[clangd] Add script to maintain list of fast clang-tidy checks

[sammccall]

Summary

The plan is to intersect this list with the checks selected per config.
This is not yet done, but the initial list is checked in as a baseline.

https://github.com/clangd/clangd/issues/1337

[kadircet]

clang-tools-extra/clangd/TidyFastChecks.py

32

file to use for benchmark?

42

what does -P do? shouldn't the latter be -DFAST?

71

comment seems to be incomplete here

71

it might be useful to include name of the file benchmarks were performed on in the output.

84

i don't see the point in including delta in the output if we're also making the decision here. is it mostly for debugging purposes? e.g. when updating the list we get to see the difference?

[sammccall]

clang-tools-extra/clangd/TidyFastChecks.py

42

-P avoids emitting # 0 /path/to/Sema.cpp lines. Added comments

Fixed -DFAST, I must have somehow mangled this after running it!

84

Yes, that's exactly the reason. Can make it a comment instead if you like, but that makes ad-hoc analysis slightly harder.

[sammccall]

review comments

[kadircet]

clang-tools-extra/clangd/TidyFastChecks.inc

3

can you also re-run the script before checking in (or update here, since re-running all might take a while, but would be a good way to test history preserving logic)

clang-tools-extra/clangd/TidyFastChecks.py

77

s/perncentage/percentage

84

if that's the case no need. just wanted to make sure about it.

[sammccall]

Script reads/writes from file rather than redirecting output.
Update .inc file to show "no-op" changes.

