Clang 10 includes the optimizations described in
https://bugs.llvm.org/show_bug.cgi?id=41761. This means that the
platform-independent implementations of {Decode,Encode}Fixed{32,64}()
compile to one instruction on the most recent Clang and GCC.

PiperOrigin-RevId: 306330166

Easier to read on sites supporting Markdown (i.e. GitHub).

PiperOrigin-RevId: 246385089

Use clang-format to correct formatting to be in agreement with the [Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html). Doing this simplifies the process of accepting changes. Also fixed a few warnings flagged by clang-tidy.

PiperOrigin-RevId: 246350737

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=240619768

This CL uses a well-known workaround for silencing arguments that may be unused, depending on the build configuration. The silenced warnings were responsible for a large amount of noise in the MSVC build on Windows.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=240357359

This CL removes AtomicPointer from leveldb's port interface. Its usage is replaced with std::atomic<> from the C++11 standard library.

AtomicPointer was used to wrap flags, numbers, and pointers, so its instances are replaced with std::atomic<bool>, std::atomic<int>, std::atomic<size_t> and std::atomic<Node*>.

This CL does not revise the memory ordering. AtomicPointer's methods are replaced mechanically with their std::atomic equivalents, even when the underlying usage is incorrect. (Example: DBImpl::has_imm_ is written using release stores, even though it is always read using relaxed ordering.) Revising the memory ordering is left for future CLs.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=237865146

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=237832823

This change adds a native Windows port (port_windows.h) and a
Windows Env (WindowsEnv).

Note1: "small" is defined when including <Windows.h> so some
parameters were renamed to avoid conflict.

Note2: leveldb::Env defines the method: "DeleteFile" which is
also a constant defined when including <Windows.h>. The solution
was to ensure this macro is defined in env.h which forces
the function, when compiled, to be either DeleteFileA or
DeleteFileW when building for MBCS or UNICODE respectively.

This resolves #519 on GitHub.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=236364778

This prevents file descriptors from leaking to child processes.

When compiled for older (pre-2.6.23) kernels which lack support for
O_CLOEXEC there is no change in behavior.  With newer kernels, child
processes will no longer inherit leveldb's file handles, which
reduces the changes of accidentally corrupting the database.

Fixes https://github.com/google/leveldb/issues/623

Apple doesn't follow POSIX specifications for fsync(). Instead, fsync() guarantees to flush the buffer cache to the device, which means the data will survive kernel panics, but may not survive power outages. Applications that need stronger guarantees (like databases) need to use fcntl(F_FULLFSYNC).

This CL switches PosixWritableFile::Sync() to get the stronger guarantees on Apple systems. The improved implementation follows the same principles as SQLite [1] and node.js [2].

Research for the fcntl() to fsync() fallback strategy:

Apple's released source code at https://opensource.apple.com/ shows at least three different error codes being returned when a filesystem does not support F_FULLFSYNC.

fcntl() is implemented in xnu-4903.221.2 in bsd/kern/kern_descrip.c, where it delegates to fcntl_nocancel(). The documentation for fcntl_nocancel() mentions error codes for some operations, but does not include F_FULLFSYNC. The F_FULLSYNC branch in fcntl_nocancel() calls VNOP_IOCTL(_, F_FULLSYNC, NULL, 0, _), whose return value sets the error
code.

VNOP_IOCTL() is implemented in bsd/vfs/kpi_vfs.c and calls the ioctl function in the vnode's operation vector. The per-filesystem function names follow the pattern _vnop_ioctl() for all the instances in opensource code: {hfs,msdosfs,nfs,ntfs,smbfs,webdav,zfs}_vnop_ioctl().

hfs-407.30.1, msdosfs-229.200.3, and nfs in xnu-4903.221.2 handle F_FULLFSYNC. ntfs-94.200.1 and smb-759.40.1 do not handle F_FULLFSYNC, and the default branch returns ENOSUP. webdav-380.200.1 also does not handle F_FULLFSYNC, but the default branch returns EINVAL. zfs-59 also does not handle F_FULLSYNC, and its default branch returns ENOTTY.

From a different angle, Apple's ntfs-94.200.1 includes utility code that uses fcntl(F_FULLFSYNC) and falls back to fsync() just like we do, supporting the hypothesis that there is no good way to detect lack of F_FULLFSYNC support. Also, Apple's fcntl() man page [3] does not mention a way to detect lack of F_FULLFSYNC support.

