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

[Keith Packard]

"Keith Packard" <keithp@keithp.com> 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
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