How to tell Bazel which compiler to use without configuring a new toolchain - c++

Question

I would like to force Bazel to use a new compiler to build my project.
From what I understand I could configure a new toolchain or use some command-line options.
I tried the following options:
-CC=clang bazel build //main:hello-world
-export CC=C:\cygwin64\bin\clang++
- bazel build //main:hello-world --client_env=CC=clang
I think these are not working because if I compile the same hello-world.cc file with
clang hello-world.cc
I get the error:
clang-8: error: linker command failed with exit code 1 (use -v to see invocation)
but with
CC=clang bazel build //main:hello-world
or
bazel build //main:hello-world --client_env=CC=clang
the build is successful.
Any idea why the result is different, and how to solve this?
Where can I check which compiler is Bazel using?
EDIT:
here the output of:
CC=C:\cygwin64\bin\clang bazel build -s //main:hello-world
Starting local Bazel server and connecting to it...
Loading:
Loading: 0 packages loaded
Analyzing: target //main:hello-world (1 packages loaded, 0 targets configured)
Analyzing: target //main:hello-world (11 packages loaded, 18 targets configured)
INFO: Analyzed target //main:hello-world (14 packages loaded, 59 targets configured).
INFO: Found 1 target...
[0 / 6] [Prepa] BazelWorkspaceStatusAction stable-status.txt ... (4 actions, 0 running)
SUBCOMMAND: # //main:hello-world [action 'Compiling main/hello-world.cc', configuration: 1f7c3bee164375155568e7845819f422152903a0240344d14b0c6474746ef185, execution platform: #local_config_platform//:host]
cd C:/cygwin64/home/user/_bazel_user/5r4lncy4/execroot/__main__
SET INCLUDE=C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.25.28610\ATLMFC\include;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.25.28610\include;C:\Program Files (x86)\Windows Kits\NETFXSDK\4.8\include\um;C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\ucrt;C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\shared;C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\um;C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\winrt;C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\cppwinrt
SET PATH=C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.25.28610\bin\HostX64\x64;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\VC\VCPackages;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\TestWindow;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\TeamFoundation\Team Explorer;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\bin\Roslyn;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Team Tools\Performance Tools\x64;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Team Tools\Performance Tools;C:\Program Files (x86)\Microsoft Visual Studio\Shared\Common\VSPerfCollectionTools\vs2019\\x64;C:\Program Files (x86)\Microsoft Visual Studio\Shared\Common\VSPerfCollectionTools\vs2019\;C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.8 Tools\x64\;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\FSharp\;C:\Program Files (x86)\Windows Kits\10\bin\10.0.18362.0\x64;C:\Program Files (x86)\Windows Kits\10\bin\x64;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\\MSBuild\Current\Bin;C:\Windows\Microsoft.NET\Framework64\v4.0.30319;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\Tools\;;C:\WINDOWS\system32;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\Ninja;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\VC\Linux\bin\ConnectionManagerExe
SET PWD=/proc/self/cwd
SET RUNFILES_MANIFEST_ONLY=1
SET TEMP=C:\cygwin64\tmp
SET TMP=C:\cygwin64\tmp
C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.25.28610/bin/HostX64/x64/cl.exe /nologo /DCOMPILER_MSVC /DNOMINMAX /D_WIN32_WINNT=0x0601 /D_CRT_SECURE_NO_DEPRECATE /D_CRT_SECURE_NO_WARNINGS /bigobj /Zm500 /EHsc /wd4351 /wd4291 /wd4250 /wd4996 /I. /Ibazel-out/x64_windows-fastbuild/bin /Iexternal/bazel_tools /Ibazel-out/x64_windows-fastbuild/bin/external/bazel_tools /showIncludes /MD /Od /Z7 /wd4117 -D__DATE__="redacted" -D__TIMESTAMP__="redacted" -D__TIME__="redacted" /Fobazel-out/x64_windows-fastbuild/bin/main/_objs/hello-world/hello-world.obj /c main/hello-world.cc
SUBCOMMAND: # //main:hello-world [action 'Linking main/hello-world.exe', configuration: 1f7c3bee164375155568e7845819f422152903a0240344d14b0c6474746ef185, execution platform: #local_config_platform//:host]
cd C:/cygwin64/home/user/_bazel_user/5r4lncy4/execroot/__main__
SET LIB=C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.25.28610\ATLMFC\lib\x64;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.25.28610\lib\x64;C:\Program Files (x86)\Windows Kits\NETFXSDK\4.8\lib\um\x64;C:\Program Files (x86)\Windows Kits\10\lib\10.0.18362.0\ucrt\x64;C:\Program Files (x86)\Windows Kits\10\lib\10.0.18362.0\um\x64;
SET PATH=C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.25.28610\bin\HostX64\x64;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\VC\VCPackages;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\TestWindow;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\TeamFoundation\Team Explorer;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Current\bin\Roslyn;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Team Tools\Performance Tools\x64;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Team Tools\Performance Tools;C:\Program Files (x86)\Microsoft Visual Studio\Shared\Common\VSPerfCollectionTools\vs2019\\x64;C:\Program Files (x86)\Microsoft Visual Studio\Shared\Common\VSPerfCollectionTools\vs2019\;C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.8 Tools\x64\;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\FSharp\;C:\Program Files (x86)\Windows Kits\10\bin\10.0.18362.0\x64;C:\Program Files (x86)\Windows Kits\10\bin\x64;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\\MSBuild\Current\Bin;C:\Windows\Microsoft.NET\Framework64\v4.0.30319;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\Tools\;;C:\WINDOWS\system32;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\CMake\bin;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\CommonExtensions\Microsoft\CMake\Ninja;C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\Common7\IDE\VC\Linux\bin\ConnectionManagerExe
SET PWD=/proc/self/cwd
SET RUNFILES_MANIFEST_ONLY=1
SET TEMP=C:\cygwin64\tmp
SET TMP=C:\cygwin64\tmp
C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/MSVC/14.25.28610/bin/HostX64/x64/link.exe #bazel-out/x64_windows-fastbuild/bin/main/hello-world.exe-2.params
Target //main:hello-world up-to-date:
bazel-bin/main/hello-world.exe
INFO: Elapsed time: 4.561s, Critical Path: 0.67s
INFO: 2 processes: 2 local.
INFO: Build completed successfully, 6 total actions
INFO: Build completed successfully, 6 total actions
Any idea why this happens? i am using windows 10. Thank you in advance

