--sysroot does not setup correct include paths

[rgrr]

Summary
See subject

To Reproduce

1. create an empty main.cpp or something else
2. invoke "clang --sysroot /home/hardy/bin/llvm-arm-none-eabi/lib/clang-runtimes/arm-none-eabi/armv8m.main_soft_nofp_unaligned_size --target=armv8m.main-none-eabi -v main.cpp"

Expected behavior
output shows correct include paths

Actual behavior
output shows formerly valid include paths which are now wrong

Environment
21.1.1, Debian testing, Release

[Additional context]
From 20.1.0 to 21.1.1 directory structure of clang-runtimes was changed: now all architectures share an include path under which common header files are kept. This avoids duplicate header files.
Unfortunately the include path created via --sysroot still shows the old structure.

-isysroot         /home/hardy/bin/llvm-arm-none-eabi/lib/clang-runtimes/arm-none-eabi/armv8m.main_soft_nofp_unaligned_size
-internal-isystem /home/hardy/bin/llvm-arm-none-eabi/lib/clang-runtimes/arm-none-eabi/armv8m.main_soft_nofp_unaligned_size/include/c++/v1
-internal-isystem /home/hardy/bin/llvm-arm-none-eabi/lib/clang-runtimes/arm-none-eabi/armv8m.main_soft_nofp_unaligned_size/include

Actually it should be

-isysroot         /home/hardy/bin/llvm-arm-none-eabi/lib/clang-runtimes/arm-none-eabi/armv8m.main_soft_nofp_unaligned_size
-internal-isystem /home/hardy/bin/llvm-arm-none-eabi/lib/clang-runtimes/arm-none-eabi/include/c++/v1
-internal-isystem /home/hardy/bin/llvm-arm-none-eabi/lib/clang-runtimes/arm-none-eabi/include

[voltur01]

Hi, yes, you are correct this change in behavior was documented here https://github.com/arm/arm-toolchain/blob/arm-software/arm-software/embedded/CHANGELOG.md#removed-1

The reason for changing was to minimize the amount of duplication and thus the size of the package - at became unusable on Windows.

Using --sysroot was not recommended before either because of active work on library variants and frequent changes of library variant folder names which where "breaking" hardcoded paths.

Please use the multilib driver logic and do let us know if you see incorrect or sub-optimal library variant selection.

[rgrr]

Thanks for the hint.

Do I understand it correctly, that now sysroot points to the root of the library (e.g. --sysroot=$HOME/bin/llvm-arm-none-eabi-21.1.1/lib/clang-runtimes/llvmlibc) and everything else happens automatically?

[rgrr]

Thank you again.

My original cause is now solved much cleaner, see raspberrypi/pico-sdk#2700

I'm closing this now.

[voltur01]

Hi, sorry, I was off for a few days.

Do I understand it correctly, that now sysroot points to the root of the library

yes, it points to the folder with the yaml file used by the driver to select the library.

One more improvement that we consider is providing an option to select the library to use https://discourse.llvm.org/t/rfc-add-command-line-option-for-selecting-c-library/87335 this will avoid the need to check for folder names under the clang-runtimes to figure out what libraries are available.