clang debug info:

(kgdb) where
#0  doadump (textdump=textdump@entry=1)
    at /mnt/jhb/work/git/kern_tls/sys/kern/kern_shutdown.c:399
#1  0xffffffff80aec390 in kern_reboot (howto=260)
    at /mnt/jhb/work/git/kern_tls/sys/kern/kern_shutdown.c:486
#2  0xffffffff80aec98f in vpanic (fmt=<optimized out>, ap=<optimized out>)
    at /mnt/jhb/work/git/kern_tls/sys/kern/kern_shutdown.c:919
#3  0xffffffff80aec543 in panic (fmt=<unavailable>)
    at /mnt/jhb/work/git/kern_tls/sys/kern/kern_shutdown.c:843
#4  0xffffffff80b77a23 in ktls_decrypt (so=<optimized out>)
    at /mnt/jhb/work/git/kern_tls/sys/kern/uipc_ktls.c:1780
#5  0xffffffff80b76c9c in ktls_work_thread (ctx=ctx@entry=0xfffff80003cebc00)
    at /mnt/jhb/work/git/kern_tls/sys/kern/uipc_ktls.c:2188
#6  0xffffffff80aaa99f in fork_exit (
    callout=0xffffffff80b76b20 <ktls_work_thread>, arg=0xfffff80003cebc00, 
    frame=0xfffffe00c931ec00)
    at /mnt/jhb/work/git/kern_tls/sys/kern/kern_fork.c:1083
#7  <signal handler called>
(kgdb) frame 4
#4  0xffffffff80b77a23 in ktls_decrypt (so=<optimized out>)
    at /mnt/jhb/work/git/kern_tls/sys/kern/uipc_ktls.c:1780
1780                            panic("oof");
(kgdb) info locals 
tls_header = "\026\000\000\000\006\000\000\000\002\000\000\000\000\370\377\377\000\000\000\000\000\000"
tgr = {tls_type = 93 ']', tls_vmajor = 190 '\276', tls_vminor = 172 '\254', 
  tls_length = 65535}
sb = <optimized out>
hdr = 0xfffffe00c931eaf0
tls = <optimized out>
data = <optimized out>
trail_len = <optimized out>
tls_len = <optimized out>
error = <optimized out>
seqno = <optimized out>
control = <optimized out>
remain = <optimized out>
m = <optimized out>
saved_vnet = <optimized out>
(kgdb) info args
so = <optimized out>

gcc6:

(kgdb) where
#0  doadump (textdump=textdump@entry=1)
    at /mnt/jhb/work/git/kern_tls/sys/kern/kern_shutdown.c:400
#1  0xffffffff80bdb822 in kern_reboot (howto=howto@entry=260)
    at /mnt/jhb/work/git/kern_tls/sys/kern/kern_shutdown.c:486
#2  0xffffffff80bdb14f in vpanic (fmt=<optimized out>, 
    ap=ap@entry=0xfffffe00c92eba78)
    at /mnt/jhb/work/git/kern_tls/sys/kern/kern_shutdown.c:919
#3  0xffffffff80bdb2bc in panic (fmt=<optimized out>)
    at /mnt/jhb/work/git/kern_tls/sys/kern/kern_shutdown.c:843
#4  0xffffffff80c79b44 in ktls_decrypt (so=0xfffff8044a164760)
    at /mnt/jhb/work/git/kern_tls/sys/kern/uipc_ktls.c:1780
#5  ktls_work_thread (ctx=ctx@entry=0xfffff80003b73c00)
    at /mnt/jhb/work/git/kern_tls/sys/kern/uipc_ktls.c:2188
#6  0xffffffff80b94a24 in fork_exit (
    callout=0xffffffff80c79800 <ktls_work_thread>, arg=0xfffff80003b73c00, 
    frame=0xfffffe00c92ebc00)
    at /mnt/jhb/work/git/kern_tls/sys/kern/kern_fork.c:1083
#7  <signal handler called>
kgdb) frame 4
#4  0xffffffff80c79b44 in ktls_decrypt (so=0xfffff8044a164760)
    at /mnt/jhb/work/git/kern_tls/sys/kern/uipc_ktls.c:1780
1780                            panic("oof");
(kgdb) info locals
saved_vnet = <optimized out>
control = <optimized out>
m = <optimized out>
remain = <optimized out>
tls_len = <optimized out>
tls_header = "\026\000\000\000\006\000\000\000\002\000\000\000\000\377\377@\001\220\"\000\376\377\377"
hdr = 0xfffffe00c92ebb61
tls = <optimized out>
tgr = {tls_type = 23 '\027', tls_vmajor = 3 '\003', tls_vminor = 3 '\003', 
  tls_length = 64}
data = <optimized out>
sb = 0xfffff8044a1648a8
seqno = <optimized out>
error = 22
trail_len = 16
saved_vnet = <optimized out>
tls_header = {<optimized out> <repeats 23 times>}
tls = <optimized out>
sb = <optimized out>
hdr = <optimized out>
tgr = {tls_type = <optimized out>, tls_vmajor = <optimized out>, 
  tls_vminor = <optimized out>, tls_length = <optimized out>}
control = <optimized out>
data = <optimized out>
m = <optimized out>
seqno = <optimized out>
error = <optimized out>
remain = <optimized out>
tls_len = <optimized out>
trail_len = <optimized out>
deref = <optimized out>
saved_vnet = <optimized out>
saved_vnet = <optimized out>
saved_vnet = <optimized out>
(kgdb) info args
so = 0xfffff8044a164760