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

[Blue Swirl]

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?

>
>> >   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