[AIX][cmake] Adjust management of -G for linking

The change in 0ba9843 changed the behaviour of the build when
using an XL build compiler because `-G` is not a pure linker option:
it also implies `-shared`. This was accounted for in the base CMake
configuration, so an analysis of the change from 0ba9843 in
relation to a build using Clang (where `-shared` is introduced by CMake)
would not identify the issue. This patch resolves this particular issue
by adding `-shared` alongside `-Wl,-G`.

At the same time, the investigation reveals that several aspects of the
various build configurations are not operating in the manner originally
intended.

The other issue related to the `-G` linker option in the build is that
the removal of it (to avoid unnecessary use of run-time linking) is not
effective for the build using the Clang compiler. This patch addresses
this by adjusting the regular expressions used to remove the broadly-
applied `-G`.

Finally, the issue of specifying the export list with `-Wl,` instead of
a compiler option is flagged with a FIXME comment.

Reviewed By: daltenty, amyk

Differential Revision: https://reviews.llvm.org/D90041

llvm/CMakeLists.txt

@@ -917,20 +917,26 @@ if (UNIX AND ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
   # configuration, it is still possible the user may force it as part of a
   # compound option.
   if(CMAKE_VERSION VERSION_LESS 3.16)
-    string(REGEX REPLACE "(^|[ \t]+)-Wl,-brtl([ \t]+|$)" "" CMAKE_EXE_LINKER_FLAGS  "${CMAKE_EXE_LINKER_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)-Wl,-brtl([ \t]+|$)" "" CMAKE_SHARED_LINKER_FLAGS  "${CMAKE_SHARED_LINKER_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)-Wl,-brtl([ \t]+|$)" "" CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)(-Wl,)?-G([ \t]+|$)" "" CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS
+    string(REGEX REPLACE "(^|[ \t]+)-Wl,-brtl([ \t]+|$)" " " CMAKE_EXE_LINKER_FLAGS  "${CMAKE_EXE_LINKER_FLAGS}")
+    string(REGEX REPLACE "(^|[ \t]+)-Wl,-brtl([ \t]+|$)" " " CMAKE_SHARED_LINKER_FLAGS  "${CMAKE_SHARED_LINKER_FLAGS}")
+    string(REGEX REPLACE "(^|[ \t]+)-Wl,-brtl([ \t]+|$)" " " CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS}")
+    string(REGEX REPLACE "(^|[ \t]+)(-Wl,)?-G([ \t]+|$)" " " CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS
       "${CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)(-Wl,)?-G([ \t]+|$)" "" CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS
+    string(REGEX REPLACE "(^|[ \t]+)(-Wl,)?-G([ \t]+|$)" " " CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS
       "${CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)(-Wl,)?-G([ \t]+|$)" "" CMAKE_SHARED_LIBRARY_CREATE_ASM_FLAGS
+    string(REGEX REPLACE "(^|[ \t]+)(-Wl,)?-G([ \t]+|$)" " " CMAKE_SHARED_LIBRARY_CREATE_ASM_FLAGS
+      "${CMAKE_SHARED_LIBRARY_CREATE_ASM_FLAGS}")
+    string(REGEX REPLACE "(^|[ \t]+)-Wl,-G," " -Wl," CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS
+      "${CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS}")
+    string(REGEX REPLACE "(^|[ \t]+)-Wl,-G," " -Wl," CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS
+      "${CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS}")
+    string(REGEX REPLACE "(^|[ \t]+)-Wl,-G," " -Wl," CMAKE_SHARED_LIBRARY_CREATE_ASM_FLAGS
       "${CMAKE_SHARED_LIBRARY_CREATE_ASM_FLAGS}")
   endif()
 
-  # Modules should be built with -G, so we can use runtime linking with
-  # plugins.
-  string(APPEND CMAKE_MODULE_LINKER_FLAGS " -Wl,-G")
+  # Modules should be built with -shared -Wl,-G, so we can use runtime linking
+  # with plugins.
+  string(APPEND CMAKE_MODULE_LINKER_FLAGS " -shared -Wl,-G")
 
   # Also set the correct flags for building shared libraries.
   string(APPEND CMAKE_SHARED_LINKER_FLAGS " -shared")

llvm/cmake/modules/AddLLVM.cmake

@@ -90,6 +90,8 @@ function(add_llvm_symbol_exports target_name export_file)
     set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                  LINK_FLAGS " -Wl,-exported_symbols_list,\"${CMAKE_CURRENT_BINARY_DIR}/${native_export_file}\"")
   elseif(${CMAKE_SYSTEM_NAME} MATCHES "AIX")
+    # FIXME: `-Wl,-bE:` bypasses whatever handling there is in the build
+    # compiler driver to defer to the specified export list.
     set(native_export_file "${export_file}")
     set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                  LINK_FLAGS " -Wl,-bE:${export_file}")