Re: Qemu and GCC-3.4 on powerpc

[Gabriel Paubert <paubert@iram.es>]

On Tue, Mar 28, 2006 at 12:56:13AM +0200, Dieter Schuster wrote:
> Hello,
> 
> the version 0.8.0 of qemu in the Debian-pool will not compile on
> PowerPC with GCC 3.4. The following patch will fix it:

And suck performance wise with exploding code size. Without 
speaking of potential atomicity issues (although all architectures 
using the byte per byte generic code may hit the problem).

> 
> --- cpu-all.h~	2005-12-19 23:51:53.000000000 +0100
> +++ cpu-all.h	2006-03-27 22:47:54.291613000 +0200
> @@ -249,15 +249,11 @@
>  
>  static inline void stl_le_p(void *ptr, int v)
>  {
> -#ifdef __powerpc__
> -    __asm__ __volatile__ ("stwbrx %1,0,%2" : "=m" (*(uint32_t *)ptr) : "r" (v), "r" (ptr));
> -#else
>      uint8_t *p = ptr;
>      p[0] = v;
>      p[1] = v >> 8;
>      p[2] = v >> 16;
>      p[3] = v >> 24;
> -#endif
>  }
>  
>  static inline void stq_le_p(void *ptr, uint64_t v)
> 
> 
> If I use GCC 3.3, then qemu compiles with the assembler instruction in
> the patch above, but qemu does not work correctly (tested with Knoppix V5.0).

Interesting, could it be an aliasing problem?

Try to compile with -fno-strict-aliasing, although 
I doubt it will change anything. 

> 
> If I try to compile qemu with GCC 3.4 without the patch I get the following error:
> 
> qemu-0.8.0/linux-user/elfload.c: In function `load_elf_binary':
> qemu-0.8.0/cpu-all.h:253: error: inconsistent operand constraints in an `asm'
> qemu-0.8.0/cpu-all.h:253: error: inconsistent operand constraints in an `asm'

Weird. CC'ed to gcc list despite the fact that the 3.4 branch
is definitely closed. I've not found anything remotely similar
from bugzilla.

> 
> But if I copy the function stl_le_p to a seperate file, the function
> will compile with GCC 3.4. 

Which gcc-3.4 (gcc -v)? 

> 
> Is this a bug in qemu, or is it a bug in GCC 3.4?

It looks like a compiler bug. But you should
provide an environment independent test case,
i.e., the preprocessed source. Please try
also to provide a minimal failing test case 
(it may be hard given the symptoms).

	Regards,
	Gabriel