Github/littlefs
1
Fork 0
Code Actions Activity

t: Adopted bit swapping to save dirty/mutated state

Browse Source 
The trick is realizing the dirty/mutated bits are redundant when
mutating. So instead of saving the current dirty bit on the stack, we
can just swap the mutated/dirty bits temporarily.

The bit swapping xor trick comes from Sean Eron Anderson's infamous bit
twiddling hacks collection, unfortunately it doesn't seem possible to
avoid the hardcoded bit locations...

Saves a bit of code:

           code          stack
  before: 35504           2680
  after:  35488 (-0.0%)   2680 (+0.0%)
This commit is contained in:
Christopher Haster
2024-07-09 14:24:06 -05:00
parent 86db4330d8
commit ee990938e1
1 changed files with 30 additions and 29 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lfs.c
+30 -29
View File
@@ -8269,6 +8269,11 @@ static inline bool lfsr_f_ismutated(uint32_t flags) {
return flags & LFS_F_MUTATED;
}
static inline uint32_t lfsr_f_swapdirty(uint32_t flags) {
uint32_t x = ((flags >> 14) ^ (flags >> 12)) & 0x1;
return flags ^ (x << 14) ^ (x << 12);
}
// needed in lfsr_mtree_traverse_
@@ -8648,26 +8653,26 @@ static int lfsr_mdir_fixorphans(lfs_t *lfs, lfsr_mdir_t *mdir);
// mutation here, upper layers should call lfs_alloc_ckpoint as needed
static int lfsr_mtree_gc(lfs_t *lfs, lfsr_traversal_t *t,
lfsr_tag_t *tag_, lfsr_bptr_t *bptr_) {
// swap dirty/mutated flags while in lfsr_mtree_gc
t->o.o.flags = lfsr_f_swapdirty(t->o.o.flags);
dropped:;
lfsr_tag_t tag;
lfsr_bptr_t bptr;
int err = lfsr_mtree_traverse(lfs, t,
&tag, &bptr);
if (err) {
return err;
goto failed;
}
// keep track of flags before mutation
uint16_t flags = t->o.o.flags;
// mkconsistencing mdirs?
if (lfsr_t_ismkconsistent(t->o.o.flags)
&& tag == LFSR_TAG_MDIR
&& lfs->hasorphans) {
lfsr_mdir_t *mdir = (lfsr_mdir_t*)bptr.data.u.buffer;
int err = lfsr_mdir_fixorphans(lfs, mdir);
err = lfsr_mdir_fixorphans(lfs, mdir);
if (err) {
return err;
goto failed;
}
// make sure we clear any zombie flags
@@ -8676,11 +8681,6 @@ dropped:;
// did this drop our mdir?
if (mdir->mid != -1 && mdir->rbyd.weight == 0) {
t->o.o.state = LFSR_TSTATE_MDIRS;
// downgrade any new dirty flags
t->o.o.flags |= (t->o.o.flags & LFS_F_DIRTY) ? LFS_F_MUTATED : 0;
t->o.o.flags = (t->o.o.flags & ~LFS_F_DIRTY)
| (flags & LFS_F_DIRTY);
goto dropped;
}
}
@@ -8705,9 +8705,9 @@ dropped:;
? lfs->cfg->gc_compact_thresh
: lfs->cfg->block_size - lfs->cfg->block_size/8);
int err = lfsr_mdir_compact(lfs, mdir);
err = lfsr_mdir_compact(lfs, mdir);
if (err) {
return err;
goto failed;
}
}
@@ -8734,18 +8734,18 @@ dropped:;
LFS_ASSERT(lfsr_omdir_isopen(lfs, &t->o.o));
if (t->o.o.state == LFSR_TSTATE_MTREE) {
int err = lfsr_btree_compact_(lfs, &t->o.bshrub.u.btree,
err = lfsr_btree_compact_(lfs, &t->o.bshrub.u.btree,
// note we may be referencing the btree root here
t->u.bt.bid, rbyd);
if (err) {
return err;
goto failed;
}
} else {
int err = lfsr_bshrub_compact_(lfs, &t->o.o.mdir, &t->o.bshrub,
err = lfsr_bshrub_compact_(lfs, &t->o.o.mdir, &t->o.bshrub,
// note we may be referencing the btree root here
t->u.bt.bid, rbyd);
if (err) {
return err;
goto failed;
}
}
@@ -8758,7 +8758,7 @@ dropped:;
} else {
// commit to mdir
uint8_t buf[LFSR_BTREE_DSIZE];
int err = lfsr_mdir_commit(lfs, &t->o.o.mdir, LFSR_ATTRS(
err = lfsr_mdir_commit(lfs, &t->o.o.mdir, LFSR_ATTRS(
(t->o.o.state == LFSR_TSTATE_MTREE)
? LFSR_ATTR(
LFSR_TAG_SUB | LFSR_TAG_MTREE, 0,
@@ -8771,7 +8771,7 @@ dropped:;
LFSR_TAG_SUB | LFSR_TAG_BTREE, 0,
LFSR_DATA_BTREE_(&t->o.bshrub.u.btree, buf))));
if (err) {
return err;
goto failed;
}
// update any open files
@@ -8790,10 +8790,8 @@ dropped:;
t->u.bt.rid = t->u.bt.bid;
}
// downgrade any new dirty flags
t->o.o.flags |= (t->o.o.flags & LFS_F_DIRTY) ? LFS_F_MUTATED : 0;
t->o.o.flags = (t->o.o.flags & ~LFS_F_DIRTY)
| (flags & LFS_F_DIRTY);
// swap back dirty/mutated flags
t->o.o.flags = lfsr_f_swapdirty(t->o.o.flags);
if (tag_) {
*tag_ = tag;
@@ -8802,6 +8800,11 @@ dropped:;
*bptr_ = bptr;
}
return 0;
failed:;
// swap back dirty/mutated flags
t->o.o.flags = lfsr_f_swapdirty(t->o.o.flags);
return err;
}
@@ -12921,18 +12924,16 @@ int lfsr_traversal_read(lfs_t *lfs, lfsr_traversal_t *t,
lfsr_mid_rid(lfs, lfs->grm.mids[0]));
}
// keep track of flags before mutation
uint16_t flags = t->o.o.flags;
// swap dirty/mutated flags while mutating
t->o.o.flags = lfsr_f_swapdirty(t->o.o.flags);
int err = lfsr_fs_fixgrm(lfs);
if (err) {
t->o.o.flags = lfsr_f_swapdirty(t->o.o.flags);
return err;
}
// downgrade any new dirty flags
t->o.o.flags |= (t->o.o.flags & LFS_F_DIRTY) ? LFS_F_MUTATED : 0;
t->o.o.flags = (t->o.o.flags & ~LFS_F_DIRTY)
| (flags & LFS_F_DIRTY);
t->o.o.flags = lfsr_f_swapdirty(t->o.o.flags);
}
while (true) {