aaron.ballman added subscribers: foad, RKSimon, sepavloff, dblaikie, chandlerc.
aaron.ballman 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; };
----------------
tbaeder wrote:
> aaron.ballman wrote:
> > tbaeder wrote:
> > > 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?
> > Let's avoid serializing the floats to strings so that we can parse the
> > string to turn it back into a float later; that's going to have poor
> > performance even if we do get all the corner cases correct regarding things
> > like rounding, etc.
> >
> > `APFloat` does not have any sort of serialization functionality beyond
> > through strings representing the value. I think you'd have to invent such
> > an interface.
> Do you know who I might talk to regrading such an interface, both the
> implementation as well as general feasibility?
I think there may be at least two ways to do this: use an `APFloat` and put the
serialization interfaces there, or use an `APValue` and put the serialization
interfaces there.
Because `APFloat` is an ADT in LLVM, I think it should probably go up on
Discourse for broader discussion. @chandlerc is still listed as the code owner
for ADTs but he's not been active in quite some time. Instead, I would
recommend talking to @dblaikie (he's got a good eye for ADT work in general)
and @foad, @RKSimon, and @sepavloff as folks who have recently been touching
`APFloat`.
`APValue` is a Clang-specific class, and it already has some amount of
serialization support, it seems
(https://github.com/llvm/llvm-project/blob/main/clang/include/clang/AST/APValue.h#L54).
From a quick look, it seems we're already using `APValue` in a reasonable
number of places in the interpreter, so it might make sense to use this object
consistently to represent all values in the new interpreter?
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D134859/new/
https://reviews.llvm.org/D134859
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