RFR: 8278954: Using clang together with devkit on linux doesn't work for building

[Gilles Duboscq]

On Fri, 17 Dec 2021 13:35:04 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

>> When running `configure`, using `--with-devkit=` to point to a typical linux devkit along with `--with-toolchain-path=` and `--with-toolchain-type=clang` to point to a clang-based toolchain results in:
>> 
>> configure:75064: checking whether the C compiler works
>> configure:75086: /dl/tools/clang -m64 -isysroot /dl/x86_64-linux-gnu-to-x86_64-linux-gnu/x86_64-linux-gnu/sysroot -isysroot /dl/x86_64-linux-gnu-to-x86_64-linux-gnu/x86_64-linux-gnu/sysroot -m64 -isysroot /dl/x86_64-linux-gnu-to-x86_64-linux-gnu/x86_64-linux-gnu/sysroot conftest.c >&5
>> d.lld: error: cannot open crt1.o: No such file or directory
>> ld.lld: error: cannot open crti.o: No such file or directory
>> ld.lld: error: cannot open crtbegin.o: No such file or directory
>> ld.lld: error: unable to find library -lgcc
>> ld.lld: error: unable to find library -lgcc_s
>> ld.lld: error: unable to find library -lc
>> ld.lld: error: unable to find library -lgcc
>> ld.lld: error: unable to find library -lgcc_s
>> ld.lld: error: cannot open crtend.o: No such file or directory
>> ld.lld: error: cannot open crtn.o: No such file or directory
>> clang-12: error: linker command failed with exit code 1 (use -v to see invocation)
>> ``` 
>> This is because clang is unable to locate the gcc installation from the devkit.
>> The gcc toolchain is not in the location clang expects in the sysroot (that's not how our devkits are structured).
>> Note that it might go unnoticed on machines that have gcc installed because clang will fallback to system-global locations.
>> 
>> We can help clang by letting it now about the gcc location in the devkit with `--gcc-toolchain=`.
>> However that's not enough and we then get:
>> 
>> ld.lld: error: cannot open crt1.o: No such file or directory
>> ld.lld: error: cannot open crti.o: No such file or directory
>> ld.lld: error: unable to find library -lc
>> ld.lld: error: cannot open crtn.o: No such file or directory
>> 
>> clang was able to locate the gcc support libraries but is not able to locate required system libraries.
>> That's because `-isysroot` is not intended for libraries (only headers) but also because `-isysroot` has no effect for clang on linux (see [llvm#11503](https://bugs.llvm.org/show_bug.cgi?id=11503)).
>> Using `--sysroot=` in this case (clang on linux) resolves this issue.
>
> Marked as reviewed by erikj (Reviewer).

Thanks for the prompt review @erikj79!

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PR: https://git.openjdk.java.net/jdk/pull/6880