Re: [Qemu-devel] [PATCH v2] cpu-exec: Fix compiler warning (-Werror=clobbered)

[Jan Kiszka <jan.kiszka@web.de>]

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On 2013-10-31 20:31, Stefan Weil wrote:
> 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.
> 
> Signed-off-by: Stefan Weil <sw@weilnetz.de>
> ---
> 
> v2: Don't remove the code which causes the warnings, but use it
>     only with clang or gcc < 4.6.
> 
>  cpu-exec.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 30cfa2a..fec20c3 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -677,14 +677,18 @@ int cpu_exec(CPUArchState *env)
>                     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.
> +             * gcc 4.7 would complain about this code (-Wclobbered). */
>              cpu = current_cpu;
>              env = cpu->env_ptr;
>  #if !(defined(CONFIG_USER_ONLY) && \
>        (defined(TARGET_M68K) || defined(TARGET_PPC) || defined(TARGET_S390X)))
>              cc = CPU_GET_CLASS(cpu);
>  #endif
> +#endif /* __clang__ or old gcc */
>          }
>      } /* for(;;) */
>  
> 

Are all clang versions affected? Then this looks reasonable.

Jan


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