[PATCH] D143074: [LangRef] improve documentation of SNaN in the default FP environment

[Mak Nazečić-Andrlon via Phabricator via llvm-commits]

Muon added a comment.

Hold on, that still says that operations //assume// that all NaNs are quiet. Doesn't that mean that passing a signaling NaN to an operation is potentially undefined behavior? Can we instead say that math operations treat all NaNs as if they were quiet NaNs? Does that run into issues with pow() and friends?

Also, "status flags may be set to arbitrary values" by what exactly? Is it assuming that the initial state is arbitrary? If it's talking about operations, it certainly doesn't imply that they are side effect-free. Quite the opposite, since it implies that operations do in fact have side effects and can do arbitrary things to the status flags, and therefore can never be reordered. This is worse than the normal semantics, since those at least ensure that operations can be freely reordered as long as the reordering doesn't cross a read or clear of the status flags.

In order to actually be side effect-free you need to declare that status flags don't exist, are treated as nonexistent, or that any attempt to read the status flags may return arbitrary values. As soon as operations are declared to interact (or not interact) with status flags, you have the problem of making sure they are (or aren't) modified by them.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D143074/new/

https://reviews.llvm.org/D143074