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From: "joseph at codesourcery dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/106117] Use of option -fexcess-precision for
 operation-by-operation emulation for _Float16 arithmetics.
Date: Tue, 28 Jun 2022 21:14:56 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106117

--- Comment #2 from joseph at codesourcery dot com <joseph at codesourcery =
dot com> ---
"none" was something I mentioned as a possible future argument when=20
originally posting -fexcess-precision=20
<https://gcc.gnu.org/legacy-ml/gcc-patches/2008-11/msg00105.html>.  I=20
still think it's the appropriate name for that case.

(Doing +-*/ operations on float and then immediately converting back to=20
_Float16 has exactly the same semantics as direct _Float16 arithmetic;=20
float has sufficient precision that no double rounding issues arise; that=20
doesn't apply to fma, however.  The effect of excess precision is that=20
e.g. in "a + b + c", the value of a + b with the range and precision of=20
float is what gets added to c; there's no intermediate truncation of a + b=
=20
to _Float16.  But (_Float16)(a + b) + c would have such a truncation,=20
because casts and conversion as if by assignment remove excess range and=20
precision.)=