clang-tools-extra/clangd/TidyFastChecks.inc

+// This file is generated, do not edit it directly!
+// Deltas are percentage regression in parsing clang/lib/Sema/Sema.cpp
+#ifndef FAST
+#define FAST(CHECK, DELTA)
+#endif
+#ifndef SLOW
+#define SLOW(CHECK, DELTA)
+#endif
+
+FAST(abseil-cleanup-ctad, -1.0)
+FAST(abseil-duration-addition, 0.0)
+FAST(abseil-duration-comparison, 1.0)
+FAST(abseil-duration-conversion-cast, 3.0)
+FAST(abseil-duration-division, -0.0)
+FAST(abseil-duration-factory-float, 1.0)
+FAST(abseil-duration-factory-scale, -0.0)
+FAST(abseil-duration-subtraction, 1.0)
+FAST(abseil-duration-unnecessary-conversion, 4.0)
+FAST(abseil-faster-strsplit-delimiter, 2.0)
+FAST(abseil-no-internal-dependencies, -1.0)
+FAST(abseil-no-namespace, -1.0)
+FAST(abseil-redundant-strcat-calls, 2.0)
+FAST(abseil-str-cat-append, 1.0)
+FAST(abseil-string-find-startswith, 1.0)
+FAST(abseil-string-find-str-contains, 1.0)
+FAST(abseil-time-comparison, -0.0)
+FAST(abseil-time-subtraction, 0.0)
+FAST(abseil-upgrade-duration-conversions, 2.0)
+SLOW(altera-id-dependent-backward-branch, 13.0)
+FAST(altera-kernel-name-restriction, -1.0)
+FAST(altera-single-work-item-barrier, -1.0)
+FAST(altera-struct-pack-align, -1.0)
+FAST(altera-unroll-loops, 2.0)
+FAST(android-cloexec-accept, -1.0)
+FAST(android-cloexec-accept4, 3.0)
+FAST(android-cloexec-creat, 0.0)
+FAST(android-cloexec-dup, 3.0)
+FAST(android-cloexec-epoll-create, -2.0)
+FAST(android-cloexec-epoll-create1, -1.0)
+FAST(android-cloexec-fopen, -0.0)
+FAST(android-cloexec-inotify-init, 1.0)
+FAST(android-cloexec-inotify-init1, 2.0)
+FAST(android-cloexec-memfd-create, 2.0)
+FAST(android-cloexec-open, -1.0)
+FAST(android-cloexec-pipe, -1.0)
+FAST(android-cloexec-pipe2, 0.0)
+FAST(android-cloexec-socket, 1.0)
+FAST(android-comparison-in-temp-failure-retry, 0.0)
+FAST(boost-use-to-string, 1.0)
+FAST(bugprone-argument-comment, 2.0)
+FAST(bugprone-assert-side-effect, 1.0)
+FAST(bugprone-assignment-in-if-condition, -0.0)
+FAST(bugprone-bad-signal-to-kill-thread, -1.0)
+FAST(bugprone-bool-pointer-implicit-conversion, 0.0)
+FAST(bugprone-branch-clone, -0.0)
+FAST(bugprone-copy-constructor-init, 1.0)
+FAST(bugprone-dangling-handle, 0.0)
+FAST(bugprone-dynamic-static-initializers, 1.0)
+FAST(bugprone-easily-swappable-parameters, 2.0)
+FAST(bugprone-exception-escape, 1.0)
+FAST(bugprone-fold-init-type, 2.0)
+FAST(bugprone-forward-declaration-namespace, 0.0)
+FAST(bugprone-forwarding-reference-overload, -0.0)
+FAST(bugprone-implicit-widening-of-multiplication-result, 3.0)
+FAST(bugprone-inaccurate-erase, -0.0)
+FAST(bugprone-incorrect-roundings, 1.0)
+FAST(bugprone-infinite-loop, 4.0)
+FAST(bugprone-integer-division, -3.0)
+FAST(bugprone-lambda-function-name, 1.0)
+FAST(bugprone-macro-parentheses, 8.0)
+FAST(bugprone-macro-repeated-side-effects, 1.0)
+FAST(bugprone-misplaced-operator-in-strlen-in-alloc, -0.0)
+FAST(bugprone-misplaced-pointer-arithmetic-in-alloc, 2.0)
+FAST(bugprone-misplaced-widening-cast, -2.0)
+FAST(bugprone-move-forwarding-reference, -2.0)
+FAST(bugprone-multiple-statement-macro, 1.0)
