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Dropped attr-size down to at most 28-bits (4 leb128 bytes)

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This really doesn't need to be this large. The extra 4-bits is not worth
the required extra byte of waste in every commit (CRCs tags are forced
to use a fully expanded leb128 due to a catch-22 issue).
This commit is contained in:
Christopher Haster
2022-12-27 13:58:16 -06:00
parent 084094116f
commit a4fd070fc1
1 changed files with 25 additions and 28 deletions
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lfs.c
+25 -28
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@@ -696,14 +696,12 @@ static void lfs_fcrc_tole32(struct lfs_fcrc *fcrc) {
struct lfs_rfcrc {
uint32_t crc;
lfs_size_t size;
// extra space for leb128 encoding
uint8_t spill[1];
};
static lfs_ssize_t lfs_rfcrc_todisk(struct lfs_rfcrc *fcrc) {
lfs_tole32_(fcrc->crc, &fcrc->crc);
lfs_ssize_t delta = lfs_toleb128(fcrc->size, (uint8_t*)&fcrc->size, 5);
lfs_ssize_t delta = lfs_toleb128(fcrc->size, (uint8_t*)&fcrc->size, 4);
if (delta < 0) {
return delta;
}
@@ -714,7 +712,7 @@ static lfs_ssize_t lfs_rfcrc_todisk(struct lfs_rfcrc *fcrc) {
static lfs_ssize_t lfs_rfcrc_fromdisk(struct lfs_rfcrc *fcrc) {
fcrc->crc = lfs_fromle32_(&fcrc->crc);
lfs_ssize_t delta = lfs_fromleb128(&fcrc->size, (uint8_t*)&fcrc->size, 5);
lfs_ssize_t delta = lfs_fromleb128(&fcrc->size, (uint8_t*)&fcrc->size, 4);
if (delta < 0) {
return delta;
}
@@ -922,7 +920,7 @@ static lfs_ssize_t lfs_rbyd_readtag(lfs_t *lfs,
*tag = 0;
// read a pair of leb128s
uint8_t buffer[2*5];
uint8_t buffer[2*4];
lfs_size_t i = 0;
// TODO allow different hint for lookup? bench this? does our hint work backwards?
@@ -935,14 +933,14 @@ static lfs_ssize_t lfs_rbyd_readtag(lfs_t *lfs,
}
lfs_rtag_t tag_;
ssize_t delta = lfs_fromleb128(&tag_, &buffer[i], 5);
ssize_t delta = lfs_fromleb128(&tag_, &buffer[i], 4);
if (delta < 0) {
return delta;
}
i += delta;
lfs_size_t size_;
delta = lfs_fromleb128(&size_, &buffer[i], 5);
delta = lfs_fromleb128(&size_, &buffer[i], 4);
if (delta < 0) {
return delta;
}
@@ -1154,7 +1152,7 @@ static lfs_srtag_t lfs_rbyd_lookup(lfs_t *lfs, const lfs_rbyd_t *rbyd,
lfs_rtag_t alt;
lfs_off_t jump;
lfs_ssize_t delta = lfs_rbyd_readtag(lfs,
&lfs->pcache, &lfs->rcache, 2*5,
&lfs->pcache, &lfs->rcache, 2*4,
rbyd->block, branch, &alt, &jump, NULL);
if (delta < 0) {
return delta;
@@ -1224,16 +1222,16 @@ static lfs_ssize_t lfs_rbyd_progtag(lfs_t *lfs,
tag |= lfs_popc(crc_) & 1;
// compress into pair of leb128s
uint8_t buffer[2*5];
uint8_t buffer[2*4];
lfs_size_t i = 0;
ssize_t delta = lfs_toleb128(tag, &buffer[i], 5);
ssize_t delta = lfs_toleb128(tag, &buffer[i], 4);
if (delta < 0) {
return delta;
}
i += delta;
delta = lfs_toleb128(size, &buffer[i], 5);
delta = lfs_toleb128(size, &buffer[i], 4);
if (delta < 0) {
return delta;