Answer 1

When not defining your own toolchain, bazel attempts to perform automatic configuration for you which indeed takes into account value of CC variable when defined. Why your bare compilation attempt has failed is not entirely clear from the part of error message posted, but when in doubt, you can add -s to your bazel command line to see commands called and with that see which compiler has been called:
$ CC=/usr/bin/clang bazel build -s //:hello
INFO: Analyzed target //:hello (1 packages loaded, 15 targets configured).
INFO: Found 1 target...
SUBCOMMAND: # //:greet [action 'Compiling greet.cpp', configuration: dd44f3279b21e91676f65783c46b7b7816026ef7e696e80c251418ec64ffdf5e]
(cd /tmp/_bzl/execroot/__main__ && \
exec env - \
PATH=/bin:/usr/bin:/usr/local/bin \
PWD=/proc/self/cwd \
/usr/bin/clang -U_FORTIFY_SOURCE -fstack-protector -Wall -Wthread-safety -Wself-assign -fcolor-diagnostics -fno-omit-frame-pointer '-std=c++0x' -I/usr/lib64/qt/include -MD -MF bazel-out/k8-fastbuild/bin/_objs/greet/greet.pic.d '-frandom-seed=bazel-out/k8-fastbuild/bin/_objs/greet/greet.pic.o' -fPIC -iquote . -iquote bazel-out/k8-fastbuild/bin -Ibazel-out/k8-fastbuild/bin/_virtual_includes/greet -no-canonical-prefixes -Wno-builtin-macro-redefined '-D__DATE__="redacted"' '-D__TIMESTAMP__="redacted"' '-D__TIME__="redacted"' -c greet.cpp -o bazel-out/k8-fastbuild/bin/_objs/greet/greet.pic.o)
SUBCOMMAND: # //:hello [action 'Compiling hello.cpp', configuration: dd44f3279b21e91676f65783c46b7b7816026ef7e696e80c251418ec64ffdf5e]
(cd /tmp/_bzl/execroot/__main__ && \
exec env - \
PATH=/bin:/usr/bin:/usr/local/bin \
PWD=/proc/self/cwd \
/usr/bin/clang -U_FORTIFY_SOURCE -fstack-protector -Wall -Wthread-safety -Wself-assign -fcolor-diagnostics -fno-omit-frame-pointer '-std=c++0x' -I/usr/lib64/qt/include -MD -MF bazel-out/k8-fastbuild/bin/_objs/hello/hello.pic.d '-frandom-seed=bazel-out/k8-fastbuild/bin/_objs/hello/hello.pic.o' -fPIC -iquote . -iquote bazel-out/k8-fastbuild/bin -iquote external/bazel_tools -iquote bazel-out/k8-fastbuild/bin/external/bazel_tools -Ibazel-out/k8-fastbuild/bin/_virtual_includes/greet -no-canonical-prefixes -Wno-builtin-macro-redefined '-D__DATE__="redacted"' '-D__TIMESTAMP__="redacted"' '-D__TIME__="redacted"' -c hello.cpp -o bazel-out/k8-fastbuild/bin/_objs/hello/hello.pic.o)
SUBCOMMAND: # //:hello [action 'Linking hello', configuration: dd44f3279b21e91676f65783c46b7b7816026ef7e696e80c251418ec64ffdf5e]
(cd /tmp/_bzl/execroot/__main__ && \
exec env - \
PATH=/bin:/usr/bin:/usr/local/bin \
PWD=/proc/self/cwd \
/usr/bin/clang #bazel-out/k8-fastbuild/bin/hello-2.params)
Target //:hello up-to-date:
bazel-bin/hello
INFO: Elapsed time: 1.606s, Critical Path: 0.44s
INFO: 3 processes: 3 linux-sandbox.
INFO: Build completed successfully, 5 total actions
Windows is a bit special / different case: I've started looking at the the corresponding rules_cc, but from there actually followed back to the docs, currently recommended method would be to add a platform definition (e.g. in your workspace root BUILD):
platform(
name = "x64_windows-clang-cl",
constraint_values = [
"#platforms//cpu:x86_64",
"#platforms//os:windows",
"#bazel_tools//tools/cpp:clang-cl",
],
)
And registering the additional toolchain in your WORKSPACE file:
register_execution_platforms(
":x64_windows-clang-cl"
)
register_toolchains(
"#local_config_cc//:cc-toolchain-x64_windows-clang-cl",
)
Then (for now) use --incompatible_enable_cc_toolchain_resolution flag with bazel.
At least at the moment, alternatively you should be able to tell bazel to use LLVM on Windows by running bazel with: --compiler=clang-cl.
You may need to set BAZEL_LLVM (this time) to a directory where LLVM is installed (or try relying on automatic resolution).
That said, it does not look like you could use cygwin LLVM compiler on Windows while still relying on automatic toolchain configuration. I suspect you would really need to define your own toolchain for that (not sure how substantial problems would you encounter crossing the boundary between bazel for Windows and cygwin based compiler each having different idea about what path names to use).