gcj 4.3.0 on mpc860 kernel 2.4.24

Fri May 9 15:15:00 GMT 2008

Ben Gardiner wrote:
> Andrew Haley wrote:
>> Linuxthreads? I don't like the sound of that. gcj doesn't have any
>> explicit dependencies on NPTL, but it doesn't get tested on Linuxthreads.
>>> I see. That's important information, thank you.
>> Where does it segfault? I mean, where *exactly*?
>>> It segfaults in _Jv_MonitorEnter at locks.h:44 (compare_and_swap); here
> is a full bt up to prims.cc:1666 ( _Jv_InitClass
> (&java::lang::ClassLoader::class$); ) :
>> -------------------------------------------------------------------------
> #0 _Jv_MonitorEnter (obj=<value optimized out>) at sysdep/locks.h:44
> addr = 264790936
> address = 0
> he = (hash_entry *) 0xff83458
> self = 16384
> was_heavy = <value optimized out>

Aha! At the segfault, do something like
x/4i $pc
> ------------------------------------------------------------------------
>> and here is my locks.h around 44:
> ------------------------------------------------------------------------
> 30 compare_and_swap (volatile obj_addr_t *addr, obj_addr_t old,
> 31 obj_addr_t new_val)
> 32 {
> 33 obj_addr_t ret;
> 34
> 35 __asm__ __volatile__ (
> 36 "0: " _LARX "%0,0,%1 \n"
> 37 " xor. %0,%3,%0\n"
> 38 " bne 1f\n"
> 39 " " _STCX "%2,0,%1\n"
> 40 " bne- 0b\n"
> 41 "1: \n"
> 42 : "=&r" (ret)
> 43 : "r" (addr), "r" (new_val), "r" (old)
> 44 : "cr0", "memory");
> 45
> 46 /* This version of __compare_and_swap is to be used when
> acquiring
> 47 a lock, so we don't need to worry about whether other memory
> 48 operations have completed, but we do need to be sure that
> any loads
> 49 after this point really occur after we have acquired the
> lock. */
> 50 __asm__ __volatile__ ("isync" : : : "memory");
> 51 return ret == 0;
> 52 }
> -------------------------------------------------------------------------
>> I did a diff and discovered that this file is the
> libjava/sysdep/powerpc/locks.h file (no surprise). I'm not an expert in
> PPC assembly and I don't know if those instructions are supported on the
> mpc860; I will look into that. In the meantime, I may try to get the '
> --enable-portable-native-sync' configure option from 4.2.3 to work with
> 4.3.0. I'm thinking I might have better luck with sysdep/generic/locks.h.

I doubt it. gcj needs something that really works.
It really, really, really is worth trying to get a real compare_and_swap
that is written for your hardware. 
Andrew.