Re: [Qt-iphone-developer] Solution to bug 3082827 (4.7 fails to link)

[Mark K. <gre...@ma...>]

I forgot to ask, did you run the test case for qatomicponter and qatomicinit just to be safe?

Regards,
Mark Kromis

On Oct 19, 2010, at 1:34 PM, jacob navia wrote:

>  Hi
> Bug 3082827 says:
> ---------------------------------------------------------------
> Hi
> 
> We have translated the 4.7 source code. There were minor problems tat we 
> got around by modifying the Makefiles. When we arrive at the link time 
> however, we are confronted to a bug that we can´t solve:
> 
> [code]
> {standard input}:66:selected processor does not support `ldrex ip,[r3]'
> {standard input}:67:lo register required -- `sub ip,ip,#1'
> {standard input}:68:selected processor does not support `strex r1,ip,[r3]'
> {standard input}:69:selected processor does not support `teq r1,#0'
> Command /Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/gcc-4.2 
> failed with exit code 1
> [/code]
> 
> There is no way to know which object code file is at fault. We suspect 
> that the C++ error handling but we cnt say for sure.
> 
> My email is
> ja...@ja...
> 
> Thanks in advance for your help
> -----------------------------------------------------------------
> 
> This problem doesn't appear when compiling for the simulator of course.
> 
> More information.
> This bug is produced by the inline assembly in qatomic_armv6.h, in the 
> function
> inline bool QBasicAtomicInt::ref()
> {
>     register int newValue;
>     register int result;
>     asm volatile("0:\n\t"
>                  "ldrex %[newValue], [%[_q_value]]\n\t"
>                  "add %[newValue], %[newValue], #1\n\t"
>                  "strex %[result], %[newValue], [%[_q_value]]\n\t"
>                  "teq %[result], #0\n\t"
>                  "bne 0b\n\t"
>                  : [newValue] "=&r" (newValue),
>                    [result] "=&r" (result),
>                    "+m" (_q_value)
>                  : [_q_value] "r" (&_q_value)
>                  : "cc", "memory");
>     return newValue != 0;
> }
> 
> The problem seems to be that the inline assembler of the gcc version is 
> completely broken.
> (1) The instruction ldrex is a valid instruction for the ARM v6
> (2) The syntax for the instruction is also valid
> (3) The inline assembler uses for [newvalue] the register ip 
> (instruction pointer). This can't work of course.
>    This led me to the belief that somehow I could try to use a fixed 
> register instead of letting the compiler
>    assign a new one for me, but that didn't work. Still the instruction 
> ldrex isn't recognized.
> (4) I tried to switch to the generic atomic instructions file 
> qatomic_arm.h, but that did not work either
>    because no assembler instruction woud be recognized.
> 
> Conclusion:
> 
> The inline assembler is broken.
> 
> 
> WORKAROUND:
> ------------
> (1) I learned ARM assembly enough to write the following assembler file:
> 
>         .globl  q_atomic_swp
> q_atomic_swp:
>         push    {r7, lr}
>         swpb r0,r2,[r3]
>         pop     {r7,pc}
> 
> (2) I changed the file qatomic_arm.h:
> 
> #if 0
> inline static char q_atomic_swp(volatile char *ptr, char newval)
> {
>     register char ret;
>     asm volatile("swpb %0,%2,[%3]"
>                  : "=&r"(ret), "=m" (*ptr)
>                  : "r"(newval), "r"(ptr)
>                  : "cc", "memory");
>     return ret;
> }
> #else
> extern "C" char q_atomic_swp(char *ptr,char newval);
> #endif
> 
> 
> (3) I added the assembler file to the Xcode project, and my project 
> linked and loaded.
> 
> Total work time: a week.
> 
> 
> 
> 
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