sepavloff marked an inline comment as done.
sepavloff 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 {
----------------
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 `llvm.convert_rounding` (unpublished yet).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.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D77379/new/
https://reviews.llvm.org/D77379
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