[1] https://www.sqlite.org/src/doc/trunk/src/os_unix.c
[2] https://github.com/libuv/libuv/blob/master/src/unix/fs.c
[3] https://developer.apple.com/library/archive/documentatiVon/System/Conceptual/ManPages_iPhoneOS/man2/fcntl.2.html
Tested:
    https://travis-ci.org/pwnall/leveldb/builds/477318498
    TAP global presubmit

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=228593729

The CMake feature-detection code used check_symbol_exists(), which
invokes the C compiler. However, some glibc versions don't expose the
fdatasync() declaration when compiled with -std=c11, but do expose it
when compiled with -std=c++11. This most likely comes down to how
_POSIX_SOURCE is defined -- it needs to be >= 201112L for <unistd.h> to
expose fdatasync().

This CL switches to check_cxx_symbol_exists(), which uses the C++
compiler. Asides from fixing the problem above, this is the right thing
to do, because we use <unistd.h> in env_posix.cc, which is compiled with
the C++ compiler.

This CL also fixes a previously introduced inconsistency, where the
macro indicating the fdatasync() feature detection result was referred
to as HAVE_FDATASYNC and HAVE_FUNC_FDATASYNC. The former appears to be
used in other libraries, so this CL switches all our references to
HAVE_FDATASYNC.

Fixes https://github.com/google/leveldb/issues/629

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=228392612

This is not an API-breaking change, because it reduces the API that the
leveldb embedder must implement. The project will build just fine
against ports that still implement InitOnce.

C++11 guarantees thread-safe initialization of static variables inside
functions. This is a more restricted form of std::call_once or
pthread_once_t (e.g., single call site), so the compiler might be able
to generate better code [1]. Equally important, having less code in
port_example.h makes it easier to port to other platforms.

Due to the change above, this CL introduces a new approach for storing
the singleton BytewiseComparatorImpl instance returned by
BytewiseComparator(). The new approach avoids a dynamic memory
allocation, which eliminates the false positive from LeakSanitizer
reported in https://github.com/google/leveldb/issues/200

[1] https://stackoverflow.com/a/27206650/

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=212348004

The porting layer implements threading primitives: atomic pointers,
condition variables, mutexes, thread-safe initialization. These are all
specified in C++11, so the reference open source port implementation can
become platform-independent.

The porting layer will remain in place to allow the use of other
implementations with more features, such as the built-in deadlock
detection in abseil's Mutex.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=193245934

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=192365747

This is an accidental leftover from the CMake migration. The macro has
been replaced with LEVELDB_IS_BIG_ENDIAN.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=192364918

C++11 requires <atomic>. This lets us remove the header detection
(LEVELDB_ATOMIC_PRESENT) and simplify port/atomic_pointer.h.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189919098

Fixes https://github.com/google/leveldb/issues/466

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189425354

External linkage is the default for function declarations in C++.

This also fixes ClangTidy errors generated by removing the "extern"
keyword as described above.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188730416

This is a stopgap for removing warnings on Mac builds, so -Werror can be
turned on. C++11 will be required in the nearby future, which guarantees
<atomic> support. Once that happens, the simplified version of this will
match https://github.com/google/leveldb/pull/503

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188553251

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188488298

The thread safety annotations used by leveldb got opensourced in Abseil
[1]. This CL replaces leveldb's stubs with the relevant definitions from
[1], and adds annotations to the Mutex classes in the POSIX port.

[1] https://github.com/abseil/abseil-cpp/blob/master/absl/base/thread_annotations.h

Maintaining a hardware-accelerated CRC32C implementation tailored for
all modern platforms deserves a repository of its own. We extracted the
implementation here into https://github.com/google/crc32c and improved
it in that repository. This CL removes the SSE-optimized implementation
from this codebase, and adds the ability to use the google/crc32c
library, if it is present on the system.

The benchmarks below show the performance impact of the change. In
summary, open source builds that use the google/crc32c library can
expect a 3x improvement in CRC32C throughput, whereas builds that do not
use the library will see a 50% drop in CRC32C throughput. This
translates in much smaller changes in overall leveldb performance.

