[PATCH] D154812: [Support] Add llvm::xxh3_64bits

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: andrewng, bkramer, peter.smith, PiotrZSL, serge-sans-paille.
Herald added subscribers: pengfei, hiraditya.
Herald added a project: All.
MaskRay requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

ld.lld SHF_MERGE|SHF_STRINGS duplicate elimination is computation heavy
and utilitizes llvm::xxHash64, a simplified version of XXH64.
Externally many sources confirm that a new variant XXH3 is much faster.

I have picked a few hash implementations and computed the
proportion of time spent on hashing in the overall link time (a debug
build of clang 16):

- llvm::xxHash64: 3.63%
- official XXH64 (`#define XXH_VECTOR XXH_SCALAR`): 3.53%
- official XXH3_64bits (`#define XXH_VECTOR XXH_SCALAR`): 1.21%
- official XXH3_64bits (default, essentially `XXH_SSE2`): 1.22%
- this patch llvm::xxh3_64bits: 1.19%

The remaining part of lld remains unchanged. Consequently, a lower ratio
indicates that hashing is faster. Therefore, it is evident that XXH3 from xxhash
is significantly faster than both the official version and our llvm::xxHash64.

(
string length: count
1-3: 393434
4-8: 2084056
9-16: 2846249
17-128: 5598928
129-240: 1317989
241-: 328058
)

This patch adds heavily simplified https://github.com/Cyan4973/xxHash,
taking account of many simplification ideas from Devin Hussey's xxhash-clean.

Important x86-64 optimization ideas:

- Make XXH3_len_129to240_64b and XXH3_hashLong_64b noinline
- Unroll XXH3_len_17to128_64b
- __restrict does not affect Clang code generation

Beside SHF_MERGE|SHF_STRINGS duplicate elimination, llvm/ADT/StringMap.h
StringMapImpl::LookupBucketFor and a few places in lld can potentially be
accelerated by switching to llvm::xxh3_64bits.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D154812

Files:
  llvm/include/llvm/Support/xxhash.h
  llvm/lib/Support/xxhash.cpp
  llvm/unittests/Support/xxhashTest.cpp

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D154812.538498.patch
Type: text/x-patch
Size: 14493 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20230710/52e3ab02/attachment.bin>