qversiontagging: merge the qt_version_tag symbols

The symbols all still exist, but they all have the same address. For
example, in my Linux debug build:

 $ eu-readelf --dyn-syms lib/libQt6Core.t.so | grep qt_version_tag
 4352: 00000000005e3e74      1 OBJECT  GLOBAL DEFAULT       18 qt_version_tag_6_0@@Qt_6
 4356: 00000000005e3e74      1 OBJECT  GLOBAL DEFAULT       18 qt_version_tag_6_1@@Qt_6
 4358: 00000000005e3e74      1 OBJECT  GLOBAL DEFAULT       18 qt_version_tag_6_2@@Qt_6
 4362: 00000000005e3e74      1 OBJECT  GLOBAL DEFAULT       18 qt_version_tag_6_3@@Qt_6
 4364: 00000000005e3e74      1 OBJECT  GLOBAL DEFAULT       18 qt_version_tag_6_4@@Qt_6
 6458: 00000000005e3e74      1 OBJECT  GLOBAL DEFAULT       18 qt_version_tag@@Qt_6.4
 6460: 00000000005e3e74      1 OBJECT  GLOBAL DEFAULT       18 qt_version_tag@Qt_6.0
 6462: 00000000005e3e74      1 OBJECT  GLOBAL DEFAULT       18 qt_version_tag@Qt_6.2
 6463: 00000000005e3e74      1 OBJECT  GLOBAL DEFAULT       18 qt_version_tag@Qt_6.3
 6466: 00000000005e3e74      1 OBJECT  GLOBAL DEFAULT       18 qt_version_tag@Qt_6.1

(The part after the @ is the ELF version; two @ indicates the default
version)

Saves N bytes where N is the Qt minor version number. The exclusion of
Darwin (macOS) is because clang says that it has the "alias" attribute,
but then fails when you use it.

Change-Id: I74249c52dc02478ba93cfffd16d249d77d6f7565
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 23 insertions, 5 deletions

diff --git a/src/corelib/global/qversiontagging.cpp b/src/corelib/global/qversiontagging.cpp
index e79cf9f1b5..1c4a3e5dff 100644
--- a/src/corelib/global/qversiontagging.cpp
+++ b/src/corelib/global/qversiontagging.cpp
@@ -40,24 +40,40 @@
 #include "qglobal.h"
 #include "qversiontagging.h"
 
+extern "C" {
 #define SYM QT_MANGLE_NAMESPACE(qt_version_tag)
-//#define SSYM QT_STRINGIFY(SYM)
+#define SSYM QT_STRINGIFY(SYM)
+
+// With compilers that have the "alias" attribute, the macro creates a global
+// variable "qt_version_tag_M_m" (M: major, m: minor) as an alias to either
+// qt_version_tag or _qt_version_tag. Everywhere else, we simply create a new
+// global variable qt_version_tag_M_m without aliasing to a single variable.
+//
+// Additionally, on systems using ELF binaries (Linux, FreeBSD, etc.), we
+// create ELF versions by way of the .symver assembly directive[1].
+//
+// Unfortunately, Clang on Darwin systems says it supports the "alias"
+// attribute, but fails when used. That's a Clang bug (as of XCode 12).
+//
+// [1] https://sourceware.org/binutils/docs/as/Symver.html
 
 #if defined(Q_CC_GNU) && defined(Q_OF_ELF)
 #  define make_versioned_symbol2(sym, m, n, separator)     \
-    Q_CORE_EXPORT extern const char sym ## _ ## m ## _ ## n = 0; \
+    Q_CORE_EXPORT extern __attribute__((alias("_" SSYM))) const char sym ## _ ## m ## _ ## n; \
     asm(".symver " QT_STRINGIFY(sym) "_" QT_STRINGIFY(m) "_" QT_STRINGIFY(n) ", " \
         QT_STRINGIFY(sym) separator "Qt_" QT_STRINGIFY(m) "." QT_STRINGIFY(n))
 
+extern const char QT_MANGLE_NAMESPACE(_qt_version_tag) = 0;
+#elif __has_attribute(alias) && !defined(Q_OS_DARWIN)
+#  define make_versioned_symbol2(sym, m, n, separator)     \
+    Q_CORE_EXPORT extern __attribute__((alias(SSYM))) const char sym ## _ ## m ## _ ## n
+extern const char SYM = 0;
 #else
 #  define make_versioned_symbol2(sym, m, n, separator)     \
     Q_CORE_EXPORT extern const char sym ## _ ## m ## _ ## n = 0;
 #endif
 #define make_versioned_symbol(sym, m, n, separator)    make_versioned_symbol2(sym, m, n, separator)
 
-QT_BEGIN_NAMESPACE
-
-extern "C" {
 #if QT_VERSION_MINOR > 0
 make_versioned_symbol(SYM, QT_VERSION_MAJOR, 0, "@");
 #endif
@@ -114,6 +130,8 @@ make_versioned_symbol(SYM, QT_VERSION_MAJOR, 15, "@");
 make_versioned_symbol(SYM, QT_VERSION_MAJOR, QT_VERSION_MINOR, "@@");
 }
 
+QT_BEGIN_NAMESPACE
+
 static_assert(std::is_trivially_destructible_v<QtPrivate::QVersionTag>);
 
 QT_END_NAMESPACE