tbaeder added inline comments.
================
Comment at: clang/lib/AST/Interp/Floating.h:27-29
+ template <unsigned ReprBits> struct Repr;
+ template <> struct Repr<32> { using Type = float; };
+ template <> struct Repr<64> { using Type = double; };
----------------
jcranmer-intel wrote:
> tbaeder wrote:
> > jcranmer-intel wrote:
> > > aaron.ballman wrote:
> > > > Er, how will this extend to `long double` where the number of bits is
> > > > rather more difficult?
> > > Or `half` and `bfloat`, which are both 16-bit floating-point types?
> > I have spent some time with this today and tried to simply always use
> > `APFloat` instead of a primitive type. Unfortunately that doesn't work
> > because what we put on the stack is not the `Floating` (or `Integral`), but
> > the underlying primitive type. So even if we do the final math (in `::add`,
> > etc) via `APFloat`, we need something we can serialize to `char[]` so we
> > can put it on the stack. Do you think that would work?
> I don't know enough about the structure of the bytecode interpreter here to
> say for sure, but this smells to me like you're baking in an assumption that
> every primitive target type has a corresponding primitive type on the host.
> This assumption just doesn't hold when it comes to floating point (only two
> of the seven types, `float` and `double`, are generally portable, and even
> then, there be dragons in some corner cases).
>
> If you do need to continue down this route, there are two requirements that
> should be upheld:
> * The representation shouldn't assume that the underlying primitive type
> exists on host (bfloat16 and float128 are better test cases here).
> * Conversion to/from host primitive types shouldn't be easy to accidentally
> do.
>
> (Worth repeating again that bit size is insufficient to distinguish floating
> point types: `bfloat` and `half` are both 16-bit, PPC `long double` and IEEE
> 754 quad precision are both 128-bit, and x86 `long double` is 80 bits stored
> as 96 bits on 32-bit and 128 bits on 64-bit.)
Well, is there a way to convert an APFloat to a char[] that would work instead
of going to float/double and storing that? The only thing I see in the docs is
`convertToHexString()` (and the docs don't mention whether the conversion is
lossy). If not, do you think adding such a conversion to `APFloat` and its
various implementations is the better way forward?
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D134859/new/
https://reviews.llvm.org/D134859
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