clang-tidy does not respect clang++.cfg

[smuzaffar]

Hi,
In order to use gcc headers from our custom gcc installation we create llvm/bin/clang++.cfg file with contents like [a] in it. This allows us to use clang++ without explicitly setting --gcc-toolchain via command line. Looks like clang-tidy does not respect llvm/bin/clang++.cfg and fails with errors like [b].

clang-tidy works if I pass --extra-arg="--gcc-toolchain=/path/to/custon/gcc". I tried creating llvm/bin/clang-tidy.cfg hoping that clang-tidy might read it but this did not work.

Is it on purpose that clang-tidy does not respect clang++.cfg file?

[a]

> cat bin/clang++.cfg
--gcc-toolchain=/path/el8_amd64_gcc12/external/gcc/12.3.1-40d504be6370b5a30e3947a6e575ca28
--target=x86_64-redhat-linux-gnu

[b]

Error while processing .../src/TestSubsystem/TestDQMAnalyzerStream/plugins/TestDQMAnalyzerStream.cc.
.../el8_amd64_gcc12/lcg/root/6.32.13-4719c3acec5bf7dbc47dbafce0eb285d/include/TString.h:39:12: error: 'compare' file not found [clang-diagnostic-error]
   39 | #  include <compare>

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Malik Shahzad Muzaffar (smuzaffar)

Hi, In order to use gcc headers from our custom gcc installation we create `llvm/bin/clang++.cfg` file with contents like [a] in it. This allows us to use `clang++` with explicitly setting `--gcc-toolchain` via command line. Looks like `clang-tidy` does not respect `llvm/bin/clang++.cfg` and fails with errors like [b].

clang-tidy works if I pass --extra-arg="--gcc-toolchain=/path/to/custon/gcc". I tried creating llvm/bin/clang-tidy.cfghoping that clang-tidy` might read it but this did not work.

Is it on purpose that clang-tidy does not respect clang++.cfg file?

[a]

> cat bin/clang++.cfg
--gcc-toolchain=/path/el8_amd64_gcc12/external/gcc/12.3.1-40d504be6370b5a30e3947a6e575ca28
--target=x86_64-redhat-linux-gnu

[b]

Error while processing .../src/TestSubsystem/TestDQMAnalyzerStream/plugins/TestDQMAnalyzerStream.cc.
.../el8_amd64_gcc12/lcg/root/6.32.13-4719c3acec5bf7dbc47dbafce0eb285d/include/TString.h:39:12: error: 'compare' file not found [clang-diagnostic-error]
   39 | #  include <compare>

[vbvictor]

Could you please provide full command of how you run clang-tidy, do you provide path to "compile_commands.json" for clang-tidy?

[smuzaffar]

@vbvictor , the clang-tidy command I run is

clang-tidy -export-fixes tmp/GetFileFormatVersion.cc.yaml -header-filter 'src/.*' src/FWCore/Version/src/GetFileFormatVersion.cc

compile_commands.json is available in current working directory containing the file and command [a]. There is also src/.clang-tidy which contains all the checks and checks options.
[a]

