[all-commits] [llvm/llvm-project] 9a4420: [clang-doc] Improve complexity of Index construction

[Paul Kirth via All-commits]

  Branch: refs/heads/users/ilovepi/clang-doc-simplify-indexing
  Home:   https://github.com/llvm/llvm-project
  Commit: 9a4420ff55c9ec6a7e03386ba8a54bda175654c3
      https://github.com/llvm/llvm-project/commit/9a4420ff55c9ec6a7e03386ba8a54bda175654c3
  Author: Paul Kirth <paulkirth at google.com>
  Date:   2026-03-02 (Mon, 02 Mar 2026)

  Changed paths:
    M clang-tools-extra/clang-doc/Generators.cpp
    M clang-tools-extra/clang-doc/JSONGenerator.cpp
    M clang-tools-extra/clang-doc/MDGenerator.cpp
    M clang-tools-extra/clang-doc/Representation.cpp
    M clang-tools-extra/clang-doc/Representation.h
    M clang-tools-extra/clang-doc/YAMLGenerator.cpp
    M clang-tools-extra/unittests/clang-doc/ClangDocTest.cpp
    M clang-tools-extra/unittests/clang-doc/GeneratorTest.cpp

  Log Message:
  -----------
  [clang-doc] Improve complexity of Index construction

The existing implementation ends up with an O(N^2) algorithm due to
repeated linear scans during index construction. Switching to a
StringMap allows us to reduce this to O(N), since we no longer need to
search the vector.

The `BM_Index_Insertion` benchmark measures the time taken to insert N
unique records into the index.

| Scale (N Items) | Baseline (ns) | Patched (ns) | Speedup | Change |
|----------------:|--------------:|-------------:|--------:|-------:|
| 10              | 9,977         | 11,004       | 0.91x   | +10.3% |
| 64              | 69,249        | 69,166       | 1.00x   | -0.1%  |
| 512             | 1,932,714     | 525,877      | 3.68x   | -72.8% |
| 4,096           | 92,411,535    | 4,589,030    | 20.1x   | -95.0% |
| 10,000          | 577,384,945   | 12,998,039   | 44.4x   | -97.7% |

The patch delivers significant improvements to scalability. At 10,000
items, index construction is **~44 times faster**, confirming the
complexity reduction from O(N^2) to O(N). The crossover point where the
new map-based approach beats the vector-based approach appears to be
around N=64.

Since the index is typically larger than 64 for files of non trivial
complexity, and users will typically be building documentation for an
entire project with many files, all normal usage should benefit from
this change.

Other benchmarks show minor regressions, though in a typical build of
LLVM documentation index construction takes up a larger amount of
runtime than any of these other components.


  Commit: 2cfc12a14a615fa5f4c432c49cf8a06ff94c762f
      https://github.com/llvm/llvm-project/commit/2cfc12a14a615fa5f4c432c49cf8a06ff94c762f
  Author: Paul Kirth <paulkirth at google.com>
  Date:   2026-03-02 (Mon, 02 Mar 2026)

  Changed paths:
    M clang-tools-extra/clang-doc/JSONGenerator.cpp
    M clang-tools-extra/clang-doc/MDGenerator.cpp
    M clang-tools-extra/clang-doc/Representation.cpp
    M clang-tools-extra/clang-doc/YAMLGenerator.cpp

  Log Message:
  -----------
  Format


Compare: https://github.com/llvm/llvm-project/compare/b204f736e11a...2cfc12a14a61

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