[cfe-dev] [RFC] Proposing an Extended Integer Type

Richard Smith via cfe-dev cfe-dev at lists.llvm.org
Wed Feb 5 18:02:07 PST 2020

On Wed, 5 Feb 2020 at 15:36, Chris Lattner via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Feb 5, 2020, at 7:23 AM, Keane, Erich <erich.keane at intel.com> wrote:
> >The first case seems unfortunate/inconsistent -- are you sure it's
> necessary to do that? When dealing with temporary values in an expression,
> LLVM should be able to determine that most of the bits aren't useful, and
> optimize based upon that. I'd expect that to be generally sufficient for
> handling the usual-int-promotion temporary values inside of an expression.
> We found that it wasn’t the case in many cases (particularly in compound
> cases where intermediate values get large), which is incredibly expensive
> on some platforms.  Additionally, it heavily discourages the use of ‘auto’,
> which is otherwise incredibly useful for these types.
> C’s promotion rules are unfortunate for almost everything :-).
> I think there are two reasonable paths here:
> 1) Go for consistency with C and do the promotions, relying on the
> optimizer to trim them out.  You’ll want a suite of narrowing optimizations
> in any case.
> 2) Go for strict equivalent with no promotion rules, requiring everything
> to be explicit.
> I agree with you that promotions are extremely extensive for FPGAs and the
> entire point of using this extension is to control widths.  If so, I think
> that argues for approach #2, which means that “int7 + int8” should be a
> compile time error, and that “int8” should be semantically different (but
> explicitly convertible) to other types like char that happen to have the
> same width.

I agree with going for option #2 for the time being. I also think this is
the part of the proposed semantics that WG14 is most likely to request
revisions to, so it seems reasonable to pick an option that rejects all
cases whose semantics might reasonably be expected to change.
Using the C rules for mixed _ExtInt / standard integer type calculations
seems like it may be reasonable.
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