+FAST(bugprone-narrowing-conversions, -1.0)
+FAST(bugprone-no-escape, 3.0)
+FAST(bugprone-not-null-terminated-result, 4.0)
+FAST(bugprone-parent-virtual-call, 2.0)
+FAST(bugprone-posix-return, 0.0)
+FAST(bugprone-redundant-branch-condition, 0.0)
+FAST(bugprone-reserved-identifier, 2.0)
+FAST(bugprone-shared-ptr-array-mismatch, 0.0)
+FAST(bugprone-signal-handler, -0.0)
+FAST(bugprone-signed-char-misuse, 1.0)
+FAST(bugprone-sizeof-container, -0.0)
+FAST(bugprone-sizeof-expression, 0.0)
+FAST(bugprone-spuriously-wake-up-functions, 3.0)
+FAST(bugprone-string-constructor, 1.0)
+FAST(bugprone-string-integer-assignment, 1.0)
+FAST(bugprone-string-literal-with-embedded-nul, 0.0)
+FAST(bugprone-stringview-nullptr, 2.0)
+FAST(bugprone-suspicious-enum-usage, -0.0)
+FAST(bugprone-suspicious-include, 1.0)
+FAST(bugprone-suspicious-memory-comparison, 0.0)
+FAST(bugprone-suspicious-memset-usage, 2.0)
+FAST(bugprone-suspicious-missing-comma, 0.0)
+FAST(bugprone-suspicious-realloc-usage, 1.0)
+FAST(bugprone-suspicious-semicolon, 0.0)
+FAST(bugprone-suspicious-string-compare, 2.0)
+FAST(bugprone-swapped-arguments, 0.0)
+FAST(bugprone-terminating-continue, 2.0)
+FAST(bugprone-throw-keyword-missing, -1.0)
+FAST(bugprone-too-small-loop-variable, 0.0)
+FAST(bugprone-unchecked-optional-access, 1.0)
+FAST(bugprone-undefined-memory-manipulation, 1.0)
+FAST(bugprone-undelegated-constructor, 0.0)
+FAST(bugprone-unhandled-exception-at-new, 0.0)
+FAST(bugprone-unhandled-self-assignment, 0.0)
+FAST(bugprone-unused-raii, 2.0)
+FAST(bugprone-unused-return-value, 0.0)
+FAST(bugprone-use-after-move, 5.0)
+FAST(bugprone-virtual-near-miss, 0.0)
+FAST(cert-con36-c, 2.0)
+FAST(cert-con54-cpp, 3.0)
+FAST(cert-dcl03-c, 2.0)
+FAST(cert-dcl16-c, -1.0)
+FAST(cert-dcl21-cpp, 0.0)
+FAST(cert-dcl37-c, 3.0)
+FAST(cert-dcl50-cpp, -1.0)
+FAST(cert-dcl51-cpp, 1.0)
+FAST(cert-dcl54-cpp, 0.0)
+FAST(cert-dcl58-cpp, 1.0)
+FAST(cert-dcl59-cpp, 0.0)
+FAST(cert-env33-c, -1.0)
+FAST(cert-err09-cpp, 1.0)
+FAST(cert-err33-c, 4.0)
+FAST(cert-err34-c, 0.0)
+FAST(cert-err52-cpp, 1.0)
+FAST(cert-err58-cpp, 0.0)
+FAST(cert-err60-cpp, 0.0)
+FAST(cert-err61-cpp, -0.0)
+FAST(cert-exp42-c, 0.0)
+FAST(cert-fio38-c, 1.0)
+FAST(cert-flp30-c, 0.0)
+FAST(cert-flp37-c, 1.0)
+FAST(cert-mem57-cpp, 0.0)
+FAST(cert-msc30-c, -0.0)
+FAST(cert-msc32-c, 1.0)
+FAST(cert-msc50-cpp, -1.0)
+FAST(cert-msc51-cpp, 1.0)
+FAST(cert-msc54-cpp, -1.0)
+FAST(cert-oop11-cpp, 1.0)
+FAST(cert-oop54-cpp, -0.0)
+FAST(cert-oop57-cpp, 1.0)
+FAST(cert-oop58-cpp, 0.0)
+FAST(cert-pos44-c, 0.0)
+FAST(cert-pos47-c, -0.0)
+FAST(cert-sig30-c, 2.0)
+FAST(cert-str34-c, 0.0)
+FAST(concurrency-mt-unsafe, 3.0)
+FAST(concurrency-thread-canceltype-asynchronous, 1.0)
+FAST(cppcoreguidelines-avoid-c-arrays, 2.0)
+FAST(cppcoreguidelines-avoid-const-or-ref-data-members, 1.0)
+FAST(cppcoreguidelines-avoid-do-while, -2.0)
+FAST(cppcoreguidelines-avoid-goto, 2.0)
+FAST(cppcoreguidelines-avoid-magic-numbers, 1.0)
+FAST(cppcoreguidelines-avoid-non-const-global-variables, -0.0)
+FAST(cppcoreguidelines-c-copy-assignment-signature, 1.0)
