[compiler-rt] Check for missing CMAKE_C_COMPILER_TARGET when using COMPILER_RT_DEFAUL...

COMPILER_RT_DEFAULT_TARGET_ONLY is the goto workaround for building
only one of a multilib setup while still being able to autodetect the native target.
For example only building 64 bit libraries on Intel instead of 32 and 64 bit.
Which may fail without 32 bit compatibility libs installed.

As reported in https://discourse.llvm.org/t/configuring-compiler-rt-to-use-default-target-only-does-not-work/62727
this build config hasn't worked in a while.

It relies on a variable usually set during cross compliation,
though my testing shows you can set it in other scenarios too.
https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_COMPILER_TARGET.html

So the options you need to provide are:
-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON -DCMAKE_C_COMPILER_TARGET="aarch64-unknown-linux-gnu"

And we should error if CMAKE_C_COMPILER_TARGET isn't set.

Reviewed By: glaubitz

Differential Revision: https://reviews.llvm.org/D147598

diff --git a/compiler-rt/cmake/Modules/CompilerRTUtils.cmake b/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
index eefc466..f15917f 100644 (file)
--- a/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
+++ b/compiler-rt/cmake/Modules/CompilerRTUtils.cmake
@@ -358,6 +358,10 @@ macro(construct_compiler_rt_default_triple)
     if(DEFINED COMPILER_RT_DEFAULT_TARGET_TRIPLE)
       message(FATAL_ERROR "COMPILER_RT_DEFAULT_TARGET_TRIPLE isn't supported when building for default target only")
     endif()
+    if ("${CMAKE_C_COMPILER_TARGET}" STREQUAL "")
+      message(FATAL_ERROR "CMAKE_C_COMPILER_TARGET must also be set when COMPILER_RT_DEFAUT_TARGET_ONLY is ON")
+    endif()
+    message(STATUS "cmake c compiler target: ${CMAKE_C_COMPILER_TARGET}")
     set(COMPILER_RT_DEFAULT_TARGET_TRIPLE ${CMAKE_C_COMPILER_TARGET})
   else()
     set(COMPILER_RT_DEFAULT_TARGET_TRIPLE ${LLVM_TARGET_TRIPLE} CACHE STRING