acmp/cas retries and null arguments

[Aleksey Shipilev]

On 09/13/2016 06:16 PM, Roman Kennke wrote:
>> acmp(%a, %b):
>>   test %a, %b
>>   je <true>
>>
>>     test %a, %a
>>     je <false>  // %a is null, guaranteed "false"
>>     mov %a, (%a - 8);
>>
>>     test %b, %b
>>     je <false>  // %b is null, guaranteed "false"
>>     mov %b, (%b - 8);
>>
>>     test %a, %b;
>>     je <true>
>>
>>     false:
>>     ret false;
>>
>>   true:
>>   ret true;
>>
>> This saves read barriers if one of the arguments is not null.
> 
> Right.
> C2 should avoid generating the special acmp and cas barriers if one of
> the args is *known* to be null.

Yes, in fact we have seen it:
 http://cr.openjdk.java.net/~shade/shenandoah/shenandoah-gc-bench/src/main/java/org/openjdk/shenandoah/acmp/ACmpBarriersKnownNulls.java

Granted, the most frequent (obj == null) test is catered by this.

> It's a little more involved if it determines at runtime that one is
> null. I'd be happy to see it optimized though :-)

Yup, but this is the essence of the trick: we nullcheck before
dereferencing through forwarding pointer anyway. We could instead break
out instantly, without trying to do other read and unnecessary
additional comparison.

I'll take a look after CASes codegen.

Thanks,
-Aleksey