+FAST(cppcoreguidelines-explicit-virtual-functions, -1.0)
+FAST(cppcoreguidelines-init-variables, 2.0)
+FAST(cppcoreguidelines-interfaces-global-init, -1.0)
+FAST(cppcoreguidelines-macro-usage, 2.0)
+FAST(cppcoreguidelines-narrowing-conversions, -2.0)
+FAST(cppcoreguidelines-no-malloc, -1.0)
+FAST(cppcoreguidelines-non-private-member-variables-in-classes, -0.0)
+FAST(cppcoreguidelines-owning-memory, 3.0)
+FAST(cppcoreguidelines-prefer-member-initializer, 1.0)
+FAST(cppcoreguidelines-pro-bounds-array-to-pointer-decay, 3.0)
+FAST(cppcoreguidelines-pro-bounds-constant-array-index, 3.0)
+FAST(cppcoreguidelines-pro-bounds-pointer-arithmetic, 0.0)
+FAST(cppcoreguidelines-pro-type-const-cast, 1.0)
+FAST(cppcoreguidelines-pro-type-cstyle-cast, -1.0)
+FAST(cppcoreguidelines-pro-type-member-init, 1.0)
+FAST(cppcoreguidelines-pro-type-reinterpret-cast, 2.0)
+FAST(cppcoreguidelines-pro-type-static-cast-downcast, 0.0)
+FAST(cppcoreguidelines-pro-type-union-access, 1.0)
+FAST(cppcoreguidelines-pro-type-vararg, 1.0)
+FAST(cppcoreguidelines-slicing, 3.0)
+FAST(cppcoreguidelines-special-member-functions, -1.0)
+FAST(cppcoreguidelines-virtual-class-destructor, 0.0)
+FAST(darwin-avoid-spinlock, 1.0)
+FAST(darwin-dispatch-once-nonstatic, -0.0)
+FAST(fuchsia-default-arguments-calls, 2.0)
+FAST(fuchsia-default-arguments-declarations, -0.0)
+FAST(fuchsia-header-anon-namespaces, -0.0)
+FAST(fuchsia-multiple-inheritance, -1.0)
+FAST(fuchsia-overloaded-operator, -0.0)
+FAST(fuchsia-statically-constructed-objects, -0.0)
+FAST(fuchsia-trailing-return, 2.0)
+FAST(fuchsia-virtual-inheritance, 1.0)
+FAST(google-build-explicit-make-pair, 2.0)
+FAST(google-build-namespaces, -1.0)
+FAST(google-build-using-namespace, 0.0)
+FAST(google-default-arguments, 1.0)
+FAST(google-explicit-constructor, 2.0)
+FAST(google-global-names-in-headers, -1.0)
+FAST(google-objc-avoid-nsobject-new, 1.0)
+FAST(google-objc-avoid-throwing-exception, 1.0)
+FAST(google-objc-function-naming, 1.0)
+FAST(google-objc-global-variable-declaration, -0.0)
+FAST(google-readability-avoid-underscore-in-googletest-name, -0.0)
+FAST(google-readability-braces-around-statements, 1.0)
+FAST(google-readability-casting, 1.0)
+FAST(google-readability-function-size, 0.0)
+FAST(google-readability-namespace-comments, 0.0)
+FAST(google-readability-todo, 1.0)
+FAST(google-runtime-int, -1.0)
+FAST(google-runtime-operator, -0.0)
+FAST(google-upgrade-googletest-case, 0.0)
+FAST(hicpp-avoid-c-arrays, 1.0)
+FAST(hicpp-avoid-goto, 0.0)
+FAST(hicpp-braces-around-statements, 1.0)
+FAST(hicpp-deprecated-headers, -1.0)
+FAST(hicpp-exception-baseclass, 0.0)
+FAST(hicpp-explicit-conversions, -1.0)
+FAST(hicpp-function-size, 1.0)
+FAST(hicpp-invalid-access-moved, 6.0)
+FAST(hicpp-member-init, 2.0)
+FAST(hicpp-move-const-arg, 1.0)
+FAST(hicpp-multiway-paths-covered, 3.0)
+FAST(hicpp-named-parameter, 1.0)
+FAST(hicpp-new-delete-operators, 0.0)
+FAST(hicpp-no-array-decay, 3.0)
+FAST(hicpp-no-assembler, -0.0)
+FAST(hicpp-no-malloc, 1.0)
+FAST(hicpp-noexcept-move, -0.0)
+FAST(hicpp-signed-bitwise, 0.0)
+FAST(hicpp-special-member-functions, 0.0)
