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qt 4.5 sigbus fault - fixed

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bobholland

Posted: Jun 01, 2009 - 09:33 AM

Joined: Jun 29, 2007
Posts: 23

Referring to...

http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=78520

I've finally tracked this down to a misaligned 16-bit access in the template function for...

qt_blend_argb24_on_rgb16()

...in qblendfunctions.cpp

It seems it is necessary to add (QT_ARCH_AVR32) into the list of arches which can't handle 16-bit non-aligned access.

I guess I was expecting this to be blindingly obvious once I got to the correct place in the code.

I've not looked at what the compiler generates here, but I believe ARV32 can do some non-aligned operations - seems not in this case.

Thanks for help on this - got there in the end & can now move on and do some applications work.

I'll post this in other places.

Bob

squidgit

Posted: Jun 01, 2009 - 11:49 AM

Joined: Sep 14, 2003
Posts: 4209
Location: Queanbeyan, Australia

{ld,st}.w can handle any alignment (with a performance penalty). I guess the compiler sees the 16-bit-ness of the load and uses the appropriate {ld,st}.[su]h instructions. Fair enough choice really but annoying that it explodes in this way..

Well done and thanks Smile

-S.

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Blag: http://www.niasdigital.com/blag

csb-ac

Posted: Jul 07, 2009 - 07:59 PM

Joined: May 11, 2009
Posts: 20

Hi!
Could you tell me how to "add (QT_ARCH_AVR32) into the list of arches which can't handle 16-bit non-aligned access."?
Can't find anything in the mkspec/ files ...
Thanks,
Sebastian

csb-ac

Posted: Jul 08, 2009 - 10:06 AM

Joined: May 11, 2009
Posts: 20

Hi!
Ok .. found the place where the patch belongs ... more below ... IT WORKS!
Thanks a lot.

I used it as a debugging/dissasm try, and found the following ... my first attempt at avr32 asm:

Code:

Program received signal SIGBUS, Bus error.

0x2acc503c in ?? ()    

(gdb) disass

No function contains program counter for selected frame.

(gdb) disass 0x2acc503c

No function contains specified address.

(gdb) x/10i 0x2acc503c

0x2acc503c:     ld.uh r5,lr[1]

0x2acc5040:     cp.b r9,r8

# 4 bytes back, cmds executed

(gdb) x/10i 0x2acc5038

0x2acc5038:     ld.ub r9,lr[0x0]

0x2acc503a:     mov r8,-1

0x2acc503c:     ld.uh r5,lr[1] # sigbus

0x2acc5040:     cp.b r9,r8

(gdb) info regis

lr             0x6e438  

(gdb) x/10x 0x6e438

0x6e438:        0x09    0x41    0x08    0x04    0x20    0x00    0x00    0x00

0x6e440:        0x00    0x12

so ... the byte pointed at by lr is loaded into r9, then, the halfword pointed at by lr+1 is loaded into r5.
It seems the variable pointed at by lr (char*?) is correctly aligned, but contains binary data that needs to be parsed - bitmap, probably. And, using this code, this will never succeed in a machine that forces alignment.

On the other hand, the compiler should never have generated code that acesses an aligned variable+1 as a word, should it?

Grr, now I see what you meant ... looking into the source code ... someone tried to optimise in a way that uses an explicitly unaligned access.

Code:

template <typename T> void qt_blend_argb24_on_rgb16(uchar *destPixels, int dbpl,

  const uchar *srcPixels, int sbpl,

  int w, int h, const T &alphaFunc)

...

#if defined(QT_ARCH_ARM) || defined(QT_ARCH_POWERPC) || (defined(QT_ARCH_WINDOWSCE) && !defined(_X86_))

  // non-16-bit aligned memory access is not possible on PowerPC &

  // ARM <v6 (QT_ARCH_ARMV6)

  quint16 spix = (quint16(src[2])<<8) + src[1];

#else

  quint16 spix = *(quint16 *) (src + 1);

#endif

I see where the patch belongs, now ...

BTW, created a debug env for avr32 ... only a tad more than 300MB for the root filesystem Wink

Thanks again,
Sebastian

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