These defines and enum are all oid-related and as such seem to make
more sense being included in hash.h. Further, moving them there
allows us to remove some includes of cache.h in other files.
The change to line-log.h might look unrelated, but line-log.h includes
diffcore.h, which previously included cache.h, which included the
kitchen sink. Since this patch makes diffcore.h no longer include
cache.h, the compiler complains about the 'struct string_list *'
function parameter. Add a forward declaration for struct string_list to
address this.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
For sanity, we should probably do one of the following:
(a) make C and header files both depend upon everything they need
(b) consistently exclude git-compat-util.h from headers and require it
be the first include in C files
Currently, we have some of the headers following (a) and others
following (b), which makes things messy. In the past I was pushed
towards (b), as per [1] and [2]. Further, during this series I
discovered that this mixture empirically will mean that we end up with C
files that do not directly include git-compat-util.h, and do include
headers that don't include git-compat-util.h, with the result that we
likely have headers included before an indirect inclusion of
git-compat-util.h. Since git-compat-util.h has tricky platform-specific
stuff that is meant to be included before everything else, this state of
affairs is risky and may lead to things breaking in subtle ways (and
only on some platforms) as per [1] and [2].
Since including git-compat-util.h in existing header files makes it
harder for us to catch C files that are missing that include, let's
switch to (b) to make the enforcement of this rule easier. Remove the
inclusion of git-compat-util.h from header files other than the ones
that have been approved as alternate first includes.
[1] https://lore.kernel.org/git/20180811173406.GA9119@sigill.intra.peff.net/
[2] https://lore.kernel.org/git/20180811174301.GA9287@sigill.intra.peff.net/
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Remove the PPC_SHA1 implementation added in a6ef3518f9 ([PATCH] PPC
assembly implementation of SHA1, 2005-04-22). When this was added
Apple consumer hardware used the PPC architecture, and the
implementation was intended to improve SHA-1 speed there.
Since it was added we've moved to using sha1collisiondetection by
default, and anyone wanting hard-rolled non-DC SHA-1 implementation
can use OpenSSL's via the OPENSSL_SHA1 knob.
The PPC_SHA1 originally originally targeted 32 bit PPC, and later the
64 bit PPC 970 (a.k.a. Apple PowerPC G5). See 926172c5e4 (block-sha1:
improve code on large-register-set machines, 2009-08-10) for a
reference about the performance on G5 (a comment in block-sha1/sha1.c
being removed here).
I can't get it to do anything but segfault on both the BE and LE POWER
machines in the GCC compile farm[1]. Anyone who's concerned about
performance on PPC these days is likely to be using the IBM POWER
processors.
There have been proposals to entirely remove non-sha1collisiondetection
implementations from the tree[2]. I think per [3] that would be a bit
overzealous. I.e. there are various set-ups git's speed is going to be
more important than the relatively implausible SHA-1 collision attack,
or where such attacks are entirely mitigated by other means (e.g. by
incoming objects being checked with DC_SHA1).
But that really doesn't apply to PPC_SHA1 in particular, which seems
to have outlived its usefulness.
As this gets rid of the only in-tree *.S assembly file we can remove
the small bits of logic from the Makefile needed to build objects
from *.S (as opposed to *.c)
The code being removed here was also throwing warnings with the
"-pedantic" flag, it could have been fixed as 544d93bc3b (block-sha1:
remove use of obsolete x86 assembly, 2022-03-10) did for block-sha1/*,
but as noted above let's remove it instead.
1. https://cfarm.tetaneutral.net/machines/list/
Tested on gcc{110,112,135,203}, a mixture of POWER [789] ppc64 and
ppc64le. All segfault in anything needing object
hashing (e.g. t/t1007-hash-object.sh) when compiled with
PPC_SHA1=Y.