+FAST(hicpp-static-assert, 1.0)
+FAST(hicpp-undelegated-constructor, 3.0)
+FAST(hicpp-uppercase-literal-suffix, 3.0)
+FAST(hicpp-use-auto, 3.0)
+FAST(hicpp-use-emplace, 1.0)
+FAST(hicpp-use-equals-default, 1.0)
+FAST(hicpp-use-equals-delete, 0.0)
+FAST(hicpp-use-noexcept, -1.0)
+FAST(hicpp-use-nullptr, 2.0)
+FAST(hicpp-use-override, -0.0)
+FAST(hicpp-vararg, -2.0)
+FAST(linuxkernel-must-check-errs, 1.0)
+FAST(llvm-else-after-return, 0.0)
+FAST(llvm-header-guard, 2.0)
+FAST(llvm-include-order, 3.0)
+FAST(llvm-namespace-comment, 1.0)
+FAST(llvm-prefer-isa-or-dyn-cast-in-conditionals, 0.0)
+FAST(llvm-prefer-register-over-unsigned, 1.0)
+FAST(llvm-qualified-auto, 0.0)
+FAST(llvm-twine-local, -2.0)
+FAST(llvmlibc-callee-namespace, 2.0)
+FAST(llvmlibc-implementation-in-namespace, 2.0)
+FAST(llvmlibc-restrict-system-libc-headers, -0.0)
+FAST(misc-confusable-identifiers, 1.0)
+SLOW(misc-const-correctness, 261.0)
+FAST(misc-definitions-in-headers, -1.0)
+FAST(misc-misleading-bidirectional, -0.0)
+FAST(misc-misleading-identifier, -1.0)
+FAST(misc-misplaced-const, 1.0)
+FAST(misc-new-delete-overloads, -1.0)
+FAST(misc-no-recursion, -2.0)
+FAST(misc-non-copyable-objects, 1.0)
+FAST(misc-non-private-member-variables-in-classes, 2.0)
+FAST(misc-redundant-expression, 0.0)
+FAST(misc-static-assert, 1.0)
+FAST(misc-throw-by-value-catch-by-reference, -1.0)
+FAST(misc-unconventional-assign-operator, 1.0)
+FAST(misc-uniqueptr-reset-release, 0.0)
+FAST(misc-unused-alias-decls, 1.0)
+FAST(misc-unused-parameters, 0.0)
+FAST(misc-unused-using-decls, 4.0)
+FAST(modernize-avoid-bind, 1.0)
+FAST(modernize-avoid-c-arrays, 2.0)
+FAST(modernize-concat-nested-namespaces, -1.0)
+FAST(modernize-deprecated-headers, -0.0)
+FAST(modernize-deprecated-ios-base-aliases, 1.0)
+FAST(modernize-loop-convert, 2.0)
+FAST(modernize-macro-to-enum, 1.0)
+FAST(modernize-make-shared, -0.0)
+FAST(modernize-make-unique, 0.0)
+FAST(modernize-pass-by-value, 1.0)
+FAST(modernize-raw-string-literal, 1.0)
+FAST(modernize-redundant-void-arg, -1.0)
+FAST(modernize-replace-auto-ptr, -1.0)
+FAST(modernize-replace-disallow-copy-and-assign-macro, -2.0)
+FAST(modernize-replace-random-shuffle, 0.0)
+FAST(modernize-return-braced-init-list, -1.0)
+FAST(modernize-shrink-to-fit, 2.0)
+FAST(modernize-unary-static-assert, -1.0)
+FAST(modernize-use-auto, 2.0)
+FAST(modernize-use-bool-literals, 1.0)
+FAST(modernize-use-default-member-init, 2.0)
+FAST(modernize-use-emplace, 1.0)
+FAST(modernize-use-equals-default, 2.0)
+FAST(modernize-use-equals-delete, 0.0)
+FAST(modernize-use-nodiscard, 1.0)
+FAST(modernize-use-noexcept, 1.0)
+FAST(modernize-use-nullptr, 2.0)
+FAST(modernize-use-override, 1.0)
+FAST(modernize-use-trailing-return-type, -0.0)
+FAST(modernize-use-transparent-functors, -1.0)
+FAST(modernize-use-uncaught-exceptions, 1.0)
+FAST(modernize-use-using, 1.0)
+FAST(objc-assert-equals, -1.0)
+FAST(objc-avoid-nserror-init, -2.0)
+FAST(objc-dealloc-in-category, -0.0)
+FAST(objc-forbidden-subclassing, 0.0)
+FAST(objc-missing-hash, 1.0)
+FAST(objc-nsdate-formatter, 1.0)
