[LLVMdev] LLVM Concurrency and Undef

[Eli Friedman]

On Mon, Aug 22, 2011 at 2:49 PM, Santosh Nagarakatte
<santosh.nagarakatte at gmail.com> wrote:
> Hi all,
>
> I have been trying to understand the use of undef in both sequential
> and concurrent programs.
>
> >From the LLVM Language Reference Manual, I see the following
> definition of undef.
> "Undef can be used anywhere a constant is expected, and indicates that
> the user of the value may receive an unspecified bit-pattern".
>  LLVM Language Reference manual also demonstrates how optimizers can
> use these undef values to  optimize the program.
>
> However, on the other hand, with the LLVM Atomics and Concurrency
> Guide states that
> If code accesses a memory location from multiple threads at the same
> time, the resulting loads return 'undef'.
> This is different from the C++ memory model, which provides undefined
> behavior. What is the rationale for returning an undef on racing
> reads?
>
> 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.

> If so, what is the purpose of returning an undef with a speculative load?
> Is it to ensure that the subsequent uses of the value of the
> speculatively introduced load is caught/detected by the optimization?

The point is primarily to allow optimizations like LICM to introduce
loads whose value is never used.  It also keeps consistent semantics
through CodeGen, where some targets widen loads.

> Is it possible to separate the "undef" in a sequential setting and
> "undef" with speculative loads in a concurrent setting with separate
> undefs?

The intention is that they should have the same semantics.

-Eli