[Bug target/102783] [powerpc] FPSCR manipulations cannot be relied upon

["joseph at codesourcery dot com" <gcc-bugzilla@gcc.gnu.org>]

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 joseph@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.)