+FAST(objc-nsinvocation-argument-lifetime, -2.0)
+FAST(objc-property-declaration, -1.0)
+FAST(objc-super-self, -0.0)
+FAST(openmp-exception-escape, 1.0)
+FAST(openmp-use-default-none, 1.0)
+FAST(performance-faster-string-find, 2.0)
+FAST(performance-for-range-copy, 2.0)
+FAST(performance-implicit-conversion-in-loop, 1.0)
+FAST(performance-inefficient-algorithm, 0.0)
+FAST(performance-inefficient-string-concatenation, -0.0)
+FAST(performance-inefficient-vector-operation, 1.0)
+FAST(performance-move-const-arg, 3.0)
+FAST(performance-move-constructor-init, 1.0)
+FAST(performance-no-automatic-move, -1.0)
+FAST(performance-no-int-to-ptr, 2.0)
+FAST(performance-noexcept-move-constructor, 1.0)
+FAST(performance-trivially-destructible, -1.0)
+FAST(performance-type-promotion-in-math-fn, 4.0)
+FAST(performance-unnecessary-copy-initialization, 4.0)
+FAST(performance-unnecessary-value-param, 2.0)
+FAST(portability-restrict-system-includes, 2.0)
+FAST(portability-simd-intrinsics, 2.0)
+FAST(portability-std-allocator-const, 2.0)
+FAST(readability-avoid-const-params-in-decls, -0.0)
+FAST(readability-braces-around-statements, 2.0)
+FAST(readability-const-return-type, -0.0)
+FAST(readability-container-contains, -0.0)
+FAST(readability-container-data-pointer, 0.0)
+SLOW(readability-container-size-empty, 16.0)
+FAST(readability-convert-member-functions-to-static, 0.0)
+FAST(readability-delete-null-pointer, 0.0)
+FAST(readability-duplicate-include, -0.0)
+FAST(readability-else-after-return, 1.0)
+FAST(readability-function-cognitive-complexity, 0.0)
+FAST(readability-function-size, 3.0)
+FAST(readability-identifier-length, -1.0)
+FAST(readability-identifier-naming, 5.0)
+FAST(readability-implicit-bool-conversion, 2.0)
+FAST(readability-inconsistent-declaration-parameter-name, 1.0)
+FAST(readability-isolate-declaration, 1.0)
+FAST(readability-magic-numbers, -1.0)
+FAST(readability-make-member-function-const, 2.0)
+FAST(readability-misleading-indentation, 0.0)
+FAST(readability-misplaced-array-index, -0.0)
+FAST(readability-named-parameter, -0.0)
+FAST(readability-non-const-parameter, 1.0)
+FAST(readability-qualified-auto, -0.0)
+FAST(readability-redundant-access-specifiers, -1.0)
+FAST(readability-redundant-control-flow, -1.0)
+FAST(readability-redundant-declaration, -0.0)
+FAST(readability-redundant-function-ptr-dereference, -1.0)
+FAST(readability-redundant-member-init, 0.0)
+FAST(readability-redundant-preprocessor, 0.0)
+FAST(readability-redundant-smartptr-get, 6.0)
+FAST(readability-redundant-string-cstr, 0.0)
+FAST(readability-redundant-string-init, 1.0)
+FAST(readability-simplify-boolean-expr, 1.0)
+FAST(readability-simplify-subscript-expr, -0.0)
+FAST(readability-static-accessed-through-instance, 1.0)
+FAST(readability-static-definition-in-anonymous-namespace, -0.0)
+FAST(readability-string-compare, -0.0)
+FAST(readability-suspicious-call-argument, 0.0)
+FAST(readability-uniqueptr-delete-release, 1.0)
+FAST(readability-uppercase-literal-suffix, 3.0)
+FAST(readability-use-anyofallof, 1.0)
+FAST(zircon-temporary-objects, 1.0)
+
+#undef FAST
+#undef SLOW