2. https://lore.kernel.org/git/20200223223758.120941-1-mh@glandium.org/
3. https://lore.kernel.org/git/20200224044732.GK1018190@coredump.intra.peff.net/
Acked-by: brian m. carlson" <sandals@crustytoothpaste.net>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
For SHA-256, we currently have support for OpenSSL and libgcrypt because
these two libraries contain optimized implementations that can take
advantage of native processor instructions. However, OpenSSL is not
suitable for linking against for Linux distros due to licensing
incompatibilities with the GPLv2, and libgcrypt has been less favored by
cryptographers due to some security-related implementation issues,
which, while not affecting our use of hash algorithms, has affected its
reputation.
Let's add another option that's compatible with the GPLv2, which is
Nettle. This is an option which is generally better than libgcrypt
because on many distros GnuTLS (which uses Nettle) is used for HTTPS and
therefore as a practical matter it will be available on most systems.
As a result, prefer it over libgcrypt and our built-in implementation.
Nettle also has recently gained support for Intel's SHA-NI instructions,
which compare very favorably to other implementations, as well as
assembly implementations for when SHA-NI is not available.
A git gc on git.git sees a 12% performance improvement with Nettle over
our block SHA-256 implementation due to general assembly improvements.
With SHA-NI, the performance of raw SHA-256 on a 2 GiB file goes from
7.296 seconds with block SHA-256 to 1.523 seconds with Nettle.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This will simplify referencing them from code that is not deeply integrated with
Git, in particular, the reftable library.
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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 <andrzej@ahunt.org>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
As with `oidcpy', the source struct will not be modified and
this will allow an upcoming const-correct caller to use it.
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
An object_id storing a SHA-1 name has some unused bytes at the end of
the hash array. Since these bytes are not used, they are usually not
initialized to any value either. However, at
parallel_checkout.c:send_one_item() the object_id of a cache entry is
copied into a buffer which is later sent to a checkout worker through a
pipe write(). This makes Valgrind complain about passing uninitialized
bytes to a syscall. The worker won't use these uninitialized bytes
either, but the warning could confuse someone trying to debug this code;
So instead of using oidcpy(), send_one_item() uses hashcpy() to only
copy the used/initialized bytes of the object_id, and leave the
remaining part with zeros.
However, since cf0983213c ("hash: add an algo member to struct
object_id", 2021-04-26), using hashcpy() is no longer sufficient here as
it won't copy the new algo field from the object_id. Let's add and use a
new function which meets both our requirements of copying all the
important object_id data while still avoiding the uninitialized bytes,
by padding the end of the hash array in the destination object_id. With
this change, we also no longer need the destination buffer from
send_one_item() to be initialized with zeros, so let's switch from
xcalloc() to xmalloc() to make this clear.
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Up until recently, object IDs did not have an algorithm member, only a
hash. Consequently, it was possible to share one null (all-zeros)
object ID among all hash algorithms. Now that we're going to be
handling objects from multiple hash algorithms, it's important to make
sure that all object IDs have a correct algorithm field.
Introduce a per-algorithm null OID, and add it to struct hash_algo.
Introduce a wrapper function as well, and use it everywhere we used to
use the null_oid constant.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Now that struct object_id has an algorithm field, we should populate it.
This will allow us to handle object IDs in any supported algorithm and
distinguish between them. Ensure that the field is written whenever we
write an object ID by storing it explicitly every time we write an
object. Set values for the empty blob and tree values as well.
In addition, use the algorithm field to compare object IDs. Note that
because we zero-initialize struct object_id in many places throughout
the codebase, we default to the default algorithm in cases where the
algorithm field is zero rather than explicitly initialize all of those
locations.
This leads to a branch on every comparison, but the alternative is to
compare the entire buffer each time and padding the buffer for SHA-1.
That alternative ranges up to 3.9% worse than this approach on the perf
t0001, t1450, and t1451.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
To avoid the penalty of having to branch in hash comparison functions,
we'll want to always compare the full hash member in a struct object_id,
which will require that SHA-1 object IDs be zero-padded. To do so, add
a function which finalizes a hash context and writes it into an object
ID that performs this padding.
