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rdonly: Dropped file.b.shrub_ when LFS3_RDONLY

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We don't need the staging shrub if we never stage shrubs!

The only hangup was reuse of the staging shrub to load bshrubs/btrees in
lfs3_file_fetch (we need to be able to fallback to the previous shrub if
we error in lfs3_file_resync), but this can be handled with a stack
allocated shrub.

If btree-leaf-caches make a return, we would need to stack allocate this
anyways due to the lopsided cost of the main/staging btrees/bshrubs
introduced to avoid wasting space on the useless
staging-shrub-leaf-cache.

This saves some code in LFS3_RDONLY, and apparently an instruction or
two in the default build (I guess stack loads/stores are cheaper?):

                   code          stack          ctx
  rdonly before:  10680            840          524
  rdonly after:   10640 (-0.4%)    816 (+0.0%)  524 (+0.0%)

  default before: 37324           2280          636
  default after:  37320 (-0.0%)   2280 (+0.0%)  636 (+0.0%)

It's not apparent in ctx because lfs3_info.name dominates (guh), but
this does save some RAM in lfs3_file_t:

  rdonly              ctx
  lfs3_file_t before: 136
  lfs3_file_t after:  112 (-17.6%)

It does add some stack cost to lfs3_file_fetch, but because this isn't
on the stack hot-path in either build, we don't really care:

  default                 code          stack          ctx
  lfs3_file_fetch before:  372            416            0
  lfs3_file_fetch after:   368 (-1.1%)    440 (+5.8%)    0 (+0.0%)
This commit is contained in:
Christopher Haster
2025-06-05 18:02:57 -05:00
parent 9eaab640e6
commit 7cc87a4fe6
2 changed files with 7 additions and 6 deletions
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lfs3.c
+5 -5
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@@ -11341,15 +11341,15 @@ static int lfs3_file_fetch(lfs3_t *lfs3, lfs3_file_t *file, bool trunc) {
}
// note many of these functions leave bshrub undefined if there
// is an error, so we first read into the staging bshrub
file->b.shrub_ = file->b.shrub;
// is an error, so we first read into a temporary bshrub/btree
lfs3_rbyd_t shrub = file->b.shrub;
// found a bshrub/btree?
if (err != LFS3_ERR_NOENT) {
// may be a bshrub (inlined btree)
if (tag == LFS3_TAG_BSHRUB) {
err = lfs3_data_readshrub(lfs3, &data, &file->b.o.mdir,
&file->b.shrub_);
&shrub);
if (err) {
return err;
}
@@ -11357,7 +11357,7 @@ static int lfs3_file_fetch(lfs3_t *lfs3, lfs3_file_t *file, bool trunc) {
// or a btree
} else if (tag == LFS3_TAG_BTREE) {
err = lfs3_data_fetchbtree(lfs3, &data,
&file->b.shrub_);
&shrub);
if (err) {
return err;
}
@@ -11368,7 +11368,7 @@ static int lfs3_file_fetch(lfs3_t *lfs3, lfs3_file_t *file, bool trunc) {
}
// update the bshrub/btree
file->b.shrub = file->b.shrub_;
file->b.shrub = shrub;
// mark as in-sync
file->b.o.flags &= ~LFS3_o_UNSYNC;