RFR(M): 8038297 Avoid placing CTI immediately following cbcond instruction on T4

[Igor Veresov]

Vladimir, thanks for the review!

Here is the corrected webrev:  http://cr.openjdk.java.net/~iveresov/8038297/webrev.01/

Thanks!
igor

On Apr 10, 2014, at 2:29 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com> wrote:

> Igor,
> 
> Please, clean up code style of if() in  MachEpilogNode methods you touched in sparc.ad. The same in output_h.cpp.
> 
> I think you should separate into 2 flags Flag_avoid_before and Flag_avoid_after:
> 
> Flag_avoid_back_to_back  = Flag_may_be_short_branch << 1, // Flag has two bits
> 
> And use them instead of new MachNode::AvoidBackToBackFlag. I don't like next expression:
> 
> ins_avoid_back_to_back() * Flag_avoid_back_to_back
> 
> thanks,
> Vladimir
> 
> On 4/10/14 11:54 AM, Igor Veresov wrote:
>> We need to put nops between cbcond instructions and CTI instructions,
>> because otherwise it causes a pipeline flush.
>> This change extends the notion of the avoid_back_to_back instruction
>> attribute to hold a 2 bits flag, with bits being AVOID_BEFORE and
>> AVOID_AFTER.  The meaning of the bits is that an instruction with the
>> AVOID_BEFORE flag cannot immediately follow one with AVOID_AFTER flag.
>> The assembler has been modified accordingly to ensure that we don’t have
>> a CTI instruction immediately following a cbcond instruction.
>> 
>> Webrev: http://cr.openjdk.java.net/~iveresov/8038297/webrev.00/
>> 
>> Testing: CTW, hotspot regression tests, JPRT
>> 
>> There is a significant performance gain on the microbenchmark that
>> exposed the problem.
>> Before with -XX:+UseCBCond: 106.985ns
>> Before with -XX:-UseCBCond: 94.8967ns
>> After with -XX:+UseCBCond: 94.5262ns
>> 
>> 
>> Other performance results (some gains with jetstream and volano):
>> ==============================================================================
>> vanilla.logs:
>>   Benchmark           Samples        Mean     Stdev             Geomean
>> Weight
>>   reference_server         20    16136.35    107.24
>>     jetstream              20      153.30      1.63             0.10
>>     scimark                20      353.47      5.78             0.15
>>     specjbb2000            20   220984.11    638.71             0.15
>>     specjbb2005            20   287778.70   3987.52             0.25
>>     specjvm98              20      295.09      1.41             0.15
>>     volano25               20   224179.95   5213.65             0.20
>> ==============================================================================
>> hack1.logs:
>>   Benchmark           Samples        Mean     Stdev   %Diff     P
>>  Significant
>>   reference_server         20    16357.48    111.46    1.37 0.000
>>    Yes
>>     jetstream              20      160.82      2.07    4.90 0.000
>>    Yes
>>     scimark                20      351.65      5.53   -0.51 0.316
>>      *
>>     specjbb2000            20   221142.43   1247.38    0.07 0.617
>>      *
>>     specjbb2005            20   288196.75   4598.83    0.15 0.760
>>      *
>>     specjvm98              20      298.11      0.89    1.02 0.000
>>    Yes
>>     volano25               20   232848.15   3952.61    3.87 0.000
>>    Yes
>> ==============================================================================