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

[Sanjoy Das]

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.
>
>
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