Re: [Qemu-devel] Debian 7.8.0 SPARC64 on qemu - anything i can do to speedup the emulation?

[Aurelien Jarno <aurelien@aurel32.net>]

On 2015-08-19 12:41, Artyom Tarasenko wrote:
> Hi Richard,
> 
> On Tue, Aug 18, 2015 at 7:55 PM, Richard Henderson <rth@twiddle.net> wrote:
> > On 08/18/2015 02:24 AM, Artyom Tarasenko wrote:
> >> The unoptimized case is a sequence of multiple cmp and branch
> >> operations (likely created by a "case" statement in the original
> >> source code), especially where cmp is in a delay slot of a branch
> >> instruction.
> >
> > Interesting.
> >
> >> I wonder whether we always have to finish a TB on a conditional jump.
> >> Maybe it would make sense to translate further if a destination of a
> >> jump is not too far from dc->pc? The definition of "not too far" is
> >> indeed tricky.
> >
> > We can only handle two chained exits from a TB.  If we continue past
> > a conditional branch, we may well encounter a second conditional branch, which
> > would leave us with three different exits from the TB.
> >
> > Something that may be interesting to play with, however, is to change the TB
> > with which the insn in a delay slot is connected.
> >
> > For instance, we currently spend some amount of effort computing and saving the
> > branch condition, so that we can then execute the delay slot, and afterwards
> > use the saved branch condition to perform the branch.
> >
> > Another way of doing this is to immediately branch, exiting the TB.  But we set
> > up PC+NPC for the next TB such that the delay slot is the first insn that is
> > executed within the next TB.  In that way, the compare in the delay slot that
> > you mention *is* in the same TB as the branch that uses it, allowing
> > the case to be optimized.
> >
> > This could wind up creating more TBs than the current solution, so it's not
> > clear that it would be a win.  One can mitigate that somewhat by noticing the
> > case where the delay slot is a nop.  I do think it's worth an experiment.
> 
> So it is possible to make a TB with non sequential instructions?
> The instruction in the delay slot would be located most likely
> elsewhere than the following instructions.
> 
> But I think I've been chasing a red herring. I see those helpers in
> perf top when running sysbench, but not when running g++ (and at the
> end g++ is much more relevant benchmark for me):
> 
> 
> Samples: 83K of event 'cpu-clock', Event count (approx.): 15333243164,
> Thread: qemu-system-spa(2743)
>  27.10%  [kernel]                 [k] retint_signal
>  12.66%  qemu-system-sparc64      [.] tcg_optimize
>   9.18%  [vdso]                   [.] 0x0000000000000998
>   8.39%  [kernel]                 [k] _raw_spin_unlock_irqrestore
>   4.76%  qemu-system-sparc64      [.] tcg_liveness_analysis
>   3.89%  qemu-system-sparc64      [.] tcg_reg_alloc_op
>   2.80%  qemu-system-sparc64      [.] tcg_out_opc
>   2.45%  qemu-system-sparc64      [.] get_physical_address_data
>   1.86%  [kernel]                 [k] native_read_tsc
>   1.62%  qemu-system-sparc64      [.] tlb_flush_page
>   1.55%  qemu-system-sparc64      [.] tcg_out_modrm_sib_offset.constprop.42
>   1.45%  [unknown]                [.] 0x00000000451c5cae
>   1.43%  qemu-system-sparc64      [.] gen_intermediate_code_pc
>   1.39%  qemu-system-sparc64      [.] tcg_temp_new_internal_i64
>   1.24%  qemu-system-sparc64      [.] tb_flush_jmp_cache
>   1.11%  qemu-system-sparc64      [.] disas_sparc_insn
>   1.08%  qemu-system-sparc64      [.] tcg_out_modrm
>   0.97%  qemu-system-sparc64      [.] tcg_reg_alloc_start
>   0.77%  qemu-system-sparc64      [.] cpu_sparc_exec
>   0.73%  qemu-system-sparc64      [.] replace_tlb_1bit_lru.isra.3
>   0.72%  qemu-system-sparc64      [.] tcg_gen_code_search_pc
>   0.72%  qemu-system-sparc64      [.] tcg_opt_gen_mov
>   0.70%  qemu-system-sparc64      [.] reset_temp
> 
> I'm not sure why I still see kernel functions when I zoom into qemu
> thread. Is this qemu signal handling?
> And then it would be interesting to know where in this listing is the
> generated code. Is it [vdso], [unknown] or is it hidden behind
> retint_signal?
> 
> Ironically a good optimization target seems to be the tcg_optimize
> function. If I zoom I see it spends most of the time in
> reset_all_temps.
> 
> Any suggestions how to improve it?
> 

Try this patch:
http://lists.nongnu.org/archive/html/qemu-devel/2015-08/msg02042.html

Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net