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Dropped alt-always as an rbyd trunk terminator

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Now that tree rebalancing is implemented and needed a null terminator
anyways, I think it's clear that the benefit of the alt-always pointers
as trunk terminator has pretty limited value.

Now a null or other tag is needed for every trunk, which simplifies
checks for end-of-trunk.

Alt-always tags are still emitted for deletes, etc, but there their
behavior is implicit, so no special checks are needed. Alt-always tags
are naturally cleaned up as a part of rbyd pruning.
This commit is contained in:
Christopher Haster
2023-06-26 00:48:36 -05:00
parent f311d1102c
commit cf588ac3fa
4 changed files with 34 additions and 62 deletions
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lfs.c
+27 -32
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@@ -625,7 +625,6 @@ enum lfsr_tag_type {
LFSR_TAG_ALTGT = 0x6000,
LFSR_TAG_ALTBGT = 0x6000,
LFSR_TAG_ALTRGT = 0x7000,
LFSR_TAG_ALTA = 0x6000,
LFSR_TAG_CRC = 0x2000,
LFSR_TAG_FCRC = 0x2100,
@@ -725,14 +724,10 @@ static inline bool lfsr_tag_isalt(lfsr_tag_t tag) {
return tag & 0x4000;
}
static inline bool lfsr_tag_istrunkstart(lfsr_tag_t tag) {
static inline bool lfsr_tag_istrunk(lfsr_tag_t tag) {
return (tag & 0x6000) != 0x2000;
}
static inline bool lfsr_tag_istrunkend(lfsr_tag_t tag) {
return !lfsr_tag_isalt(tag) || tag == LFSR_TAG_ALTA;
}
static inline lfsr_tag_t lfsr_tag_next(lfsr_tag_t tag) {
return tag + 0x1;
}
@@ -1718,7 +1713,7 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
}
// found a trunk of a tree?
if (lfsr_tag_istrunkstart(tag)
if (lfsr_tag_istrunk(tag)
&& (!trunk || trunk >= off-d || wastrunk)) {
// start of trunk?
if (!wastrunk) {
@@ -1738,7 +1733,7 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
weight_ += w;
// end of trunk?
if (lfsr_tag_istrunkend(tag)) {
if (!lfsr_tag_isalt(tag)) {
wastrunk = false;
// update current weight
weight = weight_;
@@ -2448,7 +2443,7 @@ static int lfsr_rbyd_append(lfs_t *lfs, lfsr_rbyd_t *rbyd,
< lfsr_tag_key(tag)))))) {
if (lfsr_tag_isrm(tag)) {
// if removed, make our tag unreachable
alt = LFSR_TAG_ALTA;
alt = LFSR_TAG_ALTGT(false, 0);
weight = upper_id - lower_id - 1 + delta;
upper_id -= weight;
} else {
@@ -2471,7 +2466,7 @@ static int lfsr_rbyd_append(lfs_t *lfs, lfsr_rbyd_t *rbyd,
> lfsr_tag_key(tag)))))) {
if (lfsr_tag_isrm(tag)) {
// if removed, make our tag unreachable
alt = LFSR_TAG_ALTA;
alt = LFSR_TAG_ALTGT(false, 0);
weight = upper_id - lower_id - 1 + delta;
upper_id -= weight;
} else {
@@ -2505,30 +2500,30 @@ static int lfsr_rbyd_append(lfs_t *lfs, lfsr_rbyd_t *rbyd,
goto failed;
}
// we don't need to write the tag if we ended with an unreachable trunk
if (alt != LFSR_TAG_ALTA) {
leaf:;
// write the actual tag
lfs_ssize_t d = lfsr_bd_progtag(lfs, rbyd->block, rbyd->off,
// rm => null, otherwise strip off control bits
(lfsr_tag_isrm(tag) ? LFSR_TAG_NULL : lfsr_tag_key(tag)),
upper_id - lower_id - 1 + delta,
lfsr_data_size(data),
&rbyd->crc);
if (d < 0) {
err = d;
goto failed;
}
rbyd->off += d;
// don't forget the data!
err = lfsr_bd_progdata(lfs, rbyd->block, rbyd->off, data,
&rbyd->crc);
if (err) {
goto failed;
}
rbyd->off += lfsr_data_size(data);
// write the actual tag
//
// note we always need a non-alt to terminate the trunk, otherwise we
// can't find trunks during fetch
lfs_ssize_t d = lfsr_bd_progtag(lfs, rbyd->block, rbyd->off,
// rm => null, otherwise strip off control bits
(lfsr_tag_isrm(tag) ? LFSR_TAG_NULL : lfsr_tag_key(tag)),
upper_id - lower_id - 1 + delta,
lfsr_data_size(data),
&rbyd->crc);
if (d < 0) {
err = d;
goto failed;
}
rbyd->off += d;
// don't forget the data!
err = lfsr_bd_progdata(lfs, rbyd->block, rbyd->off, data,
&rbyd->crc);
if (err) {
goto failed;
}
rbyd->off += lfsr_data_size(data);
return 0;