dblaikie 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:
> aaron.ballman wrote:
> > 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?
> Going through `APInt` is already possible in `APFloat`, and that might have
> some methods to go to char arrays.
Yeah - IR gets serialized APFloats somehow (maybe through some layers of
indirection) so I'd suggest looking at how, say, a global float named constant
gets lowered to bitcode to see how that APFloat is serialized.
If that's not enough of a pointer for you/others to find something helpful, I
can look further to see what I can find (I got some of the way - a small
example with just `extern const float v; const float v = 3.14;` and breaking on
APFloat's ctor immediately finds the APFLoat being constructed from that
literal - then I looked at llvm::GlobalVariable's ctor, which does break on the
creation of `v`, but didn't continue further to figure out how the parsed value
gets into the IR constant)
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D134859/new/
https://reviews.llvm.org/D134859
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