DFSan's set label function should avoid writing to the shadow memory when the write would not change the value in memory.

[skerner]

Summary

When writing a label to shadow memory, don't write if the value is already set to the value being written. This dramatically reduces real memory consumption in programs with sparse use of labels.

lib/dfsan/dfsan.cc

dfsan_label dfsan_create_label(const char *desc, void *userdata) {
   __dfsan_label_info[label].l1 = __dfsan_label_info[label].l2 = 0;
   __dfsan_label_info[label].desc = desc;
   __dfsan_label_info[label].userdata = userdata;
   return label;
 }
 
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
 void __dfsan_set_label(dfsan_label label, void *addr, uptr size) {
-  for (dfsan_label *labelp = shadow_for(addr); size != 0; --size, ++labelp)
+  for (dfsan_label *labelp = shadow_for(addr); size != 0; --size, ++labelp) {
+    // Don't write the label if it is already the value we need it to be.
+    // In a program where most addresses are not labeled, it is common that
+    // a page of shadow memory is entirely zeroed.  The Linux copy-on-write
+    // implementation will share all of the zeroed pages, making a copy of a
+    // page when any value is written.  The un-sharing will happen even if
+    // the value written does not change the value in memory.  Avoiding the
+    // write when both |label| and |*labelp| are zero dramatically reduces
+    // the amount of real memory used by large programs.
+    if (label == *labelp)
+      continue;
+
     *labelp = label;
   }
+}
 
 SANITIZER_INTERFACE_ATTRIBUTE
 void dfsan_set_label(dfsan_label label, void *addr, uptr size) {
   __dfsan_set_label(label, addr, size);
 }
 
 SANITIZER_INTERFACE_ATTRIBUTE
 void dfsan_add_label(dfsan_label label, void *addr, uptr size) {