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

[jacob n. <ja...@ja...>]

  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.