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.Eli, 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 behavior). 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. sincerely, Peter Lawrence. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20110910/6b98b621/attachment.html>
On Sat, Sep 10, 2011 at 7:13 PM, Peter Lawrence <peterl95124 at sbcglobal.net> wrote:> > 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. > > Eli, > 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 > behavior).Yes.> 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.You can't use regular load and store instructions, but I have recently added load and store instructions with the appropriate semantics for Java to the LLVM IR. See http://llvm.org/docs/Atomics.html#o_unordered . -Eli
Eli,
ah, excellent.
-Peter.
On Sep 10, 2011, at 7:35 PM, Eli Friedman wrote:>
>
>> 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.
>
> You can't use regular load and store instructions, but I have recently
> added load and store instructions with the appropriate semantics for
> Java to the LLVM IR. See
> http://llvm.org/docs/Atomics.html#o_unordered .
>
> -Eli
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