rjmccall added inline comments.
================
Comment at: llvm/include/llvm/ADT/FloatingPointMode.h:26
+/// assigned to the rounding modes must agree with the values used by
FLT_ROUNDS
+/// (C11, 5.2.4.2.2p8).
+enum class RoundingMode : int8_t {
----------------
sepavloff wrote:
> rjmccall wrote:
> > sepavloff wrote:
> > > rjmccall wrote:
> > > > I agree that we should use one enum across LLVM and Clang. I'm not
> > > > sure that using the `FLT_ROUNDS` values is worthwhile, especially since
> > > > (1) `FLT_ROUNDS` doesn't specify a value for some of these (like
> > > > `NearestTiesToAway`) and (2) some of the values it does use (e.g. for
> > > > "indeterminable") make this actively more awkward to store. And the
> > > > most useful thing we could do — matching the values of `FE_TONEAREST`
> > > > and so on — isn't possible because those values are unportable. I'd
> > > > rather we just pick arbitrary, non-ABI-stable values, like we normally
> > > > would, and then make the places that rely on matching some external
> > > > schema translate.
> > > > (1) FLT_ROUNDS doesn't specify a value for some of these (like
> > > > NearestTiesToAway)
> > >
> > > In the recent C standard draft
> > > (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2454.pdf), there is
> > > support of all 5 rounding modes including values returned by `FLT_ROUNDS`
> > > (5.2.4.2.2p11), values used by `fegetround` and `fesetround`
> > > (FE_TONEARESTFROMZERO in 7.6p13)
> > >
> > > > (2) some of the values it does use (e.g. for "indeterminable") make
> > > > this actively more awkward to store.
> > >
> > > This is not a rounding mode value, it is just error indicator returned by
> > > intrinsic functions, it does not need to be stored. Added comment about
> > > that.
> > >
> > > > And the most useful thing we could do — matching the values of
> > > > FE_TONEAREST and so on — isn't possible because those values are
> > > > unportable.
> > >
> > > I am working on patch that implements `fesetround` as intrinsic function.
> > > It introduces two intrinsic functions, one is `llvm.set_rounding` (D74729
> > > [FPEnv] Intrinsic for setting rounding mode) and the other is
> > > `llvm.convert_rounding` (unpublished yet). The latter translates
> > > platform-dependent values like FE_DOWNWARD into platform independent
> > > representation, which is the same as used by FLT_ROUNDS.
> > >
> > > Actually the motivation for this patch was just the need to have
> > > platform-independent representation of rounding mode that could be used
> > > in IR, which is more or less platform-independent. The representation
> > > used by `FLT_ROUNDS` fits these purposes because:
> > > * it is platform-neutral,
> > > * it is defined by standard, and
> > > * it encodes all IEEE rounding modes.
> > Okay. I'm just worried that trying to match `FLT_ROUNDS` in our internal
> > representation is ultimately going to cause unnecessary problems. If C has
> > standardized a value for NearesetTiesToAway, that certainly helps avoid
> > that. `FLT_ROUNDS` allows targets to add implementation-defined values;
> > are there any targets that support other rounding modes that aren't
> > currently described?
> There are 10 possible "arithmetic" rounding modes
> (https://upload.wikimedia.org/wikipedia/commons/8/8a/Comparison_rounding_graphs_SMIL.svg).
> Current implementation of glibc does not define anything beyond the
> standard. I know there are cores that use non-IEEE modes "stochastic rounding
> to nearest" and "away from zero", don't know if they expose these modes
> through standard interface like FLT_ROUNDS. It looks like there is no
> precedent of extending the value set returned by FLT_ROUNDS.
Okay, thanks, that's the kind of thing I meant. I have no objection to
matching FLT_ROUNDS for the basic IEEE set of rounding modes that we have today
— hey, we might as well — but we should acknowledge that fundamentally this is
not the same as FLT_ROUNDS, and that places that need to generate a FLT_ROUNDS
value may need a conversion, even if that conversion is likely very simple.
For example, if targets with non-IEEE rounding modes wanted to support them in
LLVM, we will just assign them the next available enumerators in
`RoundingMode`, and they can separately make a policy decision about what they
want to return from FLT_ROUNDS (either -1 or some implementation-defined value).
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D77379/new/
https://reviews.llvm.org/D77379
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