}
@@ -1412,7 +1410,7 @@ static int lfs_rbyd_commit(lfs_t *lfs, lfs_rbyd_t *rbyd,
lfs_rtag_t alt;
lfs_off_t jump;
lfs_ssize_t delta = lfs_rbyd_readtag(lfs,
&lfs->pcache, &lfs->rcache, 2*5,
&lfs->pcache, &lfs->rcache, 2*4,
block, branch, &alt, &jump, NULL);
if (delta < 0) {
return delta;
@@ -1607,19 +1605,19 @@ static int lfs_rbyd_commit(lfs_t *lfs, lfs_rbyd_t *rbyd,
// 1-byte fcrc tag
// + 1-byte fcrc len (worst case)
// + 4-byte fcrc crc
// + 5-byte fcrc crc-len (worst case)
// + 4-byte fcrc crc-len (worst case)
// + 1-byte crc tag
// + 5-byte crc len (worst case)
// + 4-byte crc len (worst case)
// + 4-byte crc crc
// = 21 bytes
// = 19 bytes
// - 3-word crc with no following prog (end of block)
// 1-byte crc tag
// + 5-byte crc len (worst case)
// + 4-byte crc len (worst case)
// + 4-byte crc crc
// = 10 bytes
// = 9 bytes
//
const lfs_off_t aligned = lfs_alignup(
lfs_min(off + 1+1+4+5 + 1+5+4, lfs->cfg->block_size),
lfs_min(off + 1+1+4+4 + 1+4+4, lfs->cfg->block_size),
lfs->cfg->prog_size);
// space for fcrc?
@@ -1671,34 +1669,33 @@ static int lfs_rbyd_commit(lfs_t *lfs, lfs_rbyd_t *rbyd,
// note padding-size depends on leb-encoding depends on padding-size, to
// get around this catch-22 we just always write a fully-expanded leb128
// encoding
uint8_t buffer[1+5+4];
uint8_t buffer[1+4+4];
buffer[0] = (LFS_MKRTAG(CRC0, 0, 0) << 1) | (lfs_popc(crc) & 1);
lfs_off_t padding = aligned - (off + 1+5);
lfs_off_t padding = aligned - (off + 1+4);
buffer[1] = 0x80 | (0x7f & (padding >> 0));
buffer[2] = 0x80 | (0x7f & (padding >> 7));
buffer[3] = 0x80 | (0x7f & (padding >> 14));
buffer[4] = 0x80 | (0x7f & (padding >> 21));
buffer[5] = 0x00 | (0x7f & (padding >> 28));
buffer[4] = 0x00 | (0x7f & (padding >> 21));
crc = lfs_crc32c(crc, buffer, 1+5);
crc = lfs_crc32c(crc, buffer, 1+4);
// we can't let the next tag appear as valid, so intentionally perturb the
// commit if this happens, note parity(crc(m)) == parity(m) with crc32c,
// so we can really change any bit to make this happen, we've reserved a bit
// in crc tags just for this purpose
if ((lfs_popc(crc) & 1) == (perturb & 1)) {
buffer[0] ^= 0x2;
crc ^= 0xdb8ca0c3; // note crc(a ^ b) == crc(a) ^ crc(b)
crc ^= 0x7022df58; // note crc(a ^ b) == crc(a) ^ crc(b)
}
lfs_tole32_(crc, &buffer[1+5]);
lfs_tole32_(crc, &buffer[1+4]);
int err = lfs_bd_prog(lfs,
&lfs->pcache, &lfs->rcache, false,
block, off, buffer, 1+5+4);
block, off, buffer, 1+4+4);
if (err) {
return err;
}
off += 1+5+4;
off += 1+4+4;
// flush our caches, finalizing the commit on-disk
err = lfs_bd_sync(lfs, &lfs->pcache, &lfs->rcache, false);
@@ -1718,7 +1715,7 @@ static int lfs_rbyd_commit(lfs_t *lfs, lfs_rbyd_t *rbyd,
if (crc_ != crc) {
// oh no, something went wrong
printf("oh no %08x != %08x\n", crc_, crc);
//return LFS_ERR_CORRUPT;
return LFS_ERR_CORRUPT;
}
// ok, everything is good, save what we've committed