Re: [Qemu-devel] [PATCH v2 0/9] Improve TCG optimizer

[Blue Swirl]

On Sat, Sep 8, 2012 at 9:12 AM, Aurelien Jarno <address@hidden> wrote:
> On Sat, Sep 08, 2012 at 09:06:52AM +0000, Blue Swirl wrote:
>> On Sat, Sep 8, 2012 at 9:01 AM, Aurelien Jarno <address@hidden> wrote:
>> > On Sat, Sep 08, 2012 at 08:18:50AM +0000, Blue Swirl wrote:
>> >> On Fri, Sep 7, 2012 at 1:16 PM, Aurelien Jarno <address@hidden> wrote:
>> >> > This patch series improves the TCG optimizer, based on patterns found
>> >> > while executing various guest. The brcond ad setcond constant folding
>> >> > are useful especially useful when they are used to avoid some argument
>> >> > values (e.g. division by 0), and thus can be optimized when this 
>> >> > argument
>> >> > is a constant.
>> >> >
>> >> > This bring around 0.5% improvement on openssl like benchmarks.
>> >> >
>> >> >
>> >> > Modifications between V1 and V2 following feedback I got:
>> >> >  - In the first patch, account for the liveness analysis time and
>> >> >    optimizing pass time separately
>> >> >  - Fixed swith/break in patch 7 to correctly throw an error
>> >> >  - Added patch 9 to make the code more readable
>> >> > Other patches are unmodified.
>> >> >
>> >> >
>> >> > Aurelien Jarno (9):
>> >> >   tcg: improve profiler
>> >> >   tcg/optimize: split expression simplification
>> >> >   tcg/optimize: simplify or/xor r, a, 0 cases
>> >> >   tcg/optimize: simplify and r, a, 0 cases
>> >> >   tcg/optimize: simplify shift/rot r, 0, a => movi r, 0 cases
>> >>
>> >> Aren't the above or/and/shift/rot simplifications (and also for
>> >> example OR with 0xfffffffff and XOR register by itself) already
>> >> handled by tcg/tcg-op.h?
>> >
>> > They are handled there when the values are known at decode time. It is
>> > not the case when the value are propagated in the TB.
>> >
>> > For example, this is optimized in tcg/tcg-op.h:
>> >   ori t0, t1, 0
>> >
>> > This is not optimized in tcg/tcg-op.h:
>> >   movi t2, 0
>> >   or t0, t1, t2
>>
>> I see. Does the optimizer pass then make the tcg/tcg-op.h optimization
>> redundant, could we do the optimizations only in optimizer?
>
> Technically yes. In practice it's a good idea to keep simple
> optimizations in tcg/tcg-op.h, as they cost less in CPU time than when
> done later.

OK. Could there be further optimizations based on tcg/tcg-op.h, for
example case OR reg, 0xffffffff -> mov reg, 0xffffffff could be
rechecked?

>
> On the other hand, we can remove such optimizations done in some
> TCG backends as they won't see this kind of ops anymore.
>
>> >
>> >> >   tcg/optimize: swap brcond/setcond arguments when possible
>> >> >   tcg/optimize: add constant folding for setcond
>> >> >   tcg/optimize: add constant folding for brcond
>> >> >   tcg/optimize: fix if/else/break coding style
>> >>
>> >> Otherwise a very nice series.
>> >>
>> >> >
>> >> >  tcg/optimize.c |  179 
>> >> > +++++++++++++++++++++++++++++++++++++++++++++++++++-----
>> >> >  tcg/tcg.c      |   12 +++-
>> >> >  tcg/tcg.h      |    1 +
>> >> >  3 files changed, 175 insertions(+), 17 deletions(-)
>> >> >
>> >> > --
>> >> > 1.7.10.4
>> >> >
>> >> >
>> >>
>> >>
>> >
>> > --
>> > Aurelien Jarno                          GPG: 1024D/F1BCDB73
>> > address@hidden                 http://www.aurel32.net
>>
>
> --
> Aurelien Jarno                          GPG: 1024D/F1BCDB73
> address@hidden                 http://www.aurel32.net