES %s -o %t && %run %t
 // RUN: %clang_dfsan -DSTRICT_DATA_DEPENDENCIES -mllvm -dfsan-args-abi %s -o %t && %run %t
 
 // Tests custom implementations of various glibc functions.
 
 #include <sanitizer/dfsan_interface.h>
 
 #include <arpa/inet.h>
void test_snprintf() {
   ASSERT_READ_LABEL(buf + 7, 2, k_label);
   ASSERT_READ_LABEL(buf + 9, 4, 0);
   ASSERT_READ_LABEL(buf + 13, 4, i_label);
   ASSERT_READ_LABEL(buf + 17, 2, 0);
   ASSERT_LABEL(r, 0);
 }
 
 int main(void) {
+#ifdef FAST_16_LABELS
+  i_label = 1;
+  j_label = 2;
+  k_label = 4;
+#else
   i_label = dfsan_create_label("i", 0);
   j_label = dfsan_create_label("j", 0);
   k_label = dfsan_create_label("k", 0);
+#endif
   i_j_label = dfsan_union(i_label, j_label);
+  assert(i_j_label != i_label);
+  assert(i_j_label != j_label);
+  assert(i_j_label != k_label);
 
   test_calloc();
   test_clock_gettime();
   test_ctime_r();
   test_dfsan_set_write_callback();
   test_dl_iterate_phdr();
   test_dlopen();
   test_fgets();