Re: [Qemu-devel] [PATCH v3 for 2.5] cpu-exec: Fix compiler warning (-Werror=clobbered)

[Paolo Bonzini]

On 27/10/2015 19:31, Stefan Weil wrote:
> Am 26.09.2015 um 13:23 schrieb Stefan Weil:
>> Reloading of local variables after sigsetjmp is only needed for some
>> buggy compilers.
>>
>> The code which should reload these variables causes compiler warnings
>> with gcc 4.7 when compiler optimizations are enabled:
>>
>> cpu-exec.c:204:15: error:
>>  variable ‘cpu’ might be clobbered by ‘longjmp’ or ‘vfork’ 
>> [-Werror=clobbered]
>> cpu-exec.c:207:15: error:
>>  variable ‘cc’ might be clobbered by ‘longjmp’ or ‘vfork’ [-Werror=clobbered]
>> cpu-exec.c:202:28: error:
>>  argument ‘env’ might be clobbered by ‘longjmp’ or ‘vfork’ 
>> [-Werror=clobbered]
>>
>> Now this code is only used for compilers which need it
>> (and gcc 4.5.x, x > 0 which does not need it but won't give warnings).
>>
>> There were bug reports for clang and gcc 4.5.0, while gcc 4.5.1
>> was reported to work fine without the reload code. For clang it
>> is not clear which versions are affected, so simply keep the status quo
>> for all clang compilations. This can be improved later.
>>
>> Signed-off-by: Stefan Weil <address@hidden>
>> ---
>>
>> v2: Don't remove the code which causes the warnings, but use it
>>     only with clang or gcc < 4.6.
>>
>> v3: Add assertions for compilers which hopefully don't smash variables
>>     (suggested by Peter Maydell).
>>
>> I started v1 of this patch two years ago to prepare support for
>> builds with compiler option -Wextra.
>>
>> See http://patchwork.ozlabs.org/patch/287593/ for the latest
>> discussion on this issue.
>>
>>
>>  cpu-exec.c | 18 +++++++++++++++---
>>  1 file changed, 15 insertions(+), 3 deletions(-)
>>
>> diff --git a/cpu-exec.c b/cpu-exec.c
>> index 8fd56a6..7dab85a 100644
>> --- a/cpu-exec.c
>> +++ b/cpu-exec.c
>> @@ -538,15 +538,27 @@ int cpu_exec(CPUState *cpu)
>>                     only be set by a memory fault) */
>>              } /* for(;;) */
>>          } else {
>> -            /* Reload env after longjmp - the compiler may have smashed all
>> -             * local variables as longjmp is marked 'noreturn'. */
>> +#if defined(__clang__) || !QEMU_GNUC_PREREQ(4, 6)
>> +            /* Some compilers wrongly smash all local variables after
>> +             * siglongjmp. There were bug reports for gcc 4.5.0 and clang.
>> +             * Reload essential local variables here for those compilers.
>> +             * Newer versions of gcc would complain about this code 
>> (-Wclobbered). */
>>              cpu = current_cpu;
>>              cc = CPU_GET_CLASS(cpu);
>> -            cpu->can_do_io = 1;
>>  #ifdef TARGET_I386
>>              x86_cpu = X86_CPU(cpu);
>>              env = &x86_cpu->env;
>>  #endif
>> +#else /* buggy compiler */
>> +            /* Assert that the compiler does not smash local variables. */
>> +            g_assert(cpu == current_cpu);
>> +            g_assert(cc == CPU_GET_CLASS(cpu));
>> +#ifdef TARGET_I386
>> +            g_assert(x86_cpu == X86_CPU(cpu));
>> +            g_assert(env == &x86_cpu->env);
>> +#endif
>> +#endif /* buggy compiler */
>> +            cpu->can_do_io = 1;
>>              tb_lock_reset();
>>          }
>>      } /* for(;;) */
>>
> 
> 
> Ping. Is there any chance to get this patch into version 2.5?
> I'd be happy to remove this 2 year old issue from my list of
> open patches.

Yes, I'll send a pull request next week.

Paolo