在 Apple M1 上使用 clang 出现“致命错误：找不到‘omp.h’文件”

Question

omp.h每当我尝试使用 openMP 标志进行编译时，Clang 都无法找到。这就是我想做的

clang++ -dynamiclib -I/opt/homebrew/Cellar/eigen/3.3.9/include/eigen3/ -Xpreprocessor -fopenmp -o libfoo.dylib foolibrary.cpp -lomp

虽然我有 omp.h /opt/homebrew/Cellar/libomp/11.0.1/include/omp.h，而 clang 则在usr/bin/clang。我需要配置什么才能让它找到它吗？

Answer 1

当使用brew本地定位M1时，brew安装的clang不在/usr/bin. 冲泡说明显示它的首选位置是/opt/homebrew。/usr/bin 中的 clang 是 Apple 命令行工具之一的链接；你可以通过使用看到这一点-v。

$ /usr/bin/clang -v
/usr/bin/clang -v
Apple clang version 12.0.0 (clang-1200.0.32.29)
Target: arm64-apple-darwin20.3.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

要安装brew，您需要按照安装时brew给您的说明进行操作：-

llvm is keg-only, which means it was not symlinked into /opt/homebrew,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

If you need to have llvm first in your PATH, run:
  echo 'export PATH="/opt/homebrew/opt/llvm/bin:$PATH"' >> ~/.profile

For compilers to find llvm you may need to set:
  export LDFLAGS="-L/opt/homebrew/opt/llvm/lib"
  export CPPFLAGS="-I/opt/homebrew/opt/llvm/include"

这样做可以解决包含问题，但至少对我来说，不能解决链接库的问题。

/opt/homebrew/opt/llvm/bin/clang -v -fopenmp hello_omp.c
clang version 11.1.0
Target: arm64-apple-darwin20.3.0
Thread model: posix
InstalledDir: /opt/homebrew/opt/llvm/bin
 "/opt/homebrew/Cellar/llvm/11.1.0/bin/clang-11" -cc1 -triple arm64-apple-macosx11.0.0 -Wundef-prefix=TARGET_OS_ -Werror=undef-prefix -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name hello_omp.c -mrelocation-model pic -pic-level 2 -mframe-pointer=non-leaf -fno-rounding-math -munwind-tables -fcompatibility-qualified-id-block-type-checking -target-cpu apple-a7 -target-feature +fp-armv8 -target-feature +neon -target-feature +crypto -target-feature +zcm -target-feature +zcz -target-feature +sha2 -target-feature +aes -target-abi darwinpcs -fallow-half-arguments-and-returns -debugger-tuning=lldb -target-linker-version 609.8 -v -resource-dir /opt/homebrew/Cellar/llvm/11.1.0/lib/clang/11.1.0 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -internal-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include -internal-isystem /opt/homebrew/Cellar/llvm/11.1.0/lib/clang/11.1.0/include -internal-externc-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -fdebug-compilation-dir /Users/jcownie/tmp -ferror-limit 19 -fmessage-length=163 -fopenmp -fopenmp-cuda-parallel-target-regions -stack-protector 1 -fblocks -fencode-extended-block-signature -fregister-global-dtors-with-atexit -fgnuc-version=4.2.1 -fmax-type-align=16 -o /var/folders/lt/nf3dtk8j16qfsl97d_vgv4dw000lts/T/hello_omp-bd4ce3.o -x c hello_omp.c
clang -cc1 version 11.1.0 based upon LLVM 11.1.0 default target arm64-apple-darwin20.3.0
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include"
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:
 /opt/homebrew/Cellar/llvm/11.1.0/lib/clang/11.1.0/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)
End of search list.
 "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld" -demangle -lto_library /opt/homebrew/Cellar/llvm/11.1.0/lib/libLTO.dylib -no_deduplicate -dynamic -arch arm64 -platform_version macos 11.0.0 0.0.0 -syslibroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -o a.out /var/folders/lt/nf3dtk8j16qfsl97d_vgv4dw000lts/T/hello_omp-bd4ce3.o -lomp -lSystem /opt/homebrew/Cellar/llvm/11.1.0/lib/clang/11.1.0/lib/darwin/libclang_rt.osx.a
ld: library not found for -lomp
clang-11: error: linker command failed with exit code 1 (use -v to see invocation)

如果您明确通过推荐的$LDFLAGS，那么事情就会起作用......

$ echo $LDFLAGS
-L/opt/homebrew/opt/llvm/lib
$ /opt/homebrew/opt/llvm/bin/clang $LDFLAGS -fopenmp hello_omp.c
/opt/homebrew/opt/llvm/bin/clang $LDFLAGS -fopenmp hello_omp.c
$ ./a.out
./a.out
Hello from thread 0
Hello from thread 5
Hello from thread 4
Hello from thread 1
Hello from thread 3
Hello from thread 2
Hello from thread 7
Hello from thread 6
$ 

我的（虽然很黑客，但有效）解决方案是创建我自己的 shell 脚本来使用适当的库路径调用编译器；你可以在How to build LLVM (clang,clang++) for Apple M1?中看到这个 hack。

请注意，在针对 M1 时，LLVM 中对 OpenMP 任务的支持一度被破坏。（请参阅“将firstprivate args传递给Apple M1上的任务时发生崩溃”）。不过，该问题已于 2021 年 3 月修复，因此请务必使用更新的 LLVM 版本。