[
  {
    "command": "clang++ -c -DCMS_MICRO_ARCH='\"x86-64-v3\"' -DGNU_GCC -D_GNU_SOURCE -DCMSSW_GIT_HASH='\"CMSSW_15_1_X_2025-05-30-1100\"' -DPROJECT_NAME='\"CMSSW\"' -DPROJECT_VERSION='\"CMSSW_15_1_X_2025-05-30-1100\"' -Isrc -I/full/release/src -O3 -pthread -pipe -Werror=main -Werror=pointer-arith -Werror=overlength-strings -Wno-vla -Werror=overflow -std=c++20 -ftree-vectorize -Werror=array-bounds -Werror=type-limits -fvisibility-inlines-hidden -fno-math-errno --param vect-max-version-for-alias-checks=50 -Xassembler --compress-debug-sections -Wno-error=array-bounds -Warray-bounds -fuse-ld=bfd -march=x86-64-v3 -felide-constructors -fmessage-length=0 -Wall -Wno-long-long -Wreturn-type -Wextra -Wpessimizing-move -Wclass-memaccess -Wno-cast-function-type -Wno-unused-but-set-parameter -Wno-ignored-qualifiers -Wno-unused-parameter -Wunused -Wparentheses -Werror=return-type -Werror=missing-braces -Werror=unused-value -Werror=unused-label -Werror=address -Werror=format -Werror=sign-compare -Werror=write-strings -Werror=delete-non-virtual-dtor -Werror=strict-aliasing -Werror=narrowing -Werror=reorder -Werror=unused-variable -Werror=conversion-null -Wnon-virtual-dtor -Werror=switch -fdiagnostics-show-option -Wno-unused-local-typedefs -Wno-attributes -Wno-psabi -Wno-c99-extensions -Wno-c++11-narrowing -D__STRICT_ANSI__ -Wno-unused-private-field -Wno-unknown-pragmas -Wno-unused-command-line-argument -Wno-unknown-warning-option -ftemplate-depth=512 -Wno-error=potentially-evaluated-expression -Wno-tautological-type-limit-compare -Wno-vla-cxx-extension -fsized-deallocation -DBOOST_DISABLE_ASSERTS -flto=auto -Wodr  -fPIC src/FWCore/Version/src/GetFileFormatVersion.cc -o tmp/el8_amd64_gcc12/src/FWCore/Version/src/FWCoreVersion/GetFileFormatVersion.cc.o",
    "directory": "/build/muz/llvm/CMSSW_15_1_X_2025-05-30-1100",
    "file": "src/FWCore/Version/src/GetFileFormatVersion.cc"
  },
]

[vbvictor]

Thank you! I see there is no mention of clang++.cfg in the compilation database, so doesn't get picked from there.
AFAIK clang-tidy only read compilation database for compile options, so it may be on purpose that clang.cfg isn't picked.
As a "workaround" you can create a file called compile_flags.txt in project root with following contents :

--gcc-toolchain=/path/el8_amd64_gcc12/external/gcc/12.3.1-40d504be6370b5a30e3947a6e575ca28
--target=x86_64-redhat-linux-gnu

It should add these flags to clang-tidy invocation.
https://clang.llvm.org/docs/JSONCompilationDatabase.html#alternatives

[smuzaffar]

thanks @vbvictor for the suggestion but creating this files blocks reading of compile_commands.json. Looks like clang-tidy prefer to read compile_flags.txt first and then fall back to compile_commands.json but it only reads one of these files. A better work-around is to directly add these flags (--gcc-toolchain=... --target=..) in the command parameters in the compile_commands.json file. But my point is that if command in the compile_commands.json is using clang++ and clang++ by default reads llvm/bin/clang++.cfg then clang-tidy should respect it and should also read llvm/bin/clang++.cfg.

[vbvictor]

I'm not sure whether some form of clang-driver configuration should propagate to tools based on LibTooling (different thing from clang-driver). LibTooling tools only depend on compile_commands.json and in compile_commands.json an option --config can be used for specifying clang++.cfg.
@AaronBallman, may you give some opinion on this issue? Basically, it sounds like this:
Should LibTooling tools try to find default clang++/clang configuration files even if it was not specified in compiler database?

[AaronBallman]

CC @HerrCai0907 @5chmidti @PiotrZSL for additional opinions on the clang-tidy side
CC @jansvoboda11 @MaskRay for additional opinions on the clang side

My intuition is that clang++.cfg is specific to the Clang driver and not something general for tooling. However, given that clang-tidy is intended to accept the same options as Clang, I think it may make sense for clang-tidy itself to support the functionality. Whether it should start by looking at a clang-tidy.cfg before falling back to a clang++.cfg (etc) is an open question.

[MaskRay]

I also think clang.cfg clang++.cfg clang-cl.cfg files are specific to the Clang driver.
clang-tidy can be used with Clang not within the same installation or a non-clang compiler (commonly GCC).
The compile_commands.json producer or the user needs to specify --config= so that clang tooling (including clang-tidy) will load the config file.

[HerrCai0907]

I think clang-tidy should follow compile_commands.json, otherwise it may be a problem for cross-compilation. Maybe it could be better to add --config= when generating compile_commands.json instead of let clang-tidy to find clang++.cfg.

[smuzaffar]

thanks for the suggestion. I am fine with generating compile_commands.json with --config option