RFR: RFR: Eliminate write-barrier assembly stub (part 2)

[Roman Kennke]

Sometimes I seem to get a C2 compilation error. I could only reproduce
it with release build so far. It happens when running TestGCBasher.
Roland, could you check this? Find attached the hs_err and replay files.

Thanks, Roman


> After I eliminated the assembly stub, it doesn't do anything anymore for
> C2, except for some shuffling of registers and doing 2 calls instead of
> one (compiled -> stub -> runtime). I strongly suspect that the register
> allocator can do the shuffling better, and avoiding the extra jump will
> also be a net win. At the very least, letting C2 call directly to the
> entry in ShenandoaBarrierSet removes a bunch of unnecessary code diffs
> against upstream:
> 
> http://cr.openjdk.java.net/~rkennke/eliminate_wb_stub-c2/webrev.01/
> 
> One thing I am not sure is this little piece in
> ShenandoahSupport::needs_barrier_impl() :
> 
>   if (n->is_CallJava() || n->Opcode() == Op_CallLeafNoFP) {
>     return true;
>   }
> 
> 
> Was this supposed to match the wb-stub-call (CallLeafNoFP) and if so,
> should it now match the runtime call (CallLeaf plus entry address)? But
> I don't really see the point: if it's a CallJava, it doesn't make a
> difference if it's also a CallLeafNoFP. Maybe the intention was to *not*
> match WB stub calls and it should have been:
> 
> 
>   if (n->is_CallJava() && n->Opcode() != Op_CallLeafNoFP) {
>     return true;
>   }
> 
> ?
> Because that would kinda make sense: we want barriers for return values
> from java calls, but not really from wb-runtime-calls. Hmmmm. This is
> why I'd propose to change it to:
> 
>   if (n->is_CallJava() && !(n->Opcode() == Op_CallLeaf &&
> n->as_Call()->entry_point() == CAST_FROM_FN_PTR(address,
> ShenandoahBarrierSet::write_barrier_JRT))) {
>     return true;
>   }
> 
> I don't think this would make much difference though: all this stuff can
> only match after WB expansion, at which point such optimizations (WB
> after WB) should already have happened.
> 
> Passes hotspot_gc_shenandoah (fastdebug/release) and specjvm
> (fastdebug/release)
> 
> Ok to go?
>