sepavloff added inline comments.
================
Comment at: clang/include/clang/Basic/LangOptions.h:622
setFPContractMode(LangOptions::FPM_Off);
setRoundingMode(static_cast<RoundingMode>(LangOptions::FPR_ToNearest));
setFPExceptionMode(LangOptions::FPE_Ignore);
----------------
efriedma wrote:
> sepavloff wrote:
> > efriedma wrote:
> > > sepavloff wrote:
> > > > efriedma wrote:
> > > > > I'm suggesting this should be
> > > > > `setRoundingMode(llvm::RoundingMode::Dynamic)`.
> > > > >
> > > > > If FENV_ACCESS is off, getEffectiveRoundingMode() converts that to
> > > > > "nearest". If FENV_ACCESS is turned on, the mode is treated as
> > > > > dynamic. This is exactly what we want.
> > > > It would change semantics. In particular, it would make impossible to
> > > > use FP arithmetic in constexpr functions. An expression like `1.0 /
> > > > 3.0` cannot be evaluated with dynamic rounding mode.
> > > Can we just change the relevant code in ExprConstant to call
> > > getEffectiveRoundingMode()?
> > What is the advantage of using FE_DYNAMIC in the case when it is known that
> > rounding mode is FE_TONEAREST?
> >
> > Probably `FENV_ROUND FE_DYNAMIC` should result in `dynamic` rounding even
> > if `FENV_ACCESS ON` is absent. Rounding mode cannot be changed in this case
> > but it can be used to inform the compiler that such function can be called
> > in environment, where rounding mode is non-default. It would prevent some
> > constant evaluations and other transformations that assume rounding mode is
> > known. Anyway the standard only allow to assume FE_TONEAREST but does not
> > require this.
> >
> > In such case `getEffectiveRoundingMode` is not needed.
> I really want to keep the state we store, and the state updates, as simple as
> possible; if that means making getEffectiveRoundingMode() or whatever more
> complicated, that's fine. It's a matter of making sure we understand all the
> relevant transitions.
>
> As far as I can tell, according to the standard, the initial state for
> FENV_ROUND is supposed to be FE_DYNAMIC, as if "#pragma STDC FENV_ROUND
> FE_DYNAMIC" were written at the beginning of every file. If we think we need
> a new state, something like FE_DYNAMIC_INITIAL, to represent the initial
> state, we can, I guess, but I don't think the standard text requires it.
> As far as I can tell, according to the standard, the initial state for
> FENV_ROUND is supposed to be FE_DYNAMIC, as if "#pragma STDC FENV_ROUND
> FE_DYNAMIC" were written at the beginning of every file.
Could you please give any reference to this statement? I thought the initial
state should be FE_TONEAREST, as it follows from F.8.3p1.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D126364/new/
https://reviews.llvm.org/D126364
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