[cfe-dev] [llvm-dev] [RFC] Introducing a byte type to LLVM

[Nicolai Hähnle via cfe-dev]

On Mon, Jun 14, 2021 at 7:34 AM John McCall via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> On 13 Jun 2021, at 11:26, Ralf Jung wrote:
>
> > Hi Johannes,
> >
> >>> I think Joshua gave a very nice motivation already.
> >>
> >> I don't dispute that but I am still not understanding the need for
> >> bytes. None of the examples I have seen so far
> >> clearly made the point that it is the byte types that provide a
> >> substantial benefit. The AA example below does neither.
> >
> > I hope
> > <https://lists.llvm.org/pipermail/llvm-dev/2021-June/151110.html>
> > makes a convincing case that under the current semantics, when one
> > does an "i64" load of a value that was stored at pointer type, we have
> > to say that this load returns poison. In particular, saying that this
> > implicitly performs a "ptrtoint" is inconsistent with optimizations
> > that are probably too important to be changed to accommodate this
> > implicit "ptrtoint".
>
> I think it is fact rather obvious that, if this optimization as
> currently written is indeed in conflict with the current semantics, it
> is the optimization that will have to give.  If the optimization is too
> important for performance to give up entirely, we will simply have to
> find some more restricted pattern that wee can still soundly optimize.
>

I tend to agree. I don't think Ralf's example alone is convincing evidence
that pointer-load of integer-store must be poison, i.e. memory must be
typed.

FWIW, the least important optimization in that example's chain, and the one
that is most obviously incorrect in an untyped memory world, is eliminating
a store of a previously loaded value. How much would we actually lose if we
disable this particular optimization? Note that this is only a small
special case of dead store elimination. The more common case where there
are two stores to memory in a row, and the first one is eliminated, is
still correct.

Cheers,
Nicolai



>
> Perhaps the clearest reason is that, if we did declare that integer
> types cannot carry pointers and so introduced byte types that could, C
> frontends would have to switch to byte types for their integer types,
> and so we would immediately lose this supposedly important optimization
> for C-like languages, and so, since optimizing C is very important, we
> would immediately need to find some restricted pattern under which we
> could soundly apply this optimization to byte types.  That’s assuming
> that this optimization is actually significant, of course.
>
> John.
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