AIX: Explicitly compute executable exports for both XL and GNU

On AIX, symbols in executables must be exported in order to be visible
to modules (plugins) they load via `dlopen`.  Prior to policy `CMP0065`,
CMake linked all executables with flags to export symbols, but the NEW
behavior for that policy is to do so only for executables that have the
`ENABLE_EXPORTS` target property set.  In both cases, CMake has always
used the AIX linker option `-bexpall` option to export symbols from
executables.

This has worked fairly well with the XL compiler, but with the GNU
compiler it works only for C ABI symbols.  The reason is that `-bexpall`
does not export symbols starting in `_` but the GNU C++ ABI mangles all
symbols with a leading `_`.  Therefore we have only supported C ABI
plugins with the GNU compiler on AIX.  Some projects have tried to work
around this by replacing `-bexpall` with `-bexpfull`, but the latter
often exports symbols that we do not want exported.

Avoid using `-bexpall` for executables by instead using by our own
internal `ExportImportList` script to compute symbol export lists from
the object files to be linked into an executable.  Pass the explicitly
computed export list to the AIX linker's `-bE:...` option.  We already
do this for shared object exports.

Issue: #19163

1 files changed, 8 insertions, 4 deletions

diff --git a/Source/cmLocalGenerator.cxx b/Source/cmLocalGenerator.cxx
index 0f730c9..4ffd6e0 100644
--- a/Source/cmLocalGenerator.cxx
+++ b/Source/cmLocalGenerator.cxx
@@ -1515,8 +1515,10 @@ std::string cmLocalGenerator::GetLinkLibsCMP0065(
         }
         CM_FALLTHROUGH;
       case cmPolicies::OLD:
-        // OLD behavior is to always add the flags
-        add_shlib_flags = true;
+        // OLD behavior is to always add the flags, except on AIX where
+        // we compute symbol exports if ENABLE_EXPORTS is on.
+        add_shlib_flags =
+          !(tgt.Target->IsAIX() && tgt.GetPropertyAsBool("ENABLE_EXPORTS"));
         break;
       case cmPolicies::REQUIRED_IF_USED:
       case cmPolicies::REQUIRED_ALWAYS:
@@ -1525,8 +1527,10 @@ std::string cmLocalGenerator::GetLinkLibsCMP0065(
           cmPolicies::GetRequiredPolicyError(cmPolicies::CMP0065));
         CM_FALLTHROUGH;
       case cmPolicies::NEW:
-        // NEW behavior is to only add the flags if ENABLE_EXPORTS is on
-        add_shlib_flags = tgt.GetPropertyAsBool("ENABLE_EXPORTS");
+        // NEW behavior is to only add the flags if ENABLE_EXPORTS is on,
+        // except on AIX where we compute symbol exports.
+        add_shlib_flags =
+          !tgt.Target->IsAIX() && tgt.GetPropertyAsBool("ENABLE_EXPORTS");
         break;
     }