Baseline, MacBookPro13,3 with Core i7 6920HQ:
LevelDB:    version 1.20
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    1000000
RawSize:    110.6 MB (estimated)
FileSize:   62.9 MB (estimated)
------------------------------------------------
fillseq      :       3.064 micros/op;   36.1 MB/s
fillsync     :      57.861 micros/op;    1.9 MB/s (1000 ops)
fillrandom   :       3.887 micros/op;   28.5 MB/s
overwrite    :       4.140 micros/op;   26.7 MB/s
readrandom   :       7.433 micros/op; (1000000 of 1000000 found)
readrandom   :       6.825 micros/op; (1000000 of 1000000 found)
readseq      :       0.244 micros/op;  453.4 MB/s
readreverse  :       0.387 micros/op;  285.8 MB/s
compact      :  449707.000 micros/op;
readrandom   :       4.196 micros/op; (1000000 of 1000000 found)
readseq      :       0.228 micros/op;  485.8 MB/s
readreverse  :       0.320 micros/op;  345.2 MB/s
fill100K     :     562.556 micros/op;  169.6 MB/s (1000 ops)
crc32c       :       0.768 micros/op; 5085.0 MB/s (4K per op)
snappycomp   :       4.220 micros/op;  925.7 MB/s (output: 55.1%)
snappyuncomp :       0.635 micros/op; 6155.7 MB/s
acquireload  :      13.054 micros/op; (each op is 1000 loads)

New with crc32c, MacBookPro13,3 with Core i7 6920HQ:
LevelDB:    version 1.20
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    1000000
RawSize:    110.6 MB (estimated)
FileSize:   62.9 MB (estimated)
------------------------------------------------
fillseq      :       2.820 micros/op;   39.2 MB/s
fillsync     :      51.988 micros/op;    2.1 MB/s (1000 ops)
fillrandom   :       3.747 micros/op;   29.5 MB/s
overwrite    :       4.047 micros/op;   27.3 MB/s
readrandom   :       7.287 micros/op; (1000000 of 1000000 found)
readrandom   :       6.927 micros/op; (1000000 of 1000000 found)
readseq      :       0.253 micros/op;  437.5 MB/s
readreverse  :       0.411 micros/op;  269.2 MB/s
compact      :  440405.000 micros/op;
readrandom   :       4.159 micros/op; (1000000 of 1000000 found)
readseq      :       0.230 micros/op;  481.1 MB/s
readreverse  :       0.320 micros/op;  345.9 MB/s
fill100K     :     558.222 micros/op;  170.9 MB/s (1000 ops)
crc32c       :       0.214 micros/op; 18263.5 MB/s (4K per op)
snappycomp   :       4.471 micros/op;  873.7 MB/s (output: 55.1%)
snappyuncomp :       0.833 micros/op; 4688.5 MB/s
acquireload  :      13.289 micros/op; (each op is 1000 loads)

New without crc32c, MacBookPro13,3 with Core i7 6920HQ
LevelDB:    version 1.20
Keys:       16 bytes each
Values:     100 bytes each (50 bytes after compression)
Entries:    1000000
RawSize:    110.6 MB (estimated)
FileSize:   62.9 MB (estimated)
------------------------------------------------
fillseq      :       3.094 micros/op;   35.8 MB/s
fillsync     :      52.160 micros/op;    2.1 MB/s (1000 ops)
fillrandom   :       4.090 micros/op;   27.0 MB/s
overwrite    :       4.006 micros/op;   27.6 MB/s
readrandom   :       6.584 micros/op; (1000000 of 1000000 found)
readrandom   :       6.676 micros/op; (1000000 of 1000000 found)
readseq      :       0.280 micros/op;  395.2 MB/s
readreverse  :       0.391 micros/op;  283.2 MB/s
compact      :  433911.000 micros/op;
readrandom   :       4.261 micros/op; (1000000 of 1000000 found)
readseq      :       0.251 micros/op;  440.5 MB/s
readreverse  :       0.356 micros/op;  310.9 MB/s
fill100K     :     584.023 micros/op;  163.3 MB/s (1000 ops)
crc32c       :       1.384 micros/op; 2822.3 MB/s (4K per op)
snappycomp   :       4.763 micros/op;  820.1 MB/s (output: 55.1%)
snappyuncomp :       0.766 micros/op; 5098.6 MB/s
acquireload  :      12.931 micros/op; (each op is 1000 loads)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171667771

This follows the general naming convention for preprocessor macros used
to detect feature (library / header file / symbol) presence.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171184641

CL 170738066 removed all instances of fread_unlocked, fwrite_unlocked
and fflush_unlocked calls from the codebase, so the feature detection
can be removed as well.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=171154269

When faced with a pointer that is misaligned by K bytes (pointer % 8 ==
K), the code previously moved forward by K bytes. In order to end up
with an aligned pointer, the code must move by 8 - K bytes.

