Entry point publishing in Truffle OptimizedCallTarget, on AArch64

[Aleksandar Prokopec]

Dear Andrew,

I have yesterday submitted a pull request to the Graal compiler, which 
should resolve a rare race condition that happens during Truffle's code 
installation. In short, the race condition existed because: after the 
code got installed with JVMCI, the "entryPoint" field in the 
"InstalledCode" class became visible to other threads, but the 
Truffle-level assumptions were not yet associated with that code [2].

My fix solves this by introducing the rule that Truffle 
OptimizedCallTarget's entry points addresses can only be jumped to if 
the lowest bit of the "entryPoint" field is set to "1". In other words, 
setting the lowest bit means that the "entryPoint" is published. The 
tail call code that gets patched at the beginning of the code of every 
OptimizedCallTarget must now not only check that the "entryPoint" is 
non-null, but also check that the lowest bit is "1".

I have implemented the fix on x86, SPARC and AArch64. However, I was 
only able to run and test this x86 and SPARC, since we have no AArch64 
machines. Since you've been involved with a lot of AArch64 maintenance 
in the past, I am assuming that you have access to a proper machine. If 
so, could you perhaps validate that my fix on AArch64 is correct (files 
under links [3] [4] [5]), or suggest changes?

Note that the new functionality, "entryPoint" tagging, is currently 
disabled [6] on AArch64, since I was not sure about my fix.

Thanks a lot,
Alex

[1] 
https://github.com/graalvm/graal/commit/28b5474a1f6882f30c883c33012cd1bac8c2a646

[2] 
https://github.com/graalvm/graal/blob/master/compiler/src/org.graalvm.compiler.truffle/src/org/graalvm/compiler/truffle/TruffleCompiler.java#L234

[3] 
https://github.com/graalvm/graal/commit/28b5474a1f6882f30c883c33012cd1bac8c2a646#diff-d53ecccd225be6a6ece41d8c64579c7a

[4] 
https://github.com/graalvm/graal/commit/28b5474a1f6882f30c883c33012cd1bac8c2a646#diff-7a3deff5793b399e8a3b008e14fb14cf

[5] 
https://github.com/graalvm/graal/commit/28b5474a1f6882f30c883c33012cd1bac8c2a646#diff-6a1d15e2ba69aa5ed34a906ecf895a1f

[6] 
https://github.com/graalvm/graal/commit/28b5474a1f6882f30c883c33012cd1bac8c2a646#diff-b171c7113ea386f9a5af29d46cd4e7bcR238