[PATCH] D45571: [ELF] - Speedup MergeInputSection::splitStrings

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.
grimar added reviewers: ruiu, espindola.
Herald added subscribers: arichardson, emaste.
grimar edited the summary of this revision.

This is for https://bugs.llvm.org//show_bug.cgi?id=37029,
which was about the experiment of using hash_value for splitting strings.

`hash_value` at some point for short strings falls back to hash_short:
https://github.com/llvm-mirror/llvm/blob/master/include/llvm/ADT/Hashing.h#L453

I think we can use it instead of `xxHash64` for `splitStrings, as this method
uses uint64_t as a return value and shows really good results it seems. 
It computes the hash of the part of the string, but this seems to be OK here.

I used scylla to profile changes and benchmarked few algorithms. 
`splitNonStrings` did not show up in profile,
so I experimented only on changing `splitStrings`. Results are below:

  * Default (xxHash64):                             CPU(%)   CPU(ms)
  - lld.exe                                                      100.00%   4254
   + lld::elf::MergeInputSection::splitStrings  21.86%    930
  
  * With use of hash_value:
  - lld.exe                                                     100.00%   4001
   + lld::elf::MergeInputSection::splitStrings  18.25%    730
  
  * With use of hashGnu:
  - lld.exe                                                     100.00 %  4469
   + lld::elf::MergeInputSection::splitStrings  25.60 %  1144
  
  * With use of hashSysV:
  - lld.exe (PID: 5716)                                   100.00 %  5080
   + lld::elf::MergeInputSection::splitStrings  33.40 %  1711
  
  * This patch:
  - lld.exe (PID: 9192)                                  100.00%   3866
   + lld::elf::MergeInputSection::splitStrings  13.24 %   512

So this change improves total CPU time by about 10% (4254/3866) for Scylla.
And makes `splitStrings` about 80% faster.
(Note that is the time that profiler shows, I did not yet try to benchmark it
in a regular way).

Seed value used was taken from:
https://github.com/llvm-mirror/llvm/blob/master/include/llvm/ADT/Hashing.h#L328
It is equal to default seed used by `hash_value`


https://reviews.llvm.org/D45571

Files:
  ELF/InputSection.cpp
  test/ELF/comment-gc.s
  test/ELF/debug-gc.s
  test/ELF/emit-relocs-mergeable-i386.s
  test/ELF/emit-relocs-mergeable.s

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