Move the definition of struct object_id and the constant definitions
higher up so we they are available for us to use.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Now that we're working with multiple hash algorithms in the same repo,
it's best if we label each object ID with its algorithm so we can
determine how to format a given object ID. Add a member called algo to
struct object_id.
Performance testing on object ID-heavy workloads doesn't reveal a clear
change in performance. Out of performance tests t0001 and t1450, there
are slight variations in performance both up and down, but all
measurements are within the margin of error.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We define git_hash_algo and object_id in hash.h, but most of the utility
functions are declared in the main cache.h. Let's move them to hash.h
along with their struct definitions. This cleans up cache.h a bit, but
also avoids circular dependencies when other headers need to know about
these functions (e.g., if oid-array.h were to have an inline that used
oideq(), it couldn't include cache.h because it is itself included by
cache.h).
No including C files should be affected, because hash.h is always
included in cache.h already.
We do have to mention repository.h at the top of hash.h, though, since
we depend on the_repository in some of our inline functions.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
For all of our SHA-1 implementations and most of our SHA-256
implementations, the hash context we use is a real struct. For these
implementations, it's possible to copy a hash context by making a copy
of the struct.
However, for our libgcrypt implementation, our hash context is a
pointer. Consequently, copying it does not lead to an independent hash
context like we intended.
Fortunately, however, libgcrypt provides us with a handy function to
copy hash contexts. Let's add a cloning function to the hash algorithm
API, and use it in the one place we need to make a hash context copy.
With this change, our libgcrypt SHA-256 implementation is fully
functional with all of our other hash implementations.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Our hashmap.h helpfully defines a sha1hash() function. But it cannot
define a similar oidhash() without including all of cache.h, which
itself wants to include hashmap.h! Let's break this circular dependency
by moving the definition to hash.h, along with the remaining RAWSZ
macros, etc. That will put them with the existing git_hash_algo
definition.
One alternative would be to move oidhash() into cache.h, but it's
already quite bloated. We're better off moving things out than in.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There are some cases, such as the dumb HTTP transport and bundles, where
we can only determine the hash algorithm in use by the length of the
object IDs. Provide a function that looks up the algorithm by length.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We already have OpenSSL routines available for SHA-1, so add routines
for SHA-256 as well.
On a Core i7-6600U, this SHA-256 implementation compares favorably to
the SHA1DC SHA-1 implementation:
SHA-1: 157 MiB/s (64 byte chunks); 337 MiB/s (16 KiB chunks)
SHA-256: 165 MiB/s (64 byte chunks); 408 MiB/s (16 KiB chunks)
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Generally, one gets better performance out of cryptographic routines
written in assembly than C, and this is also true for SHA-256. In
addition, most Linux distributions cannot distribute Git linked against
OpenSSL for licensing reasons.
Most systems with GnuPG will also have libgcrypt, since it is a
dependency of GnuPG. libgcrypt is also faster than the SHA1DC
implementation for messages of a few KiB and larger.
For comparison, on a Core i7-6600U, this implementation processes 16 KiB
chunks at 355 MiB/s while SHA1DC processes equivalent chunks at 337
MiB/s.
In addition, libgcrypt is licensed under the LGPL 2.1, which is
compatible with the GPL. Add an implementation of SHA-256 that uses
libgcrypt.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
SHA-1 is weak and we need to transition to a new hash function. For
some time, we have referred to this new function as NewHash. Recently,
we decided to pick SHA-256 as NewHash. The reasons behind the choice of
SHA-256 are outlined in the thread starting at [1] and in the commit
history for the hash function transition document.
Add a basic implementation of SHA-256 based off libtomcrypt, which is in
the public domain. Optimize it and restructure it to meet our coding
standards. Pull in the update and final functions from the SHA-1 block
implementation, as we know these function correctly with all compilers.
This implementation is slower than SHA-1, but more performant
implementations will be introduced in future commits.
Wire up SHA-256 in the list of hash algorithms, and add a test that the
algorithm works correctly.
