Re: [Qemu-devel] [PATCH] Added more entries to the LEON processor configuration register

[Ronald Hecht <ronald.hecht@gmx.de>]

On 09/19/2012 08:56 PM, Blue Swirl wrote:
>> --- a/target-sparc/translate.c
>> +++ b/target-sparc/translate.c
>> @@ -2590,13 +2590,9 @@ static void disas_sparc_insn(DisasContext * dc, unsigned int insn)
>>                                          microSPARC II */
>>                       /* Read Asr17 */
>>                       if (rs1 == 0x11&&  dc->def->features&  CPU_FEATURE_ASR17) {
>> -                        TCGv r_const;
>> -
>> -                        /* Read Asr17 for a Leon3 monoprocessor */
>> -                        r_const = tcg_const_tl((1<<  8)
>> -                                               | (dc->def->nwindows - 1));
>> -                        gen_movl_TN_reg(rd, r_const);
>> -                        tcg_temp_free(r_const);
>> +                        /* Read Asr17 on LEON3 */
>> +                        gen_helper_rdasr17(cpu_dst, cpu_env);
>>      
> Is the register is read very often? If it is, we could avoid the
> helper call by performing the load of env->cpu_index, the shift and OR
> with the above constant with TCG ops. The arithmetic could be even
> avoided completely by storing precalculated values in cpu_index, then
> other accesses to cpu_index need to shift right by 28 to read the
> index (depends on balance of other cpu_index uses vs. this
> instruction).
>    

The register is read quite often, but not as often as the PSR. It is 
normally read when entering the OS (Linux, PikeOS ...) Kernel to know on 
which CPU we are running a thread. But polluting the cpu_index in env 
sounds a bit strange and overkill to me. Do you really mean the 
cpu_index in the structure CPUSPARCState? Or could we place it in 
dc->def as a constant?

Best regards,
Ronald