Re: [Qemu-devel] [PATCH RFC 00/11] AREG0 elimination

[Blue Swirl]

On Sun, May 15, 2011 at 2:14 PM, Aurelien Jarno <address@hidden> wrote:
> On Sun, May 15, 2011 at 02:03:40PM +0300, Blue Swirl wrote:
>> On Sun, May 15, 2011 at 12:27 PM, Laurent Desnogues
>> <address@hidden> wrote:
>> > On Sun, May 15, 2011 at 9:15 AM, Blue Swirl <address@hidden> wrote:
>> >> On Sun, May 15, 2011 at 1:04 AM, Aurelien Jarno <address@hidden> wrote:
>> >>> On Sun, May 15, 2011 at 12:52:35AM +0300, Blue Swirl wrote:
>> >>>> On Sun, May 15, 2011 at 12:16 AM, Aurelien Jarno <address@hidden> wrote:
>> >>>> > On Sat, May 14, 2011 at 10:35:20PM +0300, Blue Swirl wrote:
>> > [...]
>> >>>> > The env register is used very often (basically for every load/store, 
>> >>>> > but
>> >>>> > also a lot of helpers), so it makes sense to reserve a register for 
>> >>>> > it.
>> >>>> >
>> >>>> > For what I understand from your patch series, you prefer to pass this
>> >>>> > register explicitly to TCG functions. This basically means this TCG
>> >>>> > global will be loaded to host register as soon as it is used, but also
>> >>>> > regularly, as globals are saved back to their canonical location 
>> >>>> > before
>> >>>> > an helper or a load/store.
>> >>>> >
>> >>>> > So it seems that this patch series will just allowing the "env 
>> >>>> > register"
>> >>>> > to change over time, though it will not spare one more register for 
>> >>>> > the
>> >>>> > TCG code, and it will emit longer TCG code to regularly reload the env
>> >>>> > global into a host register.
>> >>>>
>> >>>> But there will be one more register available in some cases. In other
>> >>>
>> >>> Inside the TCG code, it will basically happens very rarely, given
>> >>> load/store are really the most used instructions, and they need to load
>> >>> the env register.
>> >>
>> >> Not exactly, from a sample run with -d op_opt:
>> >> $ egrep -v -e '^$' -v -e 'OP after' -v -e ' end' -v -e 'Search PC'
>> >> /tmp/qemu.log | awk '{print $1}' | sort | uniq -c|sort -rn
>> >> 1673966 movi_i32
>> >>  653931 ld_i32
>> >>  607432 mov_i32
>> >>  428684 st_i32
>> >>  326878 movi_i64
>> >>  308626 add_i32
>> >>  283186 call
>> >>  256817 exit_tb
>> >>  207232 nopn
>> >>  189388 goto_tb
>> >>  122398 and_i32
>> >>  117997 shr_i32
>> >>  89107 qemu_ld32
>> >>  82926 set_label
>> >>  82713 brcond_i32
>> >>  67169 qemu_st32
>> >>  55109 or_i32
>> >>  46536 ext32u_i64
>> >>  44288 xor_i32
>> >>  38103 sub_i32
>> >>  26361 shl_i32
>> >>  23218 shl_i64
>> >>  23218 qemu_st64
>> >>  23218 or_i64
>> >>  20474 shr_i64
>> >>  20445 qemu_ld64
>> >>  11161 qemu_ld8u
>> >>  10409 qemu_st8
>> >>   5013 qemu_ld16u
>> >>   3795 qemu_st16
>> >>   2776 qemu_ld8s
>> >>   1915 sar_i32
>> >>   1414 qemu_ld16s
>> >>    839 not_i32
>> >>    579 setcond_i32
>> >>    213 br
>> >>     42 ext32s_i64
>> >>     30 mul_i64
>> >
>> > Unless I missed something, this doesn't show the usage of
>> > ld/st per TB, which is what Aurélien was looking for if I
>> > understood correctly.  All I can say is that you had at
>> > most 256817 TB's and 234507 qemu_ld/st, so about one per
>> > TB.
>>
>> The question was ratio of loads/stores to other instructions. The
>> statistics are not per TB. There were about 174880 TBs.
>>
>> > Anyway I must be thick, because I fail to see how
>> > generated code could access guest CPU registers without a
>> > pointer to the CPU env :-)
>> >
>> > IIUC the SPARC translator uses ld_i32/st_i32 mainly for
>> > accessing the guest CPU registers, which due to register
>> > windows is held in a dedicated global temp.  Is that
>> > correct?  If so this is kind of hiding accesses to the
>> > CPU env;  all other targets read/write registers by using
>> > CPU env (through the use global temps in most cases).
>> >
>> > So I think most (if not almost all) TB will need a pointer
>> > to CPU env, which is why I think Aurélien's proposal to
>> > keep a dedicated register that'd be loaded in the prologue
>> > is the only way to not degrade performance of the
>> > generated code (I'd add that this dedicated register
>> > should be the one defined by the ABI as holding the first
>> > parameter value, if that's possible;  I'm afraid this is
>> > not necessarily a good idea).
>>
>> CPU env will be used, but the register could be made available for
>> other uses too.
>>
>
> What other uses? It is needed for almost every TB, so there is not other
> possible use. Let's take an example, a simple TB from mips on x86_64:
>
> | IN: start_kernel
> | 0x80510958:  jal        0x8051d50c
> | 0x8051095c:  nop
> |
> | OP after liveness analysis:
> |  movi_i32 ra,$0x80510960
> |  movi_i32 PC,$0x8051d50c
> |  exit_tb $0x0
> |  end
>
> | OUT: [size=28]
> | 0x40f22960:  mov    $0x80510960,%ebp
> | 0x40f22965:  mov    %ebp,0x7c(%r14)
> | 0x40f22969:  mov    $0x8051d50c,%ebp
> | 0x40f2296e:  mov    %ebp,0x80(%r14)
> | 0x40f22975:  xor    %eax,%eax
> | 0x40f22977:  jmpq   0x115042e
>
> x86_64 uses r14 as TCG_AREG0. Despite the instructions being quite
> simple (only 2 movi_i32), the resulting code makes 2 access to env to
> save the two registers. Having to reload the env pointer each time to a
> register would clearly increase the size of this TB.

I don't think TCG would be that simple, instead the pointer would be
loaded only once in this case.

> Another question, imagine env is not in a register anymore. What kind of
> code do you plan to use to load the env pointer into a register? This is
> something we need to know to evaluate the cost of not having a fixed
> register for env withing the TCG code.

The prologue could save it to the first temp dedicated for this.