[LLVMdev] readonly and infinite loops

[Hal Finkel]

----- Original Message -----
> From: "Sanjoy Das" <sanjoy at playingwithpointers.com>
> To: "Hal J. Finkel" <hfinkel at anl.gov>
> Cc: "Nuno Lopes" <nunoplopes at sapo.pt>, "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Saturday, June 27, 2015 7:17:16 PM
> Subject: Re: [LLVMdev] readonly and infinite loops
> 
> On Sat, Jun 27, 2015 at 3:36 PM, Finkel, Hal J. <hfinkel at anl.gov>
> wrote:
> > No; the standard says that we get to assume that the loop will
> > eventually
> > terminate, or do I/O, or a few other possible things. I can find
> > the text
> > for you later if you'd like.
> 
> Actually, I'm more interested in what LLVM (not C/C++) says about
> "while(1);" loops.  Such loops are legal in Java (and possibly other
> languages that use LLVM) so it's problematic if LLVM specifies them
> to
> have UB.

I believe our general policy is not to declare such behavior undefined, at the IR level. However, we also need to be careful to specify what the observable effect of such a program would be, and how changes to that state should be interpreted.

 -Hal

> 
> -- Sanjoy
> 
> 
> 
> 
> 
> >
> > -Hal
> >
> > Sent from my Verizon Wireless 4G LTE DROID
> >
> >
> > Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
> >
> > On Sat, Jun 27, 2015 at 2:16 PM, Nuno Lopes <nunoplopes at sapo.pt>
> > wrote:
> >> At least in C/C++ that's UB, yes.
> >
> > So you cannot map every turing machine to a valid C/C++ program
> > then. :)
> >
> > Also, does this mean that "daemon" programs that run continuously
> > till
> > they're killed by the OS (using a mechanism that is not visible in
> > C)
> > are effectively undefined?
> >
> > -- Sanjoy
> >
> >> Some years ago there was a lot of discussion about this on the ML.
> >>  Se
> >> Nick's patch, for example:
> >>
> >> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20100705/103670.html
> >> The patch was eventually dropped, I don't recall why.  I think
> >> it's fine
> >> to
> >> assume that readnone functions always terminate.  It would
> >> probably fine
> >> for
> >> readonly functions as well.
> >>
> >> Nuno
> >>
> >>
> >> -----Original Message-----
> >> From: Sanjoy Das
> >> Sent: Saturday, June 27, 2015 9:29 PM
> >> Subject: [LLVMdev] readonly and infinite loops
> >>
> >> Running -early-cse on
> >>
> >> declare void @rn() readnone nounwind
> >>
> >> define void @f() {
> >> entry:
> >>  call void @rn()
> >>  ret void
> >> }
> >>
> >> removes the call to @rn().  But @rn() could have had an infinite
> >> loop
> >> in it in which case @f() went from being a non-terminating
> >> program to an terminating no-op.  Is this intentional?
> >>
> >> The only way I can see this transform being legal is if infinite
> >> loops
> >> are declared to have undefined behavior, but I could not find
> >> anything
> >> in the LLVM specification that mentions this.
> >>
> >> -- Sanjoy
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> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory