Remove mjData.qLDiagSqrtInv, add corresponding argument to mj_solveM2.

- `qLDiagSqrtInv` is only required for the dual solvers. It is now computed as-needed rather than unconditionally.
- `mj_solveM2` now requires a new input array `sqrtInvD` which contains the square root of the inverse diagonal D (formerly saved in `qLDiagSqrtInv`).

PiperOrigin-RevId: 710805133
Change-Id: I0622d6a8da3882916824e9c10bad9223c122c321

doc/changelog.rst

@@ -13,6 +13,15 @@ Python bindings
   from multiple threads. To run multiple threaded rollouts simultaneously, use the new class ``Rollout`` which
   encapsulates the thread pool. Contribution by :github:user:`aftersomemath`.
 
+General
+^^^^^^^
+
+.. admonition:: Breaking API changes (minor)
+   :class: attention
+
+   - The field ``mjData.qLDiagSqrtInv`` has been removed. This field is only required for the dual solvers. It is now
+     computed as-needed rather than unconditionally. Relatedly, added the corresponding argument to :ref:`mj_solveM2`.
+
 Bug fixes
 ^^^^^^^^^
 - Fixed a bug in the box-sphere collider, depth was incorrect for deep penetrations (:github:issue:`2206`).

doc/includes/references.h

@@ -273,7 +273,6 @@ struct mjData_ {
   // computed by mj_fwdPosition/mj_factorM
   mjtNum* qLD;               // L'*D*L factorization of M (sparse)               (nM x 1)
   mjtNum* qLDiagInv;         // 1/diag(D)                                        (nv x 1)
-  mjtNum* qLDiagSqrtInv;     // 1/sqrt(diag(D))                                  (nv x 1)
 
   // computed by mj_collisionTree
   mjtNum*  bvh_aabb_dyn;     // global bounding box (center, size)               (nbvhdynamic x 6)
@@ -3221,7 +3220,8 @@ void mj_transmission(const mjModel* m, mjData* d);
 void mj_crb(const mjModel* m, mjData* d);
 void mj_factorM(const mjModel* m, mjData* d);
 void mj_solveM(const mjModel* m, mjData* d, mjtNum* x, const mjtNum* y, int n);
-void mj_solveM2(const mjModel* m, mjData* d, mjtNum* x, const mjtNum* y, int n);
+void mj_solveM2(const mjModel* m, mjData* d, mjtNum* x, const mjtNum* y,
+                const mjtNum* sqrtInvD, int n);
 void mj_comVel(const mjModel* m, mjData* d);
 void mj_passive(const mjModel* m, mjData* d);
 void mj_subtreeVel(const mjModel* m, mjData* d);