Note that with this patch, it is still not possible to switch to using
SHA-256 in Git. Additional patches are needed to prepare the code to
handle a larger hash algorithm and further test fixes are needed.
[1] https://public-inbox.org/git/20180609224913.GC38834@genre.crustytoothpaste.net/
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There is one place we need the hash algorithm block size: the HMAC code
for push certs. Expose this constant in struct git_hash_algo and expose
values for SHA-1 and for the largest value of any hash.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There are several ways we might refer to a hash algorithm: by name, such
as in the config file; by format ID, such as in a pack; or internally,
by a pointer to the hash_algos array. Provide functions to look up hash
algorithms based on these various forms and return the internal constant
used for them. If conversion to another form is necessary, this
internal constant can be used to look up the proper data in the
hash_algos array.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
We moved away from SHA1_HEADER to a preprocessor if chain, but didn't
update the comment discussing the platform defines. Update this comment
so it reflects the current state of our codebase.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In various parts of our code, we want to allocate a structure
representing the internal state of a hash algorithm. The original
implementation of the hash algorithm abstraction assumed we would do
that using heap allocations, and added a context size element to struct
git_hash_algo. However, most of the existing code uses stack
allocations and conversion would needlessly complicate various parts of
the code. Add a union for the purpose of allocating hash contexts on
the stack and a typedef for ease of use. Use this union for defining
the init, update, and final functions to avoid casts. Remove the ctxsz
element for struct git_hash_algo, which is no longer very useful.
This does mean that stack allocations will grow slightly as additional
hash functions are added, but this should not be a significant problem,
since we don't allocate many hash contexts. The improved usability and
benefits from avoiding dynamic allocation outweigh this small downside.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Most of the other code dealing with SHA-1 and other hashes is located in
hash.h, which is in turn loaded by cache.h. Move the SHA-1 macros to
hash.h as well, so we can use them in additional hash-related items in
the future.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Since in the future we want to support an additional hash algorithm, add
a structure that represents a hash algorithm and all the data that must
go along with it. Add a constant to allow easy enumeration of hash
algorithms. Implement function typedefs to create an abstract API that
can be used by any hash algorithm, and wrappers for the existing SHA1
functions that conform to this API.
Expose a value for hex size as well as binary size. While one will
always be twice the other, the two values are both used extremely
commonly throughout the codebase and providing both leads to improved
readability.
Don't include an entry in the hash algorithm structure for the null
object ID. As this value is all zeros, any suitably sized all-zero
object ID can be used, and there's no need to store a given one on a
per-hash basis.
The current hash function transition plan envisions a time when we will
accept input from the user that might be in SHA-1 or in the NewHash
format. Since we cannot know which the user has provided, add a
constant representing the unknown algorithm to allow us to indicate that
we must look the correct value up. Provide dummy API functions that die
in this case.
Finally, include git-compat-util.h in hash.h so that the required types
are available. This aids people using automated tools their editors.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The plumbing code between sha1dc and git is defined in
sha1dc_git.[ch], but these aren't compiled / included directly but
only via the indirect inclusion from sha1dc code. This is slightly
confusing when you try to trace the build flow.
This patch brings the following changes for simplification:
- Make sha1dc_git.c stand-alone and build from Makefile
- sha1dc_git.h is the common header to include further sha1.h
depending on the build condition
- Move comments for plumbing codes from the header to definitions
This is also meant as a preliminary work for further plumbing with
external sha1dc shlib.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add an option to use the sha1collisiondetection library from the
submodule in sha1collisiondetection/ instead of in the copy in the
sha1dc/ directory.
This allows us to try out the submodule in sha1collisiondetection
without breaking the build for anyone who's not expecting them as we
work out any kinks.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This knob lets you use the sha1dc implementation from:
https://github.com/cr-marcstevens/sha1collisiondetection
which can detect certain types of collision attacks (even
when we only see half of the colliding pair). So it
mitigates any attack which consists of getting the "good"
half of a collision into a trusted repository, and then
later replacing it with the "bad" half. The "good" half is
rejected by the victim's version of Git (and even if they
run an old version of Git, any sha1dc-enabled git will
complain loudly if it ever has to interact with the object).
