Commit Graph

5 Commits (master)

Author SHA1 Message Date
Calvin Wan 91c080dff5 git-compat-util: move alloc macros to git-compat-util.h
alloc_nr, ALLOC_GROW, and ALLOC_GROW_BY are commonly used macros for
dynamic array allocation. Moving these macros to git-compat-util.h with
the other alloc macros focuses alloc.[ch] to allocation for Git objects
and additionally allows us to remove inclusions to alloc.h from files
that solely used the above macros.

Signed-off-by: Calvin Wan <>
Signed-off-by: Junio C Hamano <>
2023-07-05 11:42:31 -07:00
Elijah Newren 8bff5ca030 treewide: ensure one of the appropriate headers is sourced first
We had several C files ignoring the rule to include one of the
appropriate headers first; fix that.

While at it, the rule in Documentation/CodingGuidelines about which
header to include has also fallen out of sync, so update the wording to
mention other allowed headers.

Unfortunately, C files in reftable/ don't actually follow the previous
or updated rule.  If you follow the #include chain in its C files,
reftable/system.h _tends_ to be first (i.e. record.c first includes
record.h, which first includes basics.h, which first includees
system.h), but not always (e.g. publicbasics.c includes another header
first that does not include system.h).  However, I'm going to punt on
making actual changes to the C files in reftable/ since I do not want to
risk bringing it out-of-sync with any version being used externally.

Signed-off-by: Elijah Newren <>
Signed-off-by: Junio C Hamano <>
2023-02-23 17:25:28 -08:00
René Scharfe 8bcda98da5 oidtree: avoid unaligned access to crit-bit tree
The flexible array member "k" of struct cb_node is used to store the key
of the crit-bit tree node.  It offers no alignment guarantees -- in fact
the current struct layout puts it one byte after a 4-byte aligned
address, i.e. guaranteed to be misaligned.

oidtree uses a struct object_id as cb_node key.  Since cf0983213c (hash:
add an algo member to struct object_id, 2021-04-26) it requires 4-byte
alignment.  The mismatch is reported by UndefinedBehaviorSanitizer at
runtime like this:

hash.h:277:2: runtime error: member access within misaligned address 0x00015000802d for type 'struct object_id', which requires 4 byte alignment
0x00015000802d: note: pointer points here
 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior hash.h:277:2 in

We can fix that by:

1. eliminating the alignment requirement of struct object_id,
2. providing the alignment in struct cb_node, or
3. avoiding the issue by only using memcpy to access "k".

Currently we only store one of two values in "algo" in struct object_id.
We could use a uint8_t for that instead and widen it only once we add
support for our twohundredth algorithm or so.  That would not only avoid
alignment issues, but also reduce the memory requirements for each
instance of struct object_id by ca. 9%.

Supporting keys with alignment requirements might be useful to spread
the use of crit-bit trees.  It can be achieved by using a wider type for
"k" (e.g. uintmax_t), using different types for the members "byte" and
"otherbits" (e.g. uint16_t or uint32_t for each), or by avoiding the use
of flexible arrays like khash.h does.

This patch implements the third option, though, because it has the least
potential for causing side-effects and we're close to the next release.
If one of the other options is implemented later as well to get their
additional benefits we can get rid of the extra copies introduced here.

Reported-by: Andrzej Hunt <>
Signed-off-by: René Scharfe <>
Signed-off-by: Junio C Hamano <>
2021-08-15 13:13:50 -07:00
Carlo Marcelo Arenas Belón 14825944d7 oidtree: avoid nested struct oidtree_node
92d8ed8ac1 (oidtree: a crit-bit tree for odb_loose_cache, 2021-07-07)
adds a struct oidtree_node that contains only an n field with a
struct cb_node.

unfortunately, while building in pedantic mode witch clang 12 (as well
as a similar error from gcc 11) it will show:

  oidtree.c:11:17: error: 'n' may not be nested in a struct due to flexible array member [-Werror,-Wflexible-array-extensions]
          struct cb_node n;

because of a constrain coded in ISO C 11¶3 that forbids using
structs that contain a flexible array as part of another struct.

use a strict cb_node directly instead.

Signed-off-by: Carlo Marcelo Arenas Belón <>
Signed-off-by: Junio C Hamano <>
2021-08-09 09:01:30 -07:00
Eric Wong 92d8ed8ac1 oidtree: a crit-bit tree for odb_loose_cache
This saves 8K per `struct object_directory', meaning it saves
around 800MB in my case involving 100K alternates (half or more
of those alternates are unlikely to hold loose objects).

This is implemented in two parts: a generic, allocation-free
`cbtree' and the `oidtree' wrapper on top of it.  The latter
provides allocation using alloc_state as a memory pool to
improve locality and reduce free(3) overhead.

Unlike oid-array, the crit-bit tree does not require sorting.
Performance is bound by the key length, for oidtree that is
fixed at sizeof(struct object_id).  There's no need to have
256 oidtrees to mitigate the O(n log n) overhead like we did
with oid-array.

Being a prefix trie, it is natively suited for expanding short
object IDs via prefix-limited iteration in

On my busy workstation, p4205 performance seems to be roughly
unchanged (+/-8%).  Startup with 100K total alternates with no
loose objects seems around 10-20% faster on a hot cache.
(800MB in memory savings means more memory for the kernel FS

The generic cbtree implementation does impose some extra
overhead for oidtree in that it uses memcmp(3) on
"struct object_id" so it wastes cycles comparing 12 extra bytes
on SHA-1 repositories.  I've not yet explored reducing this
overhead, but I expect there are many places in our code base
where we'd want to investigate this.

More information on crit-bit trees:

Signed-off-by: Eric Wong <>
Signed-off-by: Junio C Hamano <>
2021-07-07 21:28:04 -07:00