C2, ThreadLocalNode, and Loom

[Kennke, Roman]

> 2) Remodel ThreadLocalNode as non-constant
> 
> What partially solves the problem: saying that ThreadLocalNode::hash() 
> is NO_HASH. AFAICS, this
> successfully prevents collapsing TLNs in the same compilation. This 
> still does not solve the case
> where a single TLN gets yanked to the earliest block and its value 
> cached in the register.
> 
> AFAIU, we only want to make sure that TLN is reloaded after the 
> potential continuation yield, which
> also serves as the point of return. Since continuation yields are 
> modeled as calls, and calls
> produce both control and memory, we might need to hook up TLN to either 
> control or memory.
> 
> I tried to hook up the current control to every TLN node [1]. It works 
> with a few wrinkles, but the
> patch shows there are ripple effects throughout C2 code, and it 
> sometimes breaks the graph. Some
> pattern matching code (for example AddP matching code in EA) also 
> asserts, probably assuming that
> TLNs have no inputs. I suspect other places might have implicit 
> dependencies like these as well.
> This would be the inevitable consequence for any patch that changes 
> ThreadLocalNode inputs/outputs.

This reminds me of a similar problem that doesn't have a good solution 
yet, and solving one might be a template for the other. In GC barriers, 
one thing that is very commonly done is 1. Load GC state from a global 
field, 2. Branch to slow- (or mid-) path if GC is supposed to take some 
action. This GC state is loaded at every barrier site, because it's 
treated as RAW memory. However, since it only ever changes at 
safepoints, it would be nice to be able to tell C2 that it doesn't need 
reloading between safepoints. I suspect that if we had a 
constant-between-safepoints notion in C2, several other uses and 
possible optimizations could be found.

Roman




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