This lands https://github.com/google/leveldb/pull/488

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=166295921

Use __APPLE__ instead of OS_MACOS when testing for the Apple platform and
remove the latter symbol from the BUILD file. This fixes incompatibility issues
when using the library on an Apple device.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=162958094

LE_LOAD64 is only used when _mm_crc32_u64 is available, on 64-bit x86 processors.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148906169

This change authored by vadimskipin and submitted via:

    https://github.com/google/leveldb/pull/309

Changes made to support iOS builds and other architectures
without support for SSE 4.2.

db_bench reports original crc32 speed at:

    crc32c : 3.610 micros/op; 1082.0 MB/s (4K per op)

with this change performance has increased to:

    crc32c : 0.843 micros/op; 4633.6 MB/s (4K per op)

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148694935

A recent CL (104348226) created the port_posix library, but omitted: port/atomic_pointer.h.

And when:

    [] test third_party/leveldb:all

was run this error was reported:

    //third_party/leveldb:port_posix does not depend on a
    module exporting 'third_party/leveldb/port/atomic_pointer.h'
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=105243399

port/atomic_pointer.h was missing an implementation for
MemoryBarrier() for this platform.

Changes are:

* Update version number to 1.18
* Replace the basic fprintf call with a call to fwrite in order to
  work around the apparent compiler optimization/rewrite failure that we are
  seeing with the new toolchain/iOS SDKs provided with Xcode6 and iOS8.
* Fix ALL the header guards.
* Createed a README.md with the LevelDB project description.
* A new CONTRIBUTING file.
* Don't implicitly convert uint64_t to size_t or int.  Either preserve it as
  uint64_t, or explicitly cast. This fixes MSVC warnings about possible value
  truncation when compiling this code in Chromium.
* Added a DumpFile() library function that encapsulates the guts of the
  "leveldbutil dump" command. This will allow clients to dump
  data to their log files instead of stdout. It will also allow clients to
  supply their own environment.
* leveldb: Remove unused function 'ConsumeChar'.
* leveldbutil: Remove unused member variables from WriteBatchItemPrinter.
* OpenBSD, NetBSD and DragonflyBSD have _LITTLE_ENDIAN, so define
  PLATFORM_IS_LITTLE_ENDIAN like on FreeBSD. This fixes:
   * issue #143
   * issue #198
   * issue #249
* Switch from <cstdatomic> to <atomic>. The former never made it into the
  standard and doesn't exist in modern gcc versions at all.  The later contains
  everything that leveldb was using from the former.
  This problem was noticed when porting to Portable Native Client where no memory
  barrier is defined.  The fact that <cstdatomic> is missing normally goes
  unnoticed since memory barriers are defined for most architectures.
* Make Hash() treat its input as unsigned.  Before this change LevelDB files
  from platforms with different signedness of char were not compatible. This
  change fixes: issue #243
* Verify checksums of index/meta/filter blocks when paranoid_checks set.
* Invoke all tools for iOS with xcrun. (This was causing problems with the new
  XCode 5.1.1 image on pulse.)
* include <sys/stat.h> only once, and fix the following linter warning:
  "Found C system header after C++ system header"
* When encountering a corrupted table file, return Status::Corruption instead of
  Status::InvalidArgument.
* Support cygwin as build platform, patch is from https://code.google.com/p/leveldb/issues/detail?id=188
* Fix typo, merge patch from https://code.google.com/p/leveldb/issues/detail?id=159
* Fix typos and comments, and address the following two issues:
  * issue #166
  * issue #241
* Add missing db synchronize after "fillseq" in the benchmark.
* Removed unused variable in SeekRandom: value (issue #201)

- switched from mmap based writing to simpler stdio based writing. Has a
  minor impact (0.5 microseconds) on microbenchmarks for asynchronous
  writes. Synchronous writes speed up from 30ms to 10ms on linux/ext4.
  Should be much more reliable on diverse platforms.
- compaction errors now immediately put the database into a read-only
  mode (until it is re-opened). As a downside, a disk going out of
  space and then space being created will require a re-open to recover
  from, whereas previously that would happen automatically. On the
  plus side, many corruption possibilities go away.
- force the DB to enter an error-state so that all future writes fail
  when a synchronous log write succeeds but the sync fails.
- repair now regenerates sstables that exhibit problems
- fix issue 218 - Use native memory barriers on OSX
- fix issue 212 - QNX build is broken
- fix build on iOS with xcode 5
- make tests compile and pass on windows

Highlights
----------
Mmap at most 1000 files on Posix to improve performance for large databases.
Support for more architectures (thanks to Alexander K.)

