[clangd] Use xxhash instead of SHA1 for background index file digests.

Summary:
Currently SHA1 is about 10% of our CPU, this patch reduces it to ~1%.

xxhash is a well-defined (stable) non-cryptographic hash optimized for
fast checksums (like crc32).
Collisions shouldn't be a problem, despite the reduced length:
 - for actual file content (used to invalidate bg index shards), there
   are only two versions that can collide (new shard and old shard).
 - for file paths in bg index shard filenames, we would need 2^32 files
   with the same filename to expect a collision. Imperfect hashing may
   reduce this a bit but it's well beyond what's plausible.

This will invalidate shards on disk (as usual; I bumped the version),
but this time the filenames are changing so the old files will stick
around :-( So this is more expensive than the usual bump, but would be
good to land before the v9 branch when everyone will start using bg index.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64306

llvm-svn: 365311

1 files changed, 1 insertions, 1 deletions

diff --git a/clang-tools-extra/clangd/index/Serialization.cpp b/clang-tools-extra/clangd/index/Serialization.cpp
index 5b3c7cf943e..188a5cc1862 100644
--- a/clang-tools-extra/clangd/index/Serialization.cpp
+++ b/clang-tools-extra/clangd/index/Serialization.cpp
@@ -444,7 +444,7 @@ readCompileCommand(Reader CmdReader, llvm::ArrayRef<llvm::StringRef> Strings) {
 // The current versioning scheme is simple - non-current versions are rejected.
 // If you make a breaking change, bump this version number to invalidate stored
 // data. Later we may want to support some backward compatibility.
-constexpr static uint32_t Version = 11;
+constexpr static uint32_t Version = 12;
 
 llvm::Expected<IndexFileIn> readRIFF(llvm::StringRef Data) {
   auto RIFF = riff::readFile(Data);