Fix cross compilation

[Leo-Yan]

When cross building riscv64, we need to specify ARCH=riscv so that this can set to $(SRCARCH) and can find the asm headers correctly.

Also install Clang and related libs.

[acmel]

Hey, have you looked at the produced binaries? I'm not understanding how installing the host compilers and the host devel packages will end up producing a binary for the target arch, can you please elaborate?

[Leo-Yan]

Hi @acmel ,

As claimed in [1]: "... Clang/LLVM is natively a cross-compiler, meaning that one set of programs can compile to all targets by setting the -target option."

So we don't need to install extra arch specific Clang/LLVM packages, as the official Clang/LLVM can support cross compilation.

I can confirm after using Clang for cross compilation arm64, I can get the binary info:

/tmp/build/perf/perf: ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-aarch64.so.1, BuildID[sha1]=9655da4ec0f34ba91b400     b4b23aa347c99ecc6b8, for GNU/Linux 3.7.0, with debug_info, not stripped

[1] https://clang.llvm.org/docs/CrossCompilation.html

[acmel]

Ok, that covers the clang, so since rx_and_build.sh already passes ARCH, no need to have the CROSS_COMPILE prefix for the clang binary, excellent.

But is ubuntu multi-lib in the sense that the devel packages have the .so files for all target architectures?

[acmel]

Reading that clang.llvm.org doc on x-compilation, this part:

Another problem is that compilers come with standard libraries only
(like compiler-rt, libcxx, libgcc, libm, etc), so you’ll have to find and make
available to the build system, every other library required to build your
software, that is specific to your target. It’s not enough to have your
host’s libraries installed.

and:

Target Specific Libraries

[Leo-Yan]

But is ubuntu multi-lib in the sense that the devel packages have the .so files for all target architectures?

Ubuntu's multi-arch libraries can be used by both GCC and Clang. For example, if installed libtraceevent:arm64 on Ubuntu, then whether we build perf with aarch64-linux-gnu-gcc or with clang, the build will find the Arm64 version's libtraceevent.

The $(CROSS_COMPILE) variable plays an important role: it extends $PKG_CONFIG_LIBDIR (e.g., to /usr/lib/aarch64-linux-gnu/pkgconfig), which guides the Makefile to find correct headers and libraries by using pkg-config.

[Leo-Yan]

Just note for Android build:

So far we can use Clang provided by Android NDK to build perf binary. If build with static linkage, we should have no concern for running perf on Android.

On the other hand, except the Clang/LLVM is included in Android NDK package, it also contains sysroot and libs:

$ ls /data_nvme1n1/niayan01/android-ndk-r27d/toolchains/llvm/prebuilt/linux-x86_64/lib/
aarch64-unknown-linux-musl    i686-w64-windows-gnu  libc++.so     liblldbIntelFeatures.so  libscanbuild  python3
arm-unknown-linux-musleabihf  libbolt_rt_instr.a    libc++.so.1   liblldb.so               libunwind.so  python3.11
clang                         libc++.a              libear        libncurses.so.6          libxml2.so    x86_64-unknown-linux-gnu
i386-unknown-linux-gnu        libc++abi.a           libedit.so.0  libpanel.so.6            libxml2.so.2  x86_64-unknown-linux-musl
i686-unknown-linux-musl       libc++abi.so.1        libform.so.6  libRemarks.so            LLVMPolly.so  x86_64-w64-windows-gnu

Because Android NDK does not provide pkgconfig, so we cannot smoothly link these libs. Later we might consider to improve this a bit if have requirement.