How to add a new pass?

[Georg-Johann Lay]

In the internals, there is no description of a backend
can add a new pass. And no backend uses
passes.c:register_pass (4.5 and 4.7), so here is the question:

How can a backend add a pass?

I tried this code in OVERRIDE_OPTIONS (simply because I
found no canonical place to put it).

    struct register_pass_info pass_info;

    pass_info.pass = &pass_phiopt.pass;
    pass_info.reference_pass_name = "mergephi";
    pass_info.ref_pass_instance_number = 1;
    pass_info.pos_op = PASS_POS_INSERT_BEFORE;
    register_pass (&pass_info);

The pass is added, but it's the last pass of all tree
passes, i.e. it's inserted after .optimized and
.statistics; at least the pass number indicate that.

When I use PASS_POS_REPLACE as .pos_op, the original
"mergephi1" is removed, but again the new pass is
added after all other passes.

What am I doing wrong? passes.c:register_pass
finds the right place in the list, but the insertion
is incorrect.

Thanks for hints, I tried with 4.5.2

Johann