efriedma 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);
----------------
sepavloff wrote:
> 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.
" If no FENV_ROUND pragma is in effect, or the specified constant rounding mode
is FE_DYNAMIC, rounding is according to the mode specified by the dynamic
floating-point environment". And all the other rules for FENV_ROUND only apply
to "an FENV_ROUND pragma establishing a mode other than FE_DYNAMIC". So no
FENV_ROUND is equivalent to "FENV_ROUND FENV_DYNAMIC".
The text also says, "If the FE_DYNAMIC mode is specified and FENV_ACCESS is
'off', the translator may assume that the default rounding mode is in effect".
But that's the same result you'd get if you didn't specify FENV_ROUND at all:
it's just combining the general rule that you're not allowed to mess with the
dynamic rounding mode outside the scope of FENV_ACCESS, with the rule that the
initial rounding mode is "nearest".
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D126364/new/
https://reviews.llvm.org/D126364
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