l3alloc: another implementation.

l3alloc.tex

\cs_new_protected:Npn \__alloc_new:nnnN #1#2#3#4
  {
    \__alloc_setup_type:nnn {#1} {#2} {#3}
    \cs_new_protected:cpn { #1 _new:N } ##1 { \__alloc_reg:nNN {#1} #4 ##1 }
  }
\cs_new_protected:Npn \__alloc_setup_type:nnn #1#2#3
  {
    \tl_new:c    { g__ #1 _allocation_tl }
    \tl_gset:cx  { g__ #1 _allocation_tl }     { \int_eval:n {#2} }
    \tl_const:cx { c__ #1 _allocation_max_tl } { \int_eval:n {#3} }
  }
\cs_new:Npn \int_eval:n #1 { \tex_number:D \etex_numexpr:D #1 \scan_stop: }
\luatex_if_engine:TF
  { \__alloc_new:nnnN { box } \c_zero \c_max_register_int \tex_chardef:D }
  { \__alloc_new:nnnN { box } \c_zero \c_max_register_int \tex_mathchardef:D }
\__alloc_new:nnnN { dim }    \c_zero \c_max_register_int \tex_dimendef:D
\__alloc_new:nnnN { int }    { 11 }  \c_max_register_int \tex_countdef:D
\__alloc_new:nnnN { muskip } \c_zero \c_max_register_int \tex_muskipdef:D
\__alloc_new:nnnN { skip }   \c_zero \c_max_register_int \tex_skipdef:D
\cs_new_protected:Npn \__alloc_reserve_setup:nnn #1#2#3
  {
    \tl_const:cx { c__ #1 _allocation_reserve_begin_tl } { \int_eval:n {#2} }
    \tl_const:cx { c__ #1 _allocation_reserve_stop_tl }  { \int_eval:n {#3} }
  }
\__alloc_reserve_setup:nnn { box }  { 221 } { 255 }
\__alloc_reserve_setup:nnn { dim }  { 221 } { 254 }
\__alloc_reserve_setup:nnn { int }  { 221 } { 254 }
\__alloc_reserve_setup:nnn { skip } { 221 } { 254 }
\cs_new_protected:Npn \__alloc_reg:nNN #1#2#3
  {
    \int_compare:nNnTF
      { \tl_use:c { g__ #1 _allocation_tl } } >
      { \tl_use:c { c__ #1 _allocation_max_tl } }
      { \__msg_kernel_fatal:nnx { kernel } { out-of-registers } {#1} }
      {
        \__chk_if_free_cs:N #3
        \tex_global:D #2 #3 \tl_use:c { g__ #1 _allocation_tl } \scan_stop:
        \iow_log:x
          {
            \token_to_str:N #3 ~=~ #1 ~register~
            \tl_use:c { g__ #1 _allocation_tl }
          }
        \__alloc_next:n {#1}
      }
  }
\cs_new_protected:Npn \__alloc_next:n #1
  {
    \tl_gset:cx { g__ #1 _allocation_tl }
      {
        \int_eval:n
          {
            \tl_if_exist:cTF { c__ #1 _allocation_reserve_begin_tl }
              {
                \int_compare:nNnTF
                  { \tl_use:c { g__ #1 _allocation_tl } + 1 } =
                  { \tl_use:c { c__ #1 _allocation_reserve_begin_tl } }
                  { \tl_use:c { c__ #1 _allocation_reserve_stop_tl } }
                  { \tl_use:c { g__ #1 _allocation_tl } }
              }
              { \tl_use:c { g__ #1 _allocation_tl } }
            + 1
          }
      }
  }