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

[Roman Kennke]

Am 12.03.2018 um 10:51 schrieb Aleksey Shipilev:
> On 03/12/2018 10:38 AM, Roman Kennke wrote:
>> Notice that the situation before was different: we'd have a fast path
>> that avoided FPU altogether, and would still call the stub with FPU
>> spilling, which was a bit braindead.
> 
> Um, yes. But we are optimizing the fast-path here: the part when we _do not_ enter the slowpath, and
> we want to enjoy FPU spills in the generated code.
> 
> It helps to back-track a bit: did eliminating AsmWB regress our workloads? It should, right, if
> saving FPU registers was the issue on slow path? It apparently wasn't the issue then, so let's
> concentrate on making the real fast-path code fast with FPU spills. If AsmWB removal was
> performance-sensitive, then we should backtrack it too, because the choice between doing the
> callee-saving vector registers and enabling FPU spills is not trivial.
> 
>>>> However, I don't really understand the FPU spilling issue. In my mind,
>>>> it *should* turn out something like:
>>>>
>>>> if (evac-in-progress && in_cset(obj)) {
>>>>   save_fpu_regs();
>>>>   call_runtime_stub();
>>>>   restore_fpu_regs();
>>>> }
>>>
>>> Maybe, but I would not testify how C2 tracks the register dependencies. The trouble is, how do we
>>> communicate that both branches do not affect XMM registers? Doing CallLeafNoFP to wb_stub is
>>> supposed to do that, I think.
>>
>> CallLeaf tells C2 that the call might spoil FPU regs. C2/register
>> allocator should be smart enough to notice the other (empty) paths are
>> free of any FPU reg usage, wouldn't it?
> 
> Ah, "magic compiler" card! But the control flow would merge anyhow at some point, and I don't think
> C2 is doing caller-saves on "rare" paths to free up registers (especially vector ones). So we are
> where we are: making the no-brainer solution with CallLeafNoFP, and figuring out the vector register
> spills in the stub itself.
> 

Ok, I guess I misunderstood some of the issues.

Let's withdraw this patch and keep the stub, to enable FPU spilling
around WBs. I'll propose a little bit of streamlining and cleanup to the
stub soon.


Roman