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rbyd-rr: Tweaked lfsr_tag_follow to make altn/alta implicit again

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In theory, checking altn/alta tags for followability should be implicit.
These are encoding as altle/altgt tag 0, which should never be requested
in normal operation:

  altn => altle 0
  alta => altgt 0

But while that's good in theory, null tags, tag 0, has a tendency to
creep into these functions and has already caused a number of headaches.

Conditionally checking for altn/alta is safer, but asserting on tag 0 is
just as safe and adds no code cost.

Both lfsr_rbyd_appendattr and lfsr_rbyd_lookupnext have
`tag = lfs_max16(tag, 0x1)` guards now to comply with this rule. But
it's still a nice safety net to assert on tag 0 in lfsr_tag_follow*.

In case you were curious if the max16 guards were more expensive than
the explicit altn/alta checks, code size says no:

           code          stack
  before: 34256           2864
  after:  34220 (-0.1%)   2864 (+0.0%)
This commit is contained in:
Christopher Haster
2024-03-30 02:13:22 -05:00
parent 0475af0415
commit dcc67d22a8
1 changed files with 8 additions and 5 deletions
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lfs.c
+8 -5
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@@ -889,16 +889,17 @@ static inline bool lfsr_tag_follow(
lfsr_tag_t alt, lfsr_rid_t weight,
lfsr_srid_t lower_rid, lfsr_srid_t upper_rid,
lfsr_srid_t rid, lfsr_tag_t tag) {
// null tags break the following logic for altns/altas
LFS_ASSERT(lfsr_tag_key(tag) != 0);
if (lfsr_tag_isgt(alt)) {
return rid > upper_rid - (lfsr_srid_t)weight - 1
|| (rid == upper_rid - (lfsr_srid_t)weight - 1
&& (lfsr_tag_isa(alt)
|| lfsr_tag_key(tag) > lfsr_tag_key(alt)));
&& lfsr_tag_key(tag) > lfsr_tag_key(alt));
} else {
return rid < lower_rid + (lfsr_srid_t)weight - 1
|| (rid == lower_rid + (lfsr_srid_t)weight - 1
&& (!lfsr_tag_isn(alt)
&& lfsr_tag_key(tag) <= lfsr_tag_key(alt)));
&& lfsr_tag_key(tag) <= lfsr_tag_key(alt));
}
}
@@ -2826,7 +2827,7 @@ static int lfsr_rbyd_appendattr(lfs_t *lfs, lfsr_rbyd_t *rbyd,
} else if (lfsr_tag_issub(tag)) {
a_tag = lfsr_tag_supkey(tag);
b_tag = lfsr_tag_supkey(tag) + 0x100;
} else if (lfsr_tag_isrm(tag) || !lfsr_tag_key(tag)) {
} else if (lfsr_tag_isrm(tag)) {
a_tag = lfsr_tag_key(tag);
b_tag = lfsr_tag_key(tag) + 1;
} else {
@@ -2834,6 +2835,8 @@ static int lfsr_rbyd_appendattr(lfs_t *lfs, lfsr_rbyd_t *rbyd,
b_tag = lfsr_tag_key(tag);
}
}
a_tag = lfs_max16(a_tag, 0x1);
b_tag = lfs_max16(b_tag, 0x1);
// keep track of diverged state
//