[PATCH] D126364: Fix interaction of pragma FENV_ACCESS with other pragmas

[Serge Pavlov via Phabricator via cfe-commits]

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.


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  https://reviews.llvm.org/D126364/new/

https://reviews.llvm.org/D126364