[PATCH] D67517: Create UsersManual section entitled 'Controlling Floating Point Behavior'

[John McCall via Phabricator via llvm-commits]

rjmccall added inline comments.


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Comment at: clang/docs/UsersManual.rst:1152
+
+  This option implies:
+
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cameron.mcinally wrote:
> mibintc wrote:
> > rjmccall wrote:
> > > To be clear, fast math is not *equivalent* to just setting all of these separately, right?
> > I think in fact ffast-math is the same as selecting all the invidivdual settings, part of the logic checks if all the features are enabled and then also sets the fast-math macro. 
> Just checked for crtfastmath.o on the link line and they do not seem to be "equivalent", unless I made a mistake.
> 
> That said, I think the language "implies" is acceptable here.
Even if the only difference was the macro, that would be quite important — the macros can result in substantially different behavior from math libraries, IIRC.  So I would appreciate a wording that carries less of a connotation that this is equivalent to this fixed set of other options.  Maybe "Some of the individual optimizations performed by fast-math can be separately enabled or disabled with the following options:"?

Melanie, are you saying that `__FAST_MATH__` is defined only if *all* of these options are set?  I think that's important to cover as well; perhaps:

  "``-ffast-math`` causes the macro ``__FAST_MATH__`` to be defined.  Some math libraries recognize this macro and change their behavior.  Using any of the options below to disable any of the individual optimizations in ``-ffast-math`` will cause ``__FAST_MATH__`` to no longer be set."


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