https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102783
--- Comment #9 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- On Tue, 19 Oct 2021, segher at gcc dot gnu.org via Gcc-bugs wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102783 > > --- Comment #8 from Segher Boessenkool <segher at gcc dot gnu.org> --- > (In reply to jos...@codesourcery.com from comment #6) > > Generically (and if the command-line options are such that floating-point > > control / status bits are to be respected by optimizations), *any* > > function call might access or modify floating-point control and status > > bits, subject to e.g. const functions not being able to access them, pure > > functions not being able to modify them, functions whose body is known > > having properties based on analysis of that body, built-in functions > > having semantics based on what the compiler knows about those functions. > > If FENV_ACCESS is OFF most of those things can be ignored as well. But > FENV_ACCESS is much too blunt a hammer for most of our uses. My recent discussions with Roger Sayle <https://gcc.gnu.org/pipermail/gcc-patches/2021-September/thread.html#580252>, and bug 54192 as referenced therein, may be helpful for more details of how FENV_ACCESS could be split up. (At present we have -ftrapping-math, on by default, and -frounding-math, off by default. I suspect that if -ftrapping-math really restricted optimizations enough to avoid all problematic code reordering / removal in the presence of function calls possibly reading and writing exception flags, it would actually inhibit optimization more than a full implementation of -frounding-math would: a full -frounding-math only means that arithmetic *reads* the rounding mode, whereas a full -ftrapping-math means that arithmetic *writes* to the exception flags.)
