[llvm] [BOLT] Enable cross compilation of runtime libraries (PR #101180)

[Gergely Bálint via llvm-commits]

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@@ -135,36 +134,168 @@ if (LLVM_INCLUDE_TESTS)
   endif()
 endif()
 
-if (BOLT_ENABLE_RUNTIME)
-  message(STATUS "Building BOLT runtime libraries for X86")
-  set(extra_args "")
-  if(CMAKE_SYSROOT)
-    list(APPEND extra_args -DCMAKE_SYSROOT=${CMAKE_SYSROOT})
+set(AARCH64_GNU_C_COMPILER aarch64-linux-gnu-gcc)
+set(AARCH64_GNU_CXX_COMPILER aarch64-linux-gnu-g++)
+set(X86_64_GNU_C_COMPILER x86_64-linux-gnu-gcc)
+set(X86_64_GNU_CXX_COMPILER x86_64-linux-gnu-g++)
+set(RISCV_GNU_C_COMPILER riscv64-linux-gnu-gcc)
+set(RISCV_GNU_CXX_COMPILER riscv64-linux-gnu-g++)
+
+function(bolt_rt_target_supported_clang target supported)
+
+    if(${target} STREQUAL ${HOST_NAME})
+      set(${supported} TRUE PARENT_SCOPE)
+      return()
+    elseif(${target} STREQUAL "X86")
+      set(CMAKE_CXX_FLAGS "--target=x86_64-linux-gnu")
+    elseif(${target} STREQUAL "AArch64")
+      set(CMAKE_CXX_FLAGS "--target=aarch64-linux-gnu")
+    elseif(${target} STREQUAL "RISCV")
+      set(CMAKE_CXX_FLAGS "--target=riscv64-linux-gnu")
+    endif()
+
+    try_compile(CROSS_COMP
+      ${CMAKE_BINARY_DIR}/CMakeFiles/CMakeScratch
+      SOURCES ${CMAKE_CURRENT_SOURCE_DIR}/cmake/test/test.cpp
+      CMAKE_FLAGS ${CMAKE_CXX_FLAGS}
+      TRY_COMP_OUTPUT)
----------------
bgergely0 wrote:

After thorough investigation, I dont think it is possible to use `CMAKE_FLAGS` to set a target, as it is only used for `-DVAR=VALUE` type flags, and there is no way to easily pass target to `try_compile`. 
Appending `--target` to `CMAKE_CXX_FLAGS` seems to be the best option, as the appended target string is only scoped to the function, and will not affect the compilation process outside of it.

https://github.com/llvm/llvm-project/pull/101180