Qt, GCC, SSE and stack alignment
Question
I'm trying to make a program compiled with GCC and using Qt and SSE intrinsics. It seems that when one of my functions is called by Qt, the stack alignment is not preserved. Here's a short example to illustrate what I mean :
#include <cstdio>
#include <emmintrin.h>
#include <QtGui/QApplication.h>
#include <QtGui/QWidget.h>
class Widget: public QWidget {
public:
void paintEvent(QPaintEvent *) {
__m128 a;
printf("a: 0x%08x\n", ((void *) &a));
}
};
int main(int argc, char** argv)
{
QApplication application(argc, argv);
Widget w;
w.paintEvent(NULL); // Called from here, my function behaves correctly
w.show();
w.update();
// Qt will call Widget::paintEvent and my __m128 will not be
// aligned on 16 bytes as it should
application.processEvents();
return 0;
}
Here's the output:
a: 0x0023ff40 // OK, that's aligned on 16 bytes
a: 0x0023d14c // Not aligned!
Configuration:
- Intel Core2
- WinXP, SP3
- GCC 4.4 (Mingw included in the Qt SDK 2010.01)
I tried to compile the example program with the same options as those I saw in the Qt makefile :
-O2 -Wall -frtti -fexceptions -mthreads
,link options:
-enable-stdcall-fixup -Wl,-enable-auto-import -Wl,-enable-runtime-pseudo-reloc -Wl,-s -mthreads
Now I don't know in which directions to search. Any hints would be appreciated. Thanks!
Fabien
Solution
You can use the option -mstackrealign
to do that without adding attributes to your source code:
-mstackrealign Realign the stack at entry. On the Intel x86, the -mstackrealign option will generate an alternate prologue and epilogue that realigns the runtime stack if necessary. This supports mixing legacy codes that keep a 4-byte aligned stack with modern codes that keep a 16-byte stack for SSE compatibility. See also the attribute force_align_arg_pointer, applicable to individual functions.
(from the GCC docs)
OTHER TIPS
__attribute__((force_align_arg_pointer)) void paintEvent(QPaintEvent *);
made it work! Does anybody have a better solution?