Building and porting
--------------------
HP/UX support (issue 126)
AtomicPointer for ia64 (issue 123)
Sparc v9 support (issue 124)
Atomic ops for powerpc
Use -fno-builtin-memcmp only when using g++
Simplify IOS build rules (issue 114)
Use CXXFLAGS instead of CFLAGS when invoking C++ compiler (issue 118)
Fix snappy shared library problem (issue 94)
Fix shared library installation path regression
Endian-ness detection tweak for FreeBSD

Bug fixes
---------
Stop ignoring FLAGS_open_files in db_bench
Make bloom test behavior agnostic to endian-ness

Performance
-----------
Limit number of mmapped files to 1000 to improve perf for large dbs
Do not delay for 1 second on shutdown path (issue 125)

Misc
----
Make InMemoryEnv return a no-op logger
C binding now has a wrapper for free (issue 117)
Add thread-safety annotations
Added an in-process lock table (issue 120)
Make RandomAccessFile and SequentialFile non-copyable

various platforms; improve android port speed.

Avoid static initializer by using a new portability interface for
thread-safe lazy initialization.  Custom ports will need to be
extended to implement InitOnce/OnceType/LEVELDB_ONCE_INIT.

Fix endian-ness detection (fixes Powerpc builds).

Build related fixes:
- Support platforms that have unversioned shared libraries.
- Fix IOS build rules.

Android improvements
- Speed up atomic pointers
- Share more code with port_posix.

Do not spin in a tight loop attempting compactions if the file system
is inaccessible (e.g., if kerberos tickets have expired or if it is out
of space).

In particular, we add a new FilterPolicy class.  An instance
of this class can be supplied in Options when opening a
database.  If supplied, the instance is used to generate
summaries of keys (e.g., a bloom filter) which are placed in
sstables.  These summaries are consulted by DB::Get() so we
can avoid reading sstable blocks that are guaranteed to not
contain the key we are looking for.

This change provides one implementation of FilterPolicy
based on bloom filters.

Other changes:
- Updated version number to 1.4.
- Some build tweaks.
- C binding for CompactRange.
- A few more benchmarks: deleteseq, deleterandom, readmissing, seekrandom.
- Minor .gitignore update.

(1) Separate out C++ and CC flags (fixes c_test compilation)
(2) Move snappy/perftools detection to script
(3) Fix db_bench_sqlite3 and db_bench_tree_db build rules

- Replace raw slice comparison with a call to user comparator.
  Added test for custom comparators.

- Fix end of namespace comments.

- Fixed bug in picking inputs for a level-0 compaction.

  When finding overlapping files, the covered range may expand
  as files are added to the input set.  We now correctly expand
  the range when this happens instead of continuing to use the
  old range.  For example, suppose L0 contains files with the
  following ranges:

      F1: a .. d
      F2:    c .. g
      F3:       f .. j

  and the initial compaction target is F3.  We used to search
  for range f..j which yielded {F2,F3}.  However we now expand
  the range as soon as another file is added.  In this case,
  when F2 is added, we expand the range to c..j and restart the
  search.  That picks up file F1 as well.

  This change fixes a bug related to deleted keys showing up
  incorrectly after a compaction as described in Issue 44.

(Sync with upstream @25072954)

Fix GCC -Wshadow warnings in LevelDB's public header files,
reported by Dustin.

Add in-memory Env implementation (helpers/memenv/*).
This enables users to create LevelDB databases in-memory.

Initialize ShardedLRUCache::last_id_ to zero.
This fixes a Valgrind warning.

(Also delete port/sha1_* which were removed upstream some time ago.)

- LevelDB patch for FreeBSD. This resolves Issue 22.
  Contributed by dforsythe (thanks!).

- Removing Chromium-specific files.
  They are now going to live in the Chromium repository.

