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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/55522] -funsafe-math-optimizations is unexpectedly
 harmful, especially w/ -shared
Date: Fri, 09 Sep 2022 10:32:49 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D55522

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org

--- Comment #17 from Richard Biener <rguenth at gcc dot gnu.org> ---
Note that when a shared library wants -ffast-math like behavior (FTZ, etc.)
_not_ linking crtfastmath.o will cause its behavior to depend on the process
loading
it and whether that sets the appropriate flags or not.  That might also cau=
se
highly surprising effects.

That said, a "true fix" would redesign how these things work and make
FTZ part of the instruction encoding.

I suppose setting/restoring the control word around all public API of a
translation unit (and with LTO a bit more optimized) would be prohibitly
expensive.

Short of dropping the feature or re-implementing it, for example in the
dynamic loader via some ELF annotations with documented behavior on
"mismatch", I don't see any fix for this bug besides asking users to
conciously not link with -ffast-math when building shared libraries - unless
that's intended.=