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

[David Spickett 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" \
----------------
DavidSpickett wrote:

Added the variables.

There's two parts to why we need so many sysroots. The first is needing C library headers at all, so I've added a note at the beginning of the doc saying that.

The second is making sure the toolchain is set up for the correct target so you don't build "bare metal" builtins that actually make some Linux assumption. That I have noted in this section in a new paragraph before the cmake command.

I was actually able to use the A profile toolchain for both builtins (M profile target) and tests (A profile target). I don't want to recommend that though because it seems like a compatibility issue waiting to happen.

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