The big downside is that it's slower than either the openssl
or block-sha1 implementations.
Here are some timings based off of linux.git:
- compute sha1 over whole packfile
sha1dc: 3.580s
blk-sha1: 2.046s (-43%)
openssl: 1.335s (-62%)
- rev-list --all --objects
sha1dc: 33.512s
blk-sha1: 33.514s (+0.0%)
openssl: 33.650s (+0.4%)
- git log --no-merges -10000 -p
sha1dc: 8.124s
blk-sha1: 7.986s (-1.6%)
openssl: 8.203s (+0.9%)
- index-pack --verify
sha1dc: 4m19s
blk-sha1: 2m57s (-32%)
openssl: 2m19s (-42%)
So overall the sha1 computation with collision detection is
about 1.75x slower than block-sha1, and 2.7x slower than
sha1. But of course most operations do more than just sha1.
Normal object access isn't really slowed at all (both the
+/- changes there are well within the run-to-run noise); any
changes are drowned out by the other work Git is doing.
The most-affected operation is `index-pack --verify`, which
is essentially just computing the sha1 on every object. This
is similar to the `index-pack` invocation that the receiver
of a push or fetch would perform. So clearly there's some
extra CPU load here.
There will also be some latency for the user, though keep in
mind that such an operation will generally be network bound
(this is about a 1.2GB packfile). Some of that extra CPU is
"free" in the sense that we use it while the pack is
streaming in anyway. But most of it comes during the
delta-resolution phase, after the whole pack has been
received. So we can imagine that for this (quite large)
push, the user might have to wait an extra 100 seconds over
openssl (which is what we use now). If we assume they can
push to us at 20Mbit/s, that's 480s for a 1.2GB pack, which
is only 20% slower.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Many developers use functionality in their editors that allows for quick
syntax checks, including warning about questionable constructs. This
functionality allows rapid development with fewer errors. However, such
functionality generally does not allow the specification of
project-specific defines or command-line options.
Since the SHA1_HEADER include is not defined in such a case,
developers see spurious errors when using these tools. Furthermore,
there are known implementations of "cc" whose '#include' is unhappy
with this construct.
Instead of using SHA1_HEADER, create a hash.h header and use #if
and #elif to select the desired header. Have the Makefile pass an
appropriate option to help the header select the right implementation to
use.
[jc: make BLK_SHA1 the fallback default as discussed on list,
e.g. <20170314201424.vccij5z2ortq4a4o@sigill.intra.peff.net>; also
remove SHA1_HEADER and SHA1_HEADER_SQ that are no longer used].
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This avoids unnecessary re-allocations and reinsertions. On webkit.git
(i.e. about 182k inserts to the name hash table), this reduces about
100ms out of 3s user time.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
For the find_exact_renames() function, this allows us to pass the
diff_options structure pointer to the low-level routines. We will use
that to distinguish between the "rename" and "copy" cases.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This is in an effort to make the source index of 'unpack_trees()' as
being const, and thus making the compiler help us verify that we only
access it for reading.
The constification also extended to some of the hashing helpers that get
called indirectly.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This implements a smarter rename detector for exact renames, which
rather than doing a pairwise comparison (time O(m*n)) will just hash the
files into a hash-table (size O(n+m)), and only do pairwise comparisons
to renames that have the same hash (time O(n+m) except for unrealistic
hash collissions, which we just cull aggressively).
Admittedly the exact rename case is not nearly as interesting as the
generic case, but it's an important case none-the-less. A similar general
approach should work for the generic case too, but even then you do need
to handle the exact renames/copies separately (to avoid the inevitable
added cost factor that comes from the _size_ of the file), so this is
worth doing.
In the expectation that we will indeed do the same hashing trick for the
general rename case, this code uses a generic hash-table implementation
that can be used for other things too. In fact, we might be able to
consolidate some of our existing hash tables with the new generic code
in hash.[ch].
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
