[LLVMdev] Question: Motivation of the semantics of the undefined value?

[Chung-Kil Hur]

Dear Sanjoy Das,

I can see the point.
Thanks a lot for the instructive example.

Best,
Chung-Kil Hur

On Tue, Mar 17, 2015 at 4:29 AM, Sanjoy Das <sanjoy at playingwithpointers.com>
wrote:

> A trap representation prevents hoisting operations over control flow.
> For instance, if we defined reading uninitialized memory to trap, then
> we could not transform
>
>   %t = alloca
>   call void @escape(%t)
>   loop n times {
>     %m = load %t
>     use(%m)
>   }
>
> to
>
>   %t = alloca
>   call void @escape(%t)
>   %m = load %t
>   loop n times {
>    use(%m)
>   }
>
> since if n was 0 and @escape did not initialize %t, we would have
> introduced a trap that was not present in the original program.
>
> On Mon, Mar 16, 2015 at 8:49 AM, Chung-Kil Hur <gil.hur at sf.snu.ac.kr>
> wrote:
> > Dear LLVMdev,
> >
> > We have a question regarding the semantics of the undefined value in LLVM
> > IR.
> >
> > As far as we understand, in order to give a semantics to the undefined
> > value, the semantics of LLVM IR is generalized to a rather unusual
> set-based
> > semantics. More specifically, the notion of value in LLVM IR is
> generalized
> > to a set of values, rather than a single value. Then the undefined value
> is
> > simply defined as the set of all values.
> >
> > Our question is simply why LLVM does not use the notion of trap
> > representation of C11 standard to represent the undefined value, rather
> than
> > using the set-based semantics.
> >
> > We'd like to understand the motivation of the set-based semantics over
> the
> > trap representation.
> >
> > Thanks.
> > Chung-Kil Hur, Steve Zdancewic & Viktor Vafeiadis.
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
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