RFR: Remove C2 write-barrier from .ad files

[Roman Kennke]

Am 09.07.2018 um 18:34 schrieb Aleksey Shipilev:
> On 07/09/2018 06:07 PM, Roman Kennke wrote:
>> I propose to get rid of write-barriers in C2 .ad files, and instead
>> always expand WBs in ideal graph. We've been doing this for quite a
>> while now by default, with seemingly no ill effects.
>>
>> In next step, this will make it easier to move
>> MacroAssembler::shenandoah_write_barrier() away into gc/shenandoah
>> subfolder.
>>
>> http://cr.openjdk.java.net/~rkennke/remove-c2-wb/webrev.01/
> 
> That makes sense, looks good. Would it make sense to leave the ShenandoahWriteBarrier matcher with
> ShouldNotReachHere()? Or are we crashing the compilation reliably (e.g. in release builds?) when
> Ideal is not matched?

I'm pretty sure that C2 blows up with 'bad AD file' if it can't match a
ShenandoahWB. (Had that problem today when I tried traversal+LVB and got
a ShenandoahEnqueueBarrier that slipped through...)

Roman