clang-tools-extra/clangd/TidyFastChecks.py

+#!/usr/bin/env python3
+#
+# Determines which clang-tidy checks are "fast enough" to run in clangd.
+# This runs clangd --check --check-tidy-time and parses the output.
+# This program outputs a header fragment specifying which checks are fast:
+#   FAST(bugprone-argument-comment, 5)
+#   SLOW(misc-const-correctness, 200)
+# If given the old header fragment as input, we lean to preserve its choices.
+#
+# This is not deterministic or hermetic, but should be run occasionally to
+# update the list of allowed checks. From llvm-project:
+#   clang-tools-extra/clangd/TidyFastChecks.py --clangd=build-opt/bin/clangd
+# Be sure to use an optimized, no-asserts, tidy-enabled build of clangd!
+
+import argparse
+import os
+import re
+import subprocess
+import sys
+
+# Checks faster than FAST_THRESHOLD are fast, slower than SLOW_THRESHOLD slow.
+# If a check is in between, we stick with our previous decision. This avoids
+# enabling/disabling checks between releases due to random measurement jitter.
+FAST_THRESHOLD = 8 # percent
+SLOW_THRESHOLD = 15
+
+parser = argparse.ArgumentParser()
+parser.add_argument('--target', help='X-macro output file. '
+    'If it exists, existing contents will be used for hysteresis',
+    default='clang-tools-extra/clangd/TidyFastChecks.inc')
+parser.add_argument('--source', help='Source file to benchmark tidy checks',
+    default='clang/lib/Sema/Sema.cpp')
+parser.add_argument('--clangd', help='clangd binary to invoke',
+    default='build/bin/clangd')
+parser.add_argument('--checks', help='check glob to run', default='*')
+parser.add_argument('--verbose', help='log clangd output', action='store_true')
+args = parser.parse_args()
+
+# Use the preprocessor to extract the list of previously-fast checks.
+def read_old_fast(path):
+    text = subprocess.check_output(["cpp",
+        "-P",            # Omit GNU line markers
+        "-nostdinc",     # Don't include stdc-predef.h
+        "-DFAST(C,T)=C", # Print fast checks only
+        path])
+    for line in text.splitlines():
+        if line.strip():
+            yield line.strip().decode('utf-8')
+old_fast = list(read_old_fast(args.target)) if os.path.exists(args.target) else []
+print(f"Old fast checks: {old_fast}", file=sys.stderr)
+
+# Runs clangd --check --check-tidy-time.
+# Yields (check, percent-overhead) pairs.
+def measure():
+    process = subprocess.Popen([args.clangd,
+        "--check=" + args.source,
+        "--check-locations=0", # Skip useless slow steps.
+        "--check-tidy-time=" + args.checks],
+        stderr=subprocess.PIPE)
+    recording = False
+    for line in iter(process.stderr.readline, b""):
+        if args.verbose:
+            print("clangd> ", line, file=sys.stderr)
+        if not recording:
+            if b'Timing AST build with individual clang-tidy checks' in line:  
+               recording = True
+            continue
+        if b'Finished individual clang-tidy checks' in line:
+            return
+        match = re.search(rb'(\S+) = (\S+)%', line)
+        if match:
+            yield (match.group(1).decode('utf-8'), float(match.group(2)))
+
+with open(args.target, 'w', buffering=1) as target:
+    # Produce an includable X-macros fragment with our decisions.
+    print(f"""// This file is generated, do not edit it directly!
+// Deltas are percentage regression in parsing {args.source}
+#ifndef FAST
+#define FAST(CHECK, DELTA)
+#endif
+#ifndef SLOW
+#define SLOW(CHECK, DELTA)
+#endif
+""", file=target)
+
+    for check, time in measure():
+        threshold = SLOW_THRESHOLD if check in old_fast else FAST_THRESHOLD
+        decision = "FAST" if time <= threshold else "SLOW"
+        print(f"{decision} {check} {time}% <= {threshold}%", file=sys.stderr)
+        print(f"{decision}({check}, {time})", file=target)
+
+    print("""
+#undef FAST
+#undef SLOW
+""", file=target)

clang-tools-extra/clangd/tool/Check.cpp

if (Opts.BuildDynamicSymbolIndex) {
       Index.updateMain(File, *AST);
     }
 
     if (!CheckTidyTime.empty()) {
       if (!CLANGD_TIDY_CHECKS) {
         elog("-{0} requires -DCLANGD_TIDY_CHECKS!", CheckTidyTime.ArgStr);
         return false;
       }
+      #ifndef NDEBUG
+      elog("Timing clang-tidy checks in asserts-mode is not representative!");
+      #endif
       checkTidyTimes();
     }
 
     return true;
   }
 
   // For each check foo, we want to build with checks=-* and checks=-*,foo.
   // (We do a full build rather than just AST matchers to meausre PPCallbacks).