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[Darwin] Fix build warnings for libstdc++ [NFC]


Hi,

GCC does not export construction vtable symbols from shared libraries**.

The symbols are marked hidden in the objects; for Darwin that makes them also external (“private_extern” is Darwin’s hidden) which means that they show up in the list of possible symbols for export from libstdc++, and there are sufficiently relaxed match conditions that they reach the exports list.   When Darwin’s static linker encounters them it generates a warning that they cannot be exported.  This patch prunes them from the list of symbols to be considered, thus eliminating the warnings.

OK for trunk?
Iain

**  This seems a design decision, rather than an ABI mandate - note that, on Darwin at least, these symbols *are* visible in libc++.

libstdc++-v3/

	* scripts/make_exports.pl (check names): Don’t try to export construction
	vtable symbols.


diff --git a/libstdc++-v3/scripts/make_exports.pl b/libstdc++-v3/scripts/make_exports.pl
index 7c9e4e31d4..93100e17dd 100644
--- a/libstdc++-v3/scripts/make_exports.pl
+++ b/libstdc++-v3/scripts/make_exports.pl
@@ -103,6 +103,14 @@ NAME: while (<NM>) {
     # Ignore undefined and local symbols.
     next if (/^([^ ]+) [Ua-z] /);
 
+    # GCC does not export construction vtables from shared libraries.
+    # However the symbols are marked hidden, for Darwin that makes them
+    # also external "private_extern", which means that they show up in
+    # this list.  When ld64 encounters them it generates a warning that
+    # they cannot be exported, so trim them from the set now.
+    next if (/^construction vtable.*$/);
+    next if (/^__ZTC.*$/);
+


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