Add XML compiler/saveinertial flag. Fixes #2405

PiperOrigin-RevId: 743607654
Change-Id: I1f2e61be89b79c1995e079f36aa285096f9fb441

src/user/user_init.c

@@ -42,6 +42,7 @@ void mjs_defaultSpec(mjSpec* spec) {
   spec->compiler.usethread = 1;
   spec->compiler.inertiafromgeom = mjINERTIAFROMGEOM_AUTO;
   spec->compiler.inertiagrouprange[1] = mjNGROUP-1;
+  spec->compiler.saveinertial = 0;
   mj_defaultLROpt(&spec->compiler.LRopt);
 
   // engine data

src/xml/xml_native_reader.cc

@@ -97,10 +97,10 @@ static void UpdateString(string& psuffix, int count, int i) {
 const char* MJCF[nMJCF][mjXATTRNUM] = {
 {"mujoco", "!", "1", "model"},
 {"<"},
-    {"compiler", "*", "19", "autolimits", "boundmass", "boundinertia", "settotalmass",
+    {"compiler", "*", "20", "autolimits", "boundmass", "boundinertia", "settotalmass",
         "balanceinertia", "strippath", "coordinate", "angle", "fitaabb", "eulerseq",
-        "meshdir", "texturedir", "discardvisual", "usethread",
-        "fusestatic", "inertiafromgeom", "inertiagrouprange", "assetdir", "alignfree"},
+        "meshdir", "texturedir", "discardvisual", "usethread", "fusestatic", "inertiafromgeom",
+        "inertiagrouprange", "saveinertial", "assetdir", "alignfree"},
     {"<"},
         {"lengthrange", "?", "10", "mode", "useexisting", "uselimit",
             "accel", "maxforce", "timeconst", "timestep",
@@ -1015,6 +1015,9 @@ void mjXReader::Compiler(XMLElement* section, mjSpec* spec) {
   if (MapValue(section, "alignfree", &n, bool_map, 2)) {
     spec->compiler.alignfree = (n == 1);
   }
+  if (MapValue(section, "saveinertial", &n, bool_map, 2)) {
+    spec->compiler.saveinertial = (n == 1);
+  }
 
   // lengthrange subelement
   XMLElement* elem = FindSubElem(section, "lengthrange");

src/xml/xml_native_writer.cc

@@ -1632,7 +1632,8 @@ void mjXWriter::Body(XMLElement* elem, mjCBody* body, mjCFrame* frame, string_vi
     WriteVector(elem, "user", body->get_userdata());
 
     // write inertial
-    if (body->explicitinertial && model->compiler.inertiafromgeom != mjINERTIAFROMGEOM_TRUE) {
+    if (model->compiler.saveinertial ||
+        (body->explicitinertial && model->compiler.inertiafromgeom != mjINERTIAFROMGEOM_TRUE)) {
       XMLElement* inertial = InsertEnd(elem, "inertial");
       WriteAttr(inertial, "pos", 3, body->ipos);
       WriteAttr(inertial, "quat", 4, body->iquat, unitq);