- Adding a benchmark page comparing LevelDB performance
  to SQLite and Kyoto Cabinet's TreeDB, along with
  code to generate the benchmarks.
  Thanks to Kevin Tseng for compiling the benchmarks,
  and Scott Hess and Mikio Hirabayashi for their
  help and advice.



git-svn-id: https://leveldb.googlecode.com/svn/trunk@40 62dab493-f737-651d-591e-8d6aee1b9529

- Removed one copy of an uncompressed block contents changing
  the signature of Snappy_Uncompress() so it uncompresses into a
  flat array instead of a std::string.
        
  Speeds up readrandom ~10%.

- Instead of a combination of Env/WritableFile, we now have a
  Logger interface that can be easily overridden applications
  that want to supply their own logging.

- Separated out the gcc and Sun Studio parts of atomic_pointer.h
  so we can use 'asm', 'volatile' keywords for Sun Studio.




git-svn-id: https://leveldb.googlecode.com/svn/trunk@39 62dab493-f737-651d-591e-8d6aee1b9529

- LevelDB patch for Sun Studio
  Based on a patch submitted by Theo Schlossnagle - thanks!
  This fixes Issue 17.

- Fix a couple of test related memory leaks.



git-svn-id: https://leveldb.googlecode.com/svn/trunk@38 62dab493-f737-651d-591e-8d6aee1b9529

Change atomic_pointer.h to prefer a memory barrier based
implementation over a <cstdatomic> based implementation for
the following reasons:
(1) On a x86-32-bit gcc-4.4 build, <ctdatomic> was corrupting
    the AtomicPointer.
(2) On a x86-64-bit gcc build, a <ctstdatomic> based acquire-load
    takes ~15ns as opposed to the ~1ns for a memory-barrier
    based implementation.

Fixes issue 9 (corruption_test fails)
http://code.google.com/p/leveldb/issues/detail?id=9

Fixes issue 16 (CorruptionTest.MissingDescriptor fails)
http://code.google.com/p/leveldb/issues/detail?id=16



git-svn-id: https://leveldb.googlecode.com/svn/trunk@36 62dab493-f737-651d-591e-8d6aee1b9529

git-svn-id: https://leveldb.googlecode.com/svn/trunk@35 62dab493-f737-651d-591e-8d6aee1b9529

This revision adds two major changes:
1. build_detect_platform which generates build_config.mk
   with platform-dependent flags for the build process
2. /port/atomic_pointer.h with anAtomicPointerimplementation
   for platforms without <cstdatomic>

Some of this code is loosely based on patches submitted to the 
LevelDB mailing list at https://groups.google.com/forum/#!forum/leveldb
Tip of the hat to Dave Smith and Edouard A, who both sent patches.

The presence of Snappy (http://code.google.com/p/snappy/) and
cstdatomic are now both detected in the build_detect_platform
script (1.) which gets executing during make.

For (2.), instead of broadly importing atomicops_* from Chromium or
the Google performance tools, we chose to just implement AtomicPointer 
and the limited atomic load and store operations it needs. 
This resulted in much less code and fewer files - everything is 
contained in atomic_pointer.h.



git-svn-id: https://leveldb.googlecode.com/svn/trunk@34 62dab493-f737-651d-591e-8d6aee1b9529

- Implemented Get() directly instead of building on top of a full
  merging iterator stack.  This speeds up the "readrandom" benchmark
  by up to 15-30%.

- Fixed an opensource compilation problem.
  Added --db=<name> flag to control where the database is placed.

- Automatically compact a file when we have done enough
  overlapping seeks to that file.

- Fixed a performance bug where we would read from at least one
  file in a level even if none of the files overlapped the key
  being read.

- Makefile fix for Mac OSX installations that have XCode 4 without XCode 3.

- Unified the two occurrences of binary search in a file-list
  into one routine.

- Found and fixed a bug where we would unnecessarily search the
  last file when looking for a key larger than all data in the
  level.

- A fix to avoid the need for trivial move compactions and
  therefore gets rid of two out of five syncs in "fillseq".

- Removed the MANIFEST file write when switching to a new
  memtable/log-file for a 10-20% improvement on fill speed on ext4.

- Adding a SNAPPY setting in the Makefile for folks who have
  Snappy installed. Snappy compresses values and speeds up writes.



git-svn-id: https://leveldb.googlecode.com/svn/trunk@32 62dab493-f737-651d-591e-8d6aee1b9529