In final_tidyup we setup the guest to call the freeres_wrapper, which will (possibly) call __gnu_cxx::__freeres() and/or __libc_freeres().

For ppc64le we do this with:

#  elif  defined(VGP_ppc64le_linux)
   /* setting GPR2 but not really needed, GPR12 is needed */
   VG_(threads)[tid].arch.vex.guest_GPR2  = freeres_wrapper;
   VG_(threads)[tid].arch.vex.guest_GPR12 = freeres_wrapper;
#  endif

And then we setup the argument as:

#  elif defined(VGA_ppc64be) || defined(VGA_ppc64le)
   VG_(threads)[tid].arch.vex.guest_GPR3 = to_run;
   VG_TRACK(post_reg_write, Vg_CoreClientReq, tid,
            offsetof(VexGuestPPC64State, guest_GPR3),
            sizeof(VG_(threads)[tid].arch.vex.guest_GPR3));

Note how in the first hunk we are not marking GPR2 and GPR12 as defined/written.

This might cause:

==21345== Use of uninitialised value of size 8
==21345==    at 0x4050794: _vgnU_freeres (vg_preloaded.c:68)
==21345==    by 0x4114A03: __run_exit_handlers (in /usr/lib64/power9/libc-2.28.so)
==21345==    by 0x4114A97: exit (in /usr/lib64/power9/libc-2.28.so)
==21345==    by 0x40F417F: (below main) (in /usr/lib64/power9/libc-2.28.so)
==21345== 
==21345== Use of uninitialised value of size 8
==21345==    at 0x40505E4: ??? (in /root/valgrind-install/lib/valgrind/vgpreload_core-ppc64le-linux.so)
==21345==    by 0x4114A03: __run_exit_handlers (in /usr/lib64/power9/libc-2.28.so)
==21345==    by 0x4114A97: exit (in /usr/lib64/power9/libc-2.28.so)
==21345==    by 0x40F417F: (below main) (in /usr/lib64/power9/libc-2.28.so)
==21345== 

This code checks r12 (first to see if a weak function symbol is set, then inside the plt entry for __libc_freeres).

So, the setup code should also mark the helper registers as defined/written for those architectures that use them:

diff --git a/coregrind/m_main.c b/coregrind/m_main.c
index 00702fc22..9052edf1e 100644
--- a/coregrind/m_main.c
+++ b/coregrind/m_main.c
@@ -2312,14 +2312,26 @@ static void final_tidyup(ThreadId tid)
    VG_(set_IP)(tid, freeres_wrapper);
 #  if defined(VGP_ppc64be_linux)
    VG_(threads)[tid].arch.vex.guest_GPR2 = r2;
+   VG_TRACK(post_reg_write, Vg_CoreClientReq, tid,
+            offsetof(VexGuestPPC64State, guest_GPR2),
+            sizeof(VG_(threads)[tid].arch.vex.guest_GPR2));
 #  elif  defined(VGP_ppc64le_linux)
    /* setting GPR2 but not really needed, GPR12 is needed */
    VG_(threads)[tid].arch.vex.guest_GPR2  = freeres_wrapper;
+   VG_TRACK(post_reg_write, Vg_CoreClientReq, tid,
+            offsetof(VexGuestPPC64State, guest_GPR2),
+            sizeof(VG_(threads)[tid].arch.vex.guest_GPR2));
    VG_(threads)[tid].arch.vex.guest_GPR12 = freeres_wrapper;
+   VG_TRACK(post_reg_write, Vg_CoreClientReq, tid,
+            offsetof(VexGuestPPC64State, guest_GPR12),
+            sizeof(VG_(threads)[tid].arch.vex.guest_GPR12));
 #  endif
    /* mips-linux note: we need to set t9 */
 #  if defined(VGP_mips32_linux) || defined(VGP_mips64_linux)
    VG_(threads)[tid].arch.vex.guest_r25 = freeres_wrapper;
+   VG_TRACK(post_reg_write, Vg_CoreClientReq, tid,
+            offsetof(VexGuestMIPS32State, guest_r25),
+            sizeof(VG_(threads)[tid].arch.vex.guest_r25));
 #  endif
 
    /* Pass a parameter to freeres_wrapper(). */

This is somewhat hard to test though because it really depends on whether or not those registers were fully defined right before. In most cases they are.