[LLVMdev] LLVM Concurrency and Undef (original) (raw)

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.

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>

More information about the llvm-dev mailing list