--- /dev/null
+# This BUILD_CONFIG option is a bit like bootstrap-debug, but rather
+# than comparing stripped object files, it compares compiler internal
+# state during stage3. Both can be used simultaneously.
+
+# This makes it slower than bootstrap-debug alone, for there's
+# additional dumping and recompilation during stage3.
+# bootstrap-debug-big can avoid the recompilation, if plenty of disk
+# space is available.
+
+STAGE2_CFLAGS += -fcompare-debug=
+STAGE3_CFLAGS += -fcompare-debug