diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 174bc09e5cf..446b0691305 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -16100,6 +16100,10 @@ a @code{libubsan} library routine.  The advantage of this is that the
 @code{libubsan} library is not needed and is not linked in, so this
 is usable even in freestanding environments.
 
+If @option{-fsanitize} has not been specified, this option implies
+@option{-fsanitize=unreachable,return}, and is enabled by default at
+@option{-O0} and @option{-Og}.
+
 @item -fsanitize-coverage=trace-pc
 @opindex fsanitize-coverage=trace-pc
 Enable coverage-guided fuzzing code instrumentation.
diff --git a/gcc/common.opt b/gcc/common.opt
index 7ca0cceed82..90e3e84913b 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1109,7 +1109,7 @@ fsanitize-address-use-after-scope
 Common Driver Var(flag_sanitize_address_use_after_scope) Init(0)
 
 fsanitize-undefined-trap-on-error
-Common Driver Var(flag_sanitize_undefined_trap_on_error) Init(0)
+Common Driver Optimization Var(flag_sanitize_undefined_trap_on_error) Init(0)
 Use trap instead of a library function for undefined behavior sanitization.
 
 fasynchronous-unwind-tables
diff --git a/gcc/opts.cc b/gcc/opts.cc
index bf06a55456a..3699eabb599 100644
--- a/gcc/opts.cc
+++ b/gcc/opts.cc
@@ -1122,6 +1122,15 @@ finish_options (struct gcc_options *opts, struct gcc_options *opts_set,
       opts->x_flag_no_inline = 1;
     }
 
+  /* At -O0 or -Og, turn __builtin_unreachable into a trap.  */
+  if ((!opts->x_optimize || opts->x_optimize_debug)
+      && !opts_set->x_flag_sanitize)
+    SET_OPTION_IF_UNSET (opts, opts_set,
+			 flag_sanitize_undefined_trap_on_error, true);
+  if (opts->x_flag_sanitize_undefined_trap_on_error)
+    SET_OPTION_IF_UNSET (opts, opts_set, flag_sanitize,
+			 SANITIZE_UNREACHABLE|SANITIZE_RETURN);
+
   /* Pipelining of outer loops is only possible when general pipelining
      capabilities are requested.  */
   if (!opts->x_flag_sel_sched_pipelining)
@@ -2613,6 +2622,7 @@ common_handle_option (struct gcc_options *opts,
       break;
 
     case OPT_fsanitize_:
+      opts_set->x_flag_sanitize = true;
       opts->x_flag_sanitize
 	= parse_sanitizer_options (arg, loc, code,
 				   opts->x_flag_sanitize, value, true);
diff --git a/gcc/testsuite/g++.dg/ubsan/return-8a.C b/gcc/testsuite/g++.dg/ubsan/return-8a.C
new file mode 100644
index 00000000000..9b2265c4bb0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ubsan/return-8a.C
@@ -0,0 +1,17 @@
+// PR c++/104642
+
+// At -O0 we default to
+//  -fsanitize=unreachable,return -fsanitize-undefined-trap-on-error
+// so the below should abort at runtime.
+
+// { dg-do run }
+// { dg-shouldfail { *-*-* } }
+// { dg-additional-options "-O0" }
+
+bool b;
+
+int f() {
+  if (b) return 42;
+}			// { dg-warning "-Wreturn-type" }
+
+int main() { f(); }
diff --git a/gcc/optc-save-gen.awk b/gcc/optc-save-gen.awk
index 233d1fbb637..38c02bcc2cf 100644
--- a/gcc/optc-save-gen.awk
+++ b/gcc/optc-save-gen.awk
@@ -84,7 +84,7 @@ print "{";
 
 n_opt_char = 4;
 n_opt_short = 0;
-n_opt_int = 0;
+n_opt_int = 1;
 n_opt_enum = 0;
 n_opt_string = 0;
 n_opt_other = 0;
@@ -96,6 +96,7 @@ var_opt_range["optimize"] = "0, 255";
 var_opt_range["optimize_size"] = "0, 2";
 var_opt_range["optimize_debug"] = "0, 1";
 var_opt_range["optimize_fast"] = "0, 1";
+var_opt_int[0] = "flag_sanitize";
 
 # Sort by size to mimic how the structure is laid out to be friendlier to the
 # cache.
@@ -1264,7 +1265,7 @@ for (i = 0; i < n_target_str; i++) {
 }
 print "}";
 
-n_opt_val = 4;
+n_opt_val = 5;
 var_opt_val[0] = "x_optimize"
 var_opt_val_type[0] = "char "
 var_opt_hash[0] = 1;
@@ -1277,6 +1278,9 @@ var_opt_hash[2] = 1;
 var_opt_val[3] = "x_optimize_fast"
 var_opt_val_type[3] = "char "
 var_opt_hash[3] = 1;
+var_opt_val[4] = "x_flag_sanitize"
+var_opt_val_type[4] = "unsigned int "
+var_opt_hash[4] = 1;
 for (i = 0; i < n_opts; i++) {
 	if (flag_set_p("(Optimization|PerFunction)", flags[i])) {
 		name = var_name(flags[i])
diff --git a/gcc/opth-gen.awk b/gcc/opth-gen.awk
index 8bba8ec4549..b3bedaa6da2 100644
--- a/gcc/opth-gen.awk
+++ b/gcc/opth-gen.awk
@@ -134,7 +134,7 @@ print "{";
 
 n_opt_char = 4;
 n_opt_short = 0;
-n_opt_int = 0;
+n_opt_int = 1;
 n_opt_enum = 0;
 n_opt_other = 0;
 n_opt_explicit = 4;
@@ -142,6 +142,7 @@ var_opt_char[0] = "unsigned char x_optimize";
 var_opt_char[1] = "unsigned char x_optimize_size";
 var_opt_char[2] = "unsigned char x_optimize_debug";
 var_opt_char[3] = "unsigned char x_optimize_fast";
+var_opt_int[0] = "unsigned int x_flag_sanitize";
 
 for (i = 0; i < n_opts; i++) {
 	if (flag_set_p("(Optimization|PerFunction)", flags[i])) {
