Re: [PATCH v2] target/riscv: Use a direct cast for better performance

[LIU Zhiwei]

On 2023/10/9 5:50, Richard W.M. Jones wrote:
> RISCV_CPU(cs) uses a checked cast.  When QOM cast debugging is enabled
> this adds about 5% total overhead when emulating RV64 on x86-64 host.
>
> Using a RISC-V guest with 16 vCPUs, 16 GB of guest RAM, virtio-blk
> disk.  The guest has a copy of the qemu source tree.  The test
> involves compiling the qemu source tree with 'make clean; time make -j16'.
>
> Before making this change the compile step took 449 & 447 seconds over
> two consecutive runs.
>
> After making this change, 428 & 422 seconds.
>
> The saving is about 5%.
>
> Thanks: Paolo Bonzini
> Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
> Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
> ---
>   target/riscv/cpu_helper.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c
> index 3a02079290..479d9863ae 100644
> --- a/target/riscv/cpu_helper.c
> +++ b/target/riscv/cpu_helper.c
> @@ -66,7 +66,11 @@ void cpu_get_tb_cpu_state(CPURISCVState *env, vaddr *pc,
>                             uint64_t *cs_base, uint32_t *pflags)
>   {
>       CPUState *cs = env_cpu(env);
> -    RISCVCPU *cpu = RISCV_CPU(cs);
> +    /*
> +     * Using the checked cast RISCV_CPU(cs) imposes ~ 5% overhead when
> +     * QOM cast debugging is enabled, so use a direct cast instead.
> +     */
> +    RISCVCPU *cpu = (RISCVCPU *)cs;

This function is very hot. Maybe we should cache the tbflags instead of 
calculate it here. Otherwise,

Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>

Zhiwei

>       RISCVExtStatus fs, vs;
>       uint32_t flags = 0;
>