[builtins] Make sure builtin compile tests respect CMAKE_C_COMPILER_TARGET

[fjricci]

Summary

Since we generate the compiler invocation on our own, we need to
manually add -target if CMAKE_C_COMPILER_TARGET has been specified.

[fjricci]

Improved cmake syntax

[compnerd]

cmake/Modules/BuiltinTests.cmake

17 ↗

(On Diff #69063)

I think I prefer the following syntax:

list(APPEND TRY_COMPILE_FLAGS -target;${CMAKE_C_COMPILER_TARGET})

[fjricci]

Update cmake style and check compiler ID

[beanz]

CMAKE_C_COMPILER_TARGET is an oddly used option. CMake only uses it in two places, one is in the tests, and the other is in the logic for finding the compiler. Setting CMAKE_C_COMPILER_TARGET and not explicitly overriding your compiler to clang, could result in undesirable behavior on systems that have GCC cross-compilers installed, so I'm a little concerned about using this option. It might make more sense to use COMPILER_RT_DEFAULT_TARGET_TRIPLE instead.

cmake/Modules/BuiltinTests.cmake

17 ↗

(On Diff #69134)

list(APPEND...) is better, but don't put a semi-colon between the elements, a space will suffice. Using a semi-colon is odd because instead of appending a list to a list you're appending a string-ified list.

[fjricci]

In order for cmake to check whether Clang can cross-compile for a given target, -target <target> needs to be set. One could do this either by explicitly adding it to CMAKE_<LANG>_FLAGS or by setting CMAKE_<LANG>_COMPILER_TARGET. The problem with using COMPILER_RT_DEFAULT_TARGET_TRIPLE is that this won't set -target.

A further complication is that COMPILER_RT_DEFAULT_TARGET_TRIPLE and the desired -target option aren't always the same. For example, to build for linux-armhf, COMPILER_RT_DEFAULT_TARGET_TRIPLE expects armhf-linux-gnueabihf, but you need to build with -target armv7-linux-gnueabihf, so there isn't a 1:1 mapping.

Presumably this could be fixed by reworking COMPILER_RT_DEFAULT_TARGET_TRIPLE, but that's more involved, and this seemed like a simpler solution.

[fjricci]

Remove semi-colon in list append

[beanz]

You’re right. My grep of CMake's sources missed the generator uses where it adds --target to the compiler invocations. I completely understand why you're using it this way.

Patch LGTM.

compiler-rt/trunk/cmake/Modules/BuiltinTests.cmake

 include(CMakeCheckCompilerFlagCommonPatterns)
 
 # This function takes an OS and a list of architectures and identifies the
 # subset of the architectures list that the installed toolchain can target.
 function(try_compile_only output)
   cmake_parse_arguments(ARG "" "" "SOURCE;FLAGS" ${ARGN})
   if(NOT ARG_SOURCE)
     set(ARG_SOURCE "int foo(int x, int y) { return x + y; }\n")
   endif()
   set(SIMPLE_C ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/src.c)
   file(WRITE ${SIMPLE_C} "${ARG_SOURCE}\n")
   string(REGEX MATCHALL "<[A-Za-z0-9_]*>" substitutions
          ${CMAKE_C_COMPILE_OBJECT})
-  string(REPLACE ";" " " extra_flags "${ARG_FLAGS}")
+
+  set(TRY_COMPILE_FLAGS "${ARG_FLAGS}")
+  if(CMAKE_C_COMPILER_ID MATCHES Clang AND CMAKE_C_COMPILER_TARGET)
+    list(APPEND TRY_COMPILE_FLAGS "-target ${CMAKE_C_COMPILER_TARGET}")
+  endif()
+
+  string(REPLACE ";" " " extra_flags "${TRY_COMPILE_FLAGS}")
 
   set(test_compile_command "${CMAKE_C_COMPILE_OBJECT}")
   foreach(substitution ${substitutions})
     if(substitution STREQUAL "<CMAKE_C_COMPILER>")
       string(REPLACE "<CMAKE_C_COMPILER>"
              "${CMAKE_C_COMPILER}" test_compile_command ${test_compile_command})
     elseif(substitution STREQUAL "<OBJECT>")
       string(REPLACE "<OBJECT>"