rjmccall added a reviewer: scanon.
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:
> > 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 `llvm.convert_rounding` (unpublished yet).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?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D77379/new/
https://reviews.llvm.org/D77379
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