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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>
bb10
Thiago Macieira 2022-02-09 17:53:23 -08:00
parent dda4186842
commit 07b176ce70
1 changed files with 23 additions and 5 deletions
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28
src/corelib/global/qversiontagging.cpp
View File

@ -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