[CMake] Remove unused variable LLDB_TEST_CXX_COMPILER

[JDevlieghere]

Summary

CMake allows you to set a custom CXX compiler for the API test suite.
However, this variable is never used, because dotest uses the same
compiler to build C and CXX sources.

I'm not sure if this variable was added with the intention of supporting
a different compiler or if this is just a remnant of old functionality.
Given that this hasn't been working for a while, I assume it's safe to
remove.

[JDevlieghere]

I did a bit of digging and there's more context in https://reviews.llvm.org/D39215. It sounds to me like the motivation for the two variables isn't relevant anymore, so I still think removing the CXX variant is desirable.

[labath]

Overall, I think it would be better to just pass the c++ compiler instead of having dotest try to guess it from the c compiler name. But, it's easy to reintroduce this option once we get to that point.

[xiaobai]

I thought this option was so that you could specify which compilers to use to build from a cache or command line? I don't think you just want to set LLDB_TEST_COMPILER to clang since clang++ is what is used to compile C++ code. Maybe I'm mis-remembering or mistaken here.

[JDevlieghere]

In D69401#1720432, @xiaobai wrote:

I thought this option was so that you could specify which compilers to use to build from a cache or command line? I don't think you just want to set LLDB_TEST_COMPILER to clang since clang++ is what is used to compile C++ code. Maybe I'm mis-remembering or mistaken here.

Yes, that's indeed the point. However, dotest expects you to set the C compiler and then infers the C++ compiler from it, which for clang is just appending ++ at the end. I'm sure it must do something sensible for gcc as well, as I believe people are running the test suite with that compiler.

[xiaobai]

In D69401#1720494, @JDevlieghere wrote:

In D69401#1720432, @xiaobai wrote:

I thought this option was so that you could specify which compilers to use to build from a cache or command line? I don't think you just want to set LLDB_TEST_COMPILER to clang since clang++ is what is used to compile C++ code. Maybe I'm mis-remembering or mistaken here.

Yes, that's indeed the point. However, dotest expects you to set the C compiler and then infers the C++ compiler from it, which for clang is just appending ++ at the end. I'm sure it must do something sensible for gcc as well, as I believe people are running the test suite with that compiler.

Right, I see. I'm not sure I like the inference in dotest, and I assume cl is left untouched here (if it's even possible to use cl as the compiler on windows for tests). I do like the simplification to the build though so ship it!

lldb/CMakeLists.txt

   set(LLDB_DEFAULT_TEST_FILECHECK "${LLVM_BINARY_DIR}/${CMAKE_CFG_INTDIR}/bin/FileCheck${CMAKE_EXECUTABLE_SUFFIX}")
 
   if (TARGET clang)
-    set(LLDB_DEFAULT_TEST_C_COMPILER "${LLVM_BINARY_DIR}/${CMAKE_CFG_INTDIR}/bin/clang${CMAKE_EXECUTABLE_SUFFIX}")
-    set(LLDB_DEFAULT_TEST_CXX_COMPILER "${LLVM_BINARY_DIR}/${CMAKE_CFG_INTDIR}/bin/clang++${CMAKE_EXECUTABLE_SUFFIX}")
+    set(LLDB_DEFAULT_TEST_COMPILER "${LLVM_BINARY_DIR}/${CMAKE_CFG_INTDIR}/bin/clang${CMAKE_EXECUTABLE_SUFFIX}")
   else()
-    set(LLDB_DEFAULT_TEST_C_COMPILER "")
-    set(LLDB_DEFAULT_TEST_CXX_COMPILER "")
+    set(LLDB_DEFAULT_TEST_COMPILER "")
   endif()
 
   set(LLDB_TEST_EXECUTABLE "${LLDB_DEFAULT_TEST_EXECUTABLE}" CACHE PATH "lldb executable used for testing")
-  set(LLDB_TEST_C_COMPILER "${LLDB_DEFAULT_TEST_C_COMPILER}" CACHE PATH "C Compiler to use for building LLDB test inferiors")
-  set(LLDB_TEST_CXX_COMPILER "${LLDB_DEFAULT_TEST_CXX_COMPILER}" CACHE PATH "C++ Compiler to use for building LLDB test inferiors")
+  set(LLDB_TEST_COMPILER "${LLDB_DEFAULT_TEST_COMPILER}" CACHE PATH "C Compiler to use for building LLDB test inferiors")
   set(LLDB_TEST_DSYMUTIL "${LLDB_DEFAULT_TEST_DSYMUTIL}" CACHE PATH "dsymutil used for generating dSYM bundles")
   set(LLDB_TEST_FILECHECK "${LLDB_DEFAULT_TEST_FILECHECK}" CACHE PATH "FileCheck used for testing purposes")
 
-  if (("${LLDB_TEST_C_COMPILER}" STREQUAL "") OR
-      ("${LLDB_TEST_CXX_COMPILER}" STREQUAL ""))
-    message(FATAL_ERROR "LLDB test compilers not specified. Tests will not run.")
+  if ("${LLDB_TEST_COMPILER}" STREQUAL "")
+    message(FATAL_ERROR "LLDB test compiler not specified. Tests will not run.")
   endif()
 
   add_custom_target(lldb-test-deps)

lldb/docs/resources/build.rst

 ::
 
   > cmake -G Ninja \
-      -DLLDB_TEST_C_COMPILER=<path to C compiler> \
-      -DLLDB_TEST_CXX_COMPILER=<path to C++ compiler> \
+      -DLLDB_TEST_COMPILER=<path to C compiler> \
       <path to root of llvm source tree>
 
 It is strongly recommend to use a release build for the compiler to speed up

   > cmake -G Ninja^
       -DLLDB_TEST_DEBUG_TEST_CRASHES=1^
       -DPYTHON_HOME=C:\Python35^
-      -DLLDB_TEST_C_COMPILER=d:\src\llvmbuild\ninja_release\bin\clang.exe^
+      -DLLDB_TEST_COMPILER=d:\src\llvmbuild\ninja_release\bin\clang.exe^
       <path to root of llvm source tree>
 
 

lldb/docs/resources/test.rst

 
 By default, the ``check-lldb`` target builds the test programs with the same
 compiler that was used to build LLDB. To build the tests with a different
-compiler, you can set the ``LLDB_TEST_C_COMPILER`` or the
-``LLDB_TEST_CXX_COMPILER`` CMake variables.
+compiler, you can set the ``LLDB_TEST_COMPILER`` CMake variable.
 
 It is possible to customize the architecture of the test binaries and compiler
 used by appending ``-A`` and ``-C`` options respectively to the CMake variable

lldb/test/API/CMakeLists.txt

   --executable ${LLDB_TEST_EXECUTABLE}
   --dsymutil ${LLDB_TEST_DSYMUTIL}
   --filecheck ${LLDB_TEST_FILECHECK}
-  -C ${LLDB_TEST_C_COMPILER}
+  --compiler ${LLDB_TEST_COMPILER}
   )
 
 if ( CMAKE_SYSTEM_NAME MATCHES "Windows" )