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From: "fxcoudert at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug fortran/91690] Slow IEEE intrinsics
Date: Wed, 29 Dec 2021 14:48:50 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D91690

Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |fxcoudert at gcc dot gnu.o=
rg

--- Comment #8 from Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> =
---
I think things are a little more complicated that they appear. The function
IEEE_IS_NAN, in itself, is not the problem: it is not slow, and does not
store/restore the floating-point state. If you take the demo program, and
replace the call to the contained function IS_NAN_IEEE directly by the
IEEE_IS_NAN function, you will actually see that it is faster (50% faster) =
than
the bit-checking function.

The problem is in the contained function IS_NAN_IEEE. By my reading of the
Fortran standard, that function is required to save its floating-point stat=
e on
entry, and restore it on exit. This is regardless of what happens inside the
function, and this is what is slowing things done.

Of course, we can still consider this a "missed optimisation": the compiler
could in theory determine that absolutely no floating-point operation is be=
ing
done, and optimise away the save/restore calls. But it is a tricky thing to=
 do,
and would have no value for "real" functions, that actually perform
floating-point operations.=