From a17f6eff4382a9461b412897709e86a58a29c920 Mon Sep 17 00:00:00 2001
From: Brad King <brad.king@kitware.com>
Date: Thu, 30 Jan 2020 09:41:51 -0500
Subject: [PATCH] AIX: Restore pre-3.16 undocumented method to suppress exports
 with XL

In commit 0f150b69d3 (AIX: Explicitly compute shared object exports for
both XL and GNU, 2019-07-11, v3.16.0-rc1~418^2~2) we dropped use of the
old `CMAKE_XL_CreateExportList` cache entry for XL exports.  However,
some people were setting the value to an empty string as a way to
disable automatic export of symbols.  Restore this behavior when the
option is explicitly set to an empty string.

Issue: #20290
---
 Modules/Platform/AIX-XL.cmake | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/Modules/Platform/AIX-XL.cmake b/Modules/Platform/AIX-XL.cmake
index 64b0bc196c..2a67c4fedb 100644
--- a/Modules/Platform/AIX-XL.cmake
+++ b/Modules/Platform/AIX-XL.cmake
@@ -18,14 +18,24 @@ macro(__aix_compiler_xl lang)
 
   set(CMAKE_${lang}_LINK_FLAGS "-Wl,-bnoipath")
 
+  set(_OBJECTS " <OBJECTS>")
+  if(DEFINED CMAKE_XL_CreateExportList AND CMAKE_XL_CreateExportList STREQUAL "")
+    # Prior to CMake 3.16, CMAKE_XL_CreateExportList held the path to the XL CreateExportList tool.
+    # Users could set it to an empty value to skip automatic exports in favor of manual -bE: flags.
+    # Preserve that behavior for compatibility (even though it was undocumented).
+    set(_OBJECTS "")
+  endif()
+
   # Construct the export list ourselves to pass only the object files so
   # that we export only the symbols actually provided by the sources.
   set(CMAKE_${lang}_CREATE_SHARED_LIBRARY
-    "\"${CMAKE_ROOT}/Modules/Platform/AIX/ExportImportList\" -o <OBJECT_DIR>/objects.exp <OBJECTS>"
+    "\"${CMAKE_ROOT}/Modules/Platform/AIX/ExportImportList\" -o <OBJECT_DIR>/objects.exp${_OBJECTS}"
     "<CMAKE_${lang}_COMPILER> <CMAKE_SHARED_LIBRARY_${lang}_FLAGS> -Wl,-bE:<OBJECT_DIR>/objects.exp <LANGUAGE_COMPILE_FLAGS> <LINK_FLAGS> <CMAKE_SHARED_LIBRARY_CREATE_${lang}_FLAGS> <SONAME_FLAG><TARGET_SONAME> -o <TARGET> <OBJECTS> <LINK_LIBRARIES>"
     )
 
   set(CMAKE_${lang}_LINK_EXECUTABLE_WITH_EXPORTS
     "\"${CMAKE_ROOT}/Modules/Platform/AIX/ExportImportList\" -o <TARGET_IMPLIB> -l . <OBJECTS>"
     "<CMAKE_${lang}_COMPILER> <FLAGS> <CMAKE_${lang}_LINK_FLAGS> -Wl,-bE:<TARGET_IMPLIB> <LINK_FLAGS> <OBJECTS> -o <TARGET> <LINK_LIBRARIES>")
+
+  unset(_OBJECTS)
 endmacro()