[PATCH] D68115: Zero initialize padding in unions

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added a comment.

In D68115#1946990 <https://reviews.llvm.org/D68115#1946990>, @jfb wrote:

> In D68115#1946757 <https://reviews.llvm.org/D68115#1946757>, @rsmith wrote:
>
> > In D68115#1946668 <https://reviews.llvm.org/D68115#1946668>, @hubert.reinterpretcast wrote:
> >
> > > It sounds like we are looking for `-fzero-union-padding`. That's been where the discussion has left off twice for months.
> >
> >
> > I believe the state of Clang prior to this patch is actually wrong.
>
>
> That's my understanding as well. I'd like it if this patch (or a follow-up) got us back to standard behavior. In either case, I'd like the proposed behavior to bee on-by-default when doing initialization of stack variables.


Your preference is noted. However, I think the majority opinion expressed on this review at this point favors not guaranteeing zero-initialization except where required by the relevant standard. That'd also be consistent with our stance on trivial auto variable initialization in general.

I'm not yet sure about whether we want separate controls for this and for `-ftrivial-auto-init`, or whether from a user's perspective there's really only one question: should bits left uninitialized be `undef`, guaranteed zero, or guaranteed to be filled with a pattern -- independent of whether they're padding bits? (And related, do we actually want control over zeroing union padding in all cases or only for trivial automatic variables? And do we want control over zeroing or pattern-filling objects allocated with `new` with trivial initialization?)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D68115/new/

https://reviews.llvm.org/D68115