[LLVM][AIX] Prefer a 32-bit default target triple on AIX

[daltenty]

Summary

If the user doesn't specify a default target triple, the LLVM CMake usually defaults us into the host triple. This is a problem when building Clang/LLVM on 64-bit AIX (i.e. powerpc64-ibm-aix), as the host toolchain (e.g. ar, ld, nm, dump) all expect the compiler to generate 32-bit objects by default (which both GCC and XL on the platform do) and will hard error if passed a 64-bit object without an explicit option or environment setting. This breaks downstream consumers, such as builds generated with build tools like CMake, which when they invoke clang, etc. without explicit bitmode flags also expect 32-bit mode.

This patch changes the default target selection when the host is powerpc64-ibm-aix to prefer powerpc-ibm-aix to avoid these issues. We don't update the runtimes/CMakeList.txt since the default is less meaningful as we assume runtimes will need to build for both targets anyways.

[hubert.reinterpretcast]

LGTM with an observation.

llvm/CMakeLists.txt

706–708

This relies on the user not explicitly configuring the host triple to include the AIX version.

[daltenty]

Slightly tweak the match and replace to preserve any custom OS versioned triple the user may have passed in.

llvm/CMakeLists.txt

 if (LLVM_BUILD_STATIC)
   set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -static")
   # Remove shared library suffixes from use in find_library
   foreach (shared_lib_suffix ${CMAKE_SHARED_LIBRARY_SUFFIX} ${CMAKE_IMPORT_LIBRARY_SUFFIX})
     list(FIND CMAKE_FIND_LIBRARY_SUFFIXES ${shared_lib_suffix} shared_lib_suffix_idx)
     if(NOT ${shared_lib_suffix_idx} EQUAL -1)
       list(REMOVE_AT CMAKE_FIND_LIBRARY_SUFFIXES ${shared_lib_suffix_idx})
     endif()
   endforeach()
 endif()
 
 # Use libtool instead of ar if you are both on an Apple host, and targeting Apple.
 if(CMAKE_HOST_APPLE AND APPLE)
   include(UseLibtool)
 endif()
 
 # Override the default target with an environment variable named by LLVM_TARGET_TRIPLE_ENV.
 set(LLVM_TARGET_TRIPLE_ENV CACHE STRING "The name of environment variable to override default target. Disabled by blank.")
 mark_as_advanced(LLVM_TARGET_TRIPLE_ENV)
 
 set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR OFF CACHE BOOL
   "Enable per-target runtimes directory")
 
 set(LLVM_PROFDATA_FILE "" CACHE FILEPATH
   "Profiling data file to use when compiling in order to improve runtime performance.")
 
 # All options referred to from HandleLLVMOptions have to be specified
 # BEFORE this include, otherwise options will not be correctly set on
 # first cmake run
 include(config-ix)
 
 # By default, we target the host, but this can be overridden at CMake
-# invocation time.
-set(LLVM_DEFAULT_TARGET_TRIPLE "${LLVM_HOST_TRIPLE}" CACHE STRING
+# invocation time. Except on 64-bit AIX, where the system toolchain
+# expect 32-bit objects by default.
+if("${LLVM_HOST_TRIPLE}" MATCHES "^powerpc64-ibm-aix")
+  string(REGEX REPLACE "^powerpc64" "powerpc" LLVM_DEFAULT_TARGET_TRIPLE_default "${LLVM_HOST_TRIPLE}")
+else()
+  set(LLVM_DEFAULT_TARGET_TRIPLE_default "${LLVM_HOST_TRIPLE}")
+endif()
+
+set(LLVM_DEFAULT_TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE_default}" CACHE STRING
   "Default target for which LLVM will generate code." )
 set(TARGET_TRIPLE "${LLVM_DEFAULT_TARGET_TRIPLE}")
 message(STATUS "LLVM host triple: ${LLVM_HOST_TRIPLE}")
 message(STATUS "LLVM default target triple: ${LLVM_DEFAULT_TARGET_TRIPLE}")
 
 if(WIN32 OR CYGWIN)
   if(BUILD_SHARED_LIBS OR LLVM_BUILD_LLVM_DYLIB)
     set(LLVM_ENABLE_PLUGINS_default ON)