I'd already coded up an XS version and used memchr for my search instead of your explicit loop. I don't know why but I found that using memchr got a function that was twice as fast. I included the searching part of the code below.

                    Rate  avar avar2 ikegami_tr avar2_pos corion morit
+z avar2_pos_inplace dio_c2 dio_c
avar               109/s    --   -6%       -18%      -40%   -51%   -70
+%              -84%   -91%  -95%
avar2              116/s    6%    --       -13%      -36%   -48%   -68
+%              -83%   -90%  -95%
ikegami_tr         133/s   22%   15%         --      -27%   -41%   -64
+%              -81%   -89%  -94%
avar2_pos          182/s   66%   56%        36%        --   -19%   -50
+%              -74%   -84%  -91%
corion             224/s  105%   93%        68%       23%     --   -39
+%              -67%   -81%  -89%
moritz             366/s  235%  215%       175%      101%    63%     -
+-              -47%   -69%  -83%
avar2_pos_inplace  686/s  527%  490%       415%      278%   206%    87
+%                --   -41%  -68%
dio_c2            1172/s  971%  908%       779%      545%   422%   220
+%               71%     --  -45%
dio_c             2118/s 1836% 1722%      1488%     1066%   844%   479
+%              209%    81%    --

dio_c2
        // Do it
        while ( dpv < dpv_end ) {
            if ( ! *dpv )
                *dpv = *spv;
            ++spv;
            ++dpv;
        }

dio_c
        // Do it
        while ( 1 ) {
            ptr = (char*)memchr( ptr, '\0', ptr_end - ptr );

            if ( ! ( ptr && ptr < ptr_end ) )
                break;

            *ptr = *(ptr - dpv + spv);

            ++ ptr;
        }
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