Microvacuum for gist. Question about GISTPageOpaqueData flag

Hi,

I'm working on microvacuum for gist access method.
Briefly microvacuum includes two steps:
1. When search tells us that the tuple is invisible to all transactions it
is marked LP_DEAD and page is marked as "has dead tuples",
2. Then, when insert touches full page which has dead tuples it calls
microvacuum instead of splitting page.
You can find a kind of review here [1].

While writing patch, I found strange GISTPageOpaqueData flag -
F_TUPLES_DELETED
<http://doxygen.postgresql.org/gist_8h.html#a23812efd70313b9b10ae61376e2594f6>
.
Its description looks like it is the same for BTP_HAS_GARBAGE
<http://doxygen.postgresql.org/nbtree_8h.html#a3b7c77849276ff8617edc1f84441c230>

#define F_TUPLES_DELETED (1 << 2) /* some tuples on the page are dead */

#define BTP_HAS_GARBAGE (1 << 6) /* page has LP_DEAD tuples */

But it's definitely not the same things. I found only two mentions of this
flag.
Function GistMarkTuplesDeleted
<http://doxygen.postgresql.org/gist_8h.html#a96fc3c6bb5aecfc8d2818b7010d68aac>
sets
the flag after dead tuples deletion.

Do anyone need it at all? I found no place where this flag is checked.
Is it correct using of the flag?

I need an advice, what would be better:
- to add new flag like F_HAS_GARBAGE,
- or to delete all mentions of F_TUPLES_DELETED and use it in gist
microvacuum.

[1]
http://www.google-melange.com/gsoc/proposal/public/google/gsoc2015/ivanitskiy_ilya/5629499534213120
--
Best regards,
Lubennikova Anastasia