Re: [PATCH 0/3] ARM with only 32-bit floats do not have fast 64-bit FMA

"Keith Packard" <[email protected]> writes:

> That implementation violates the spec though because it does two
> binary operations involving two roundings, so you get a different answer
> than you would with a true fma.

Hrm. C99 and C17 both have macros to detect whether fma is 'fast' or
not: FP_FAST_FMA, FP_FAST_FMAF and FP_FAST_FMAL. This page:

        https://en.cppreference.com/w/cpp/numeric/math/fma

has a nice parenthetical comment:

 "If ... defined, the function std::fma evaluates faster (in addition to
  being more precise) than the expression x*y+z."

If C99 or C17 included 'in addition to being more precise', it would
be much more obvious to me that we should include the fall-back fma
implementation.

So, we should at least change the CPP defines that we have in
math_config.h to match the C99 and C17 specs.

Is it reasonable to assume that applications which care about accuracy
will also be checking these defines and using them as the C++ standard
appears to?

-- 
-keith

-----BEGIN PGP SIGNATURE-----
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=7ElC
-----END PGP SIGNATURE-----