Use greg7mdp/parallel-hashmap for faster execution

[atetubou]

This implements option 1 in #2462 (comment).

This uses https://github.com/greg7mdp/parallel-hashmap instead of unordered_map for faster ninja execution.
Some more context is in https://crbug.com/346910589.

This makes noop build for chromium

• 12% faster on Linux
• 2% slower on Windows
• 96% faster on macOS

Linux

$ hyperfine --warmup 1 \
   '~/ninja_master -C out/non_component build:buildflag_header_h' \
   '~/ninja_faster_hash -C out/non_component build:buildflag_header_h'
Benchmark 1: ~/ninja_master -C out/non_component build:buildflag_header_h
  Time (mean ± σ):      5.251 s ±  0.027 s    [User: 4.641 s, System: 0.608 s]
  Range (min … max):    5.222 s …  5.309 s    10 runs
 
Benchmark 2: ~/ninja_faster_hash -C out/non_component build:buildflag_header_h
  Time (mean ± σ):      4.689 s ±  0.022 s    [User: 4.055 s, System: 0.633 s]
  Range (min … max):    4.648 s …  4.737 s    10 runs
 
Summary
  ~/ninja_faster_hash -C out/non_component build:buildflag_header_h ran
    1.12 ± 0.01 times faster than ~/ninja_master -C out/non_component build:buildflag_header_h

Windows

Benchmark 1: C:\Users\tikuta\source\repos\ninja-build\ninja\ninja_master -C out/non_component build:buildflag_header_h
  Time (mean ± σ):     13.161 s ±  0.105 s    [User: 10.657 s, System: 2.194 s]
  Range (min … max):   13.005 s … 13.328 s    10 runs

Benchmark 2: C:\Users\tikuta\source\repos\ninja-build\ninja\ninja_faster_hash -C out/non_component build:buildflag_header_h
  Time (mean ± σ):     13.206 s ±  0.210 s    [User: 10.653 s, System: 2.194 s]
  Range (min … max):   12.951 s … 13.578 s    10 runs

Summary
  C:\Users\tikuta\source\repos\ninja-build\ninja\ninja_master -C out/non_component build:buildflag_header_h ran
    1.00 ± 0.02 times faster than C:\Users\tikuta\source\repos\ninja-build\ninja\ninja_faster_hash -C out/non_component build:buildflag_header_h

macOS

$ hyperfine --warmup 1 '~/ninja_master -C out/non_component build:buildflag_header_h' '~/ninja_faster_hash -C out/non_component build:buildflag_header_h'
Benchmark 1: ~/ninja_master -C out/non_component build:buildflag_header_h
  Time (mean ± σ):     14.360 s ±  0.706 s    [User: 9.006 s, System: 1.681 s]
  Range (min … max):   12.832 s … 15.297 s    10 runs

Benchmark 2: ~/ninja_faster_hash -C out/non_component build:buildflag_header_h
  Time (mean ± σ):      7.340 s ±  0.260 s    [User: 6.263 s, System: 0.828 s]
  Range (min … max):    6.948 s …  7.799 s    10 runs

Summary
  ~/ninja_faster_hash -C out/non_component build:buildflag_header_h ran
    1.96 ± 0.12 times faster than ~/ninja_master -C out/non_component build:buildflag_header_h

[atetubou]

@digit-google @jhasse how do you think about this PR instead of #2462?

[digit-google]

Thanks, I have tested this CL with a local Fuchsia build on Linux, I get a consistent 2% (x1.02) speedup there.

Can you fuse all commits into a single one though? I'll add a few inline comments too.

I suggest adding a top-level .clang-format-ignore file to avoid editors to reformat third_party/*. Unfortunately, it seems that this file is ignored by git-clang-format for now, so that's not a hard-requirement.

[atetubou]

Can you fuse all commits into a single one though? I'll add a few inline comments too.

Done.

I suggest adding a top-level .clang-format-ignore file to avoid editors to reformat third_party/*.

Done.

[digit-google]

Thanks, the latest commit is much better, but please change the commit message to something informative. This is what will be submitted into the final git repository, so the current message will be useless for people looking at the git log in the future. And the github conversation here, and its description message at the top, will essentially be lost in the wind after that.

@jhasse , what do you think?

[atetubou]

Thank you for your review, I updated commit message.