[LLVMdev] LLVM Concurrency and Undef

Peter Lawrence peterl95124 at sbcglobal.net
Sat Sep 10 19:13:24 PDT 2011

On Aug 22, 2011, at 5:29 PM, Eli Friedman wrote:

>> LLVM Atomics and Concurrency guide also states the following
>> "Note that speculative loads are allowed; a load which is part of a
>> race returns undef, but does not have undefined behavior"
>> If the speculative loads returns an undef and the returned value is
>> used, then it results in an undefined behavior. Am I correct?
> It behaves like any other undef value... which do often lead to
> undefined behavior.

       Java by necessity has to be well defined in spite of data- 
races (because, IIUC, detecting
data-races in just about anything and java-byte-codes in particular  
is an N-P-complete problem,
so cannot be part of the byte-code validation process, therefore any  
byte-code which does have
a data-race and does pass validation must still execute with defined  

so, again IIUC, it appears that LLVM-IR could not be used for Java if  
we import C/C++'s
strict "data-race implies undefined" semantics into our IR.

Peter Lawrence.
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