[llvm] [llvm][docs] Update CMake commands for cross compiling Arm builtins (PR #151544)

[Eli Friedman via llvm-commits]

================
@@ -235,31 +252,55 @@ into a binary and execute the tests correctly but it will not catch if the
 builtins use instructions that are supported on Armv7-A but not Armv6-M,
 Armv7-M and Armv7E-M.
 
-To get the cmake compile test to pass you will need to pass the libraries
-needed to successfully link the cmake test via ``CMAKE_CFLAGS``::
-
- -DCMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY \
- -DCOMPILER_RT_OS_DIR="baremetal" \
- -DCOMPILER_RT_BUILD_BUILTINS=ON \
- -DCOMPILER_RT_BUILD_SANITIZERS=OFF \
- -DCOMPILER_RT_BUILD_XRAY=OFF \
- -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
- -DCOMPILER_RT_BUILD_PROFILE=OFF \
- -DCMAKE_C_COMPILER=${host_install_dir}/bin/clang \
- -DCMAKE_C_COMPILER_TARGET="your *-none-eabi target" \
- -DCMAKE_ASM_COMPILER_TARGET="your *-none-eabi target" \
- -DCMAKE_AR=/path/to/llvm-ar \
- -DCMAKE_NM=/path/to/llvm-nm \
- -DCMAKE_RANLIB=/path/to/llvm-ranlib \
- -DCOMPILER_RT_BAREMETAL_BUILD=ON \
- -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON \
- -DLLVM_CONFIG_PATH=/path/to/llvm-config \
- -DCMAKE_C_FLAGS="build-c-flags" \
- -DCMAKE_ASM_FLAGS="build-c-flags" \
- -DCOMPILER_RT_EMULATOR="qemu-arm -L /path/to/armv7-A/sysroot" \
- -DCOMPILER_RT_INCLUDE_TESTS=ON \
- -DCOMPILER_RT_TEST_COMPILER="/path/to/clang" \
- -DCOMPILER_RT_TEST_COMPILER_CFLAGS="test-c-flags"
+Below is an example that builds the builtins for Armv7-M, but runs the tests
+as Armv7-A. It is presented in full, but is very similar to the earlier
+command for Armv7-A build and test::
+
+  LLVM_TOOLCHAIN=<path-to-llvm-install>/
+  TARGET_TRIPLE=arm-none-eabi
+  GCC_TOOLCHAIN=<path-to-gcc-toolchain>
+  SYSROOT=${GCC_TOOLCHAIN}/${TARGET_TRIPLE}/libc
+  COMPILE_FLAGS="-march=armv7-m -mfpu=vfpv2"
+
+  cmake ../llvm-project/compiler-rt \
+    -G Ninja \
+    -DCMAKE_AR=${LLVM_TOOLCHAIN}/bin/llvm-ar \
+    -DCMAKE_NM=${LLVM_TOOLCHAIN}/bin/llvm-nm \
+    -DCMAKE_RANLIB=${LLVM_TOOLCHAIN}/bin/llvm-ranlib \
+    -DLLVM_CMAKE_DIR="${LLVM_TOOLCHAIN}/lib/cmake/llvm" \
+    -DCMAKE_SYSROOT="${SYSROOT}" \
+    -DCMAKE_ASM_COMPILER_TARGET="${TARGET_TRIPLE}" \
+    -DCMAKE_ASM_FLAGS="${COMPILE_FLAGS}" \
+    -DCMAKE_C_COMPILER_TARGET="${TARGET_TRIPLE}" \
+    -DCMAKE_C_COMPILER_EXTERNAL_TOOLCHAIN=${GCC_TOOLCHAIN} \
+    -DCMAKE_C_COMPILER=${LLVM_TOOLCHAIN}/bin/clang \
+    -DCMAKE_C_FLAGS="${COMPILE_FLAGS}" \
+    -DCMAKE_CXX_COMPILER_TARGET="${TARGET_TRIPLE}" \
+    -DCMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN=${GCC_TOOLCHAIN} \
+    -DCMAKE_CXX_COMPILER=${LLVM_TOOLCHAIN}/bin/clang \
+    -DCMAKE_CXX_FLAGS="${COMPILE_FLAGS}" \
+    -DCMAKE_EXE_LINKER_FLAGS="-fuse-ld=lld" \
+    -DCOMPILER_RT_BUILD_BUILTINS=ON \
+    -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
+    -DCOMPILER_RT_BUILD_MEMPROF=OFF \
+    -DCOMPILER_RT_BUILD_PROFILE=OFF \
+    -DCOMPILER_RT_BUILD_CTX_PROFILE=OFF \
+    -DCOMPILER_RT_BUILD_SANITIZERS=OFF \
+    -DCOMPILER_RT_BUILD_XRAY=OFF \
+    -DCOMPILER_RT_BUILD_ORC=OFF \
+    -DCOMPILER_RT_BUILD_CRT=OFF \
+    -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON \
+    -DCOMPILER_RT_EMULATOR="qemu-arm -L <path to arm-none-linux-gnueabihf toolchain>/arm-none-linux-gnueabihf/libc" \
----------------
efriedma-quic wrote:

Add a variable for `<path to arm-none-linux-gnueabihf toolchain>`, since you're using it in multiple places?

And maybe a brief explanation of why you need the baremetal toolchain, since it isn't obvious (ref #127227).

https://github.com/llvm/llvm-project/pull/151544