Re: Weird conflict with gcc-15

[Paolo Bonzini <pbonzini@redhat.com>]

On 8/27/25 10:26, Christian Ehrhardt wrote:
> Hi,
> in testing qemu 10.1 since rc levels I've found yet another odd
> behavior. As far as I can tell it's not your fault, but I wanted to
> tell you for awareness and potentially to help me get the right debug
> data.
> 
> A test that triggered when checking Ubuntu was the one of EDK2,
> emulating riscv when running on ppc64, example log [1]. Yep, yet again
> not the most common setup :-) and the same emulation in all other
> Ubuntu architectures as host works fine.
> Gladly this was reproducible and it eventually led me to a wild
> journey which now makes me consider gcc-15 (15.2.0-1ubuntu1) as the
> suspect here.
> 
> Since I know the arch, the function and the tunable - I can use a
> rather surgical mitigation like this.
> 
> diff --git a/target/riscv/pmu.c b/target/riscv/pmu.c
> index a68809eef3..5317d8be57 100644
> --- a/target/riscv/pmu.c
> +++ b/target/riscv/pmu.c
> @@ -189,6 +189,13 @@ static int riscv_pmu_incr_ctr_rv64(RISCVCPU *cpu,
> uint32_t ctr_idx)
>    * env->priv and env->virt_enabled contain old priv and old virt and
>    * new priv and new virt values are passed in as arguments.
>    */
> +#if defined(__powerpc64__) || defined(__ppc64__)
> + #define NO_GCSE_ATTR __attribute__((optimize("no-gcse")))
> +#else
> + #define NO_GCSE_ATTR
> +#endif
> +
> +NO_GCSE_ATTR
>   static void riscv_pmu_icount_update_priv(CPURISCVState *env,
>                                            target_ulong newpriv, bool new_virt)
>   {
> 
> But a mitigation is all that it is, ideally, I'd report this as a gcc bug.
> Yet the - understandable - hard requirement of getting the
> pre-processed files makes this quite complex. As I can't even exactly
> point to where exactly things go wrong.
> I'd ask if one of you has experience in providing gcc-bugs out of a
> qemu build. Is it as obvious as throwing -save-temps into *flags or is
> there more to consider get what would be needed?

Yes, it's like that.  Run "ninja -v 
./libqemu-riscv64-softmmu.a.p/target_riscv_pmu.c.o", stick -save-temps 
at the end and submit the resulting .i file as an attachment to the GCC bug.

Paolo