Hello,

as discussed in the PR, this seems like a simple enough approach to handle 
FENV functionality safely, while keeping it possible to implement 
optimizations in the future.

Some key missing things:
- handle C, not just C++ (I don't care, but some people probably do)
- handle vectors (for complex, I don't know what it means)

Then flag_trapping_math should also enable this path, meaning that we 
should stop making it the default, or performance will suffer.

Nice to have:
- parse the fenv_access pragma and make it set flag_rounding_math or similar.
- sqrt

All the optimizations can come later (I count having different functions 
for flag_rounding_math and flag_trapping_math as one such optimization).


I put the lowering in its own pass, because it needs to run at -O0 and 
there aren't that many passes at -O0 where I could put it. It would 
probably be better to handle this directly during expansion, but with my 
knowledge of the compiler it was easier to lower it before.

This patch passes bootstrap+regtest on x86_64. I expect it may break a few 
testcases on some targets (arm?) that check that we optimize some things 
even with -frounding-math, but as far as I am concerned those do not count 
as regressions because -frounding-math was never really implemented, so I 
would encourage target maintainers to xfail those for now.

I'd like to handle this incrementally, rather than wait for a mega-patch 
that does everything, if that's ok. For instance, I didn't handle vectors 
in this first patch because the interaction with vector lowering was not 
completely obvious. Plus it may help get others to implement some parts of 
it ;-)

2019-06-24  Marc Glisse  <marc.glisse@inria.fr>

         PR middle-end/34678
gcc/cp/
         * typeck.c (cp_build_binary_op): Generate internal functions for float
         operations with -frounding-math.

gcc/
         * Makefile.in: Handle new file gimple-lower-fenv.cc.
         * gimple-lower-fenv.cc: New file.
         * internal-fn.c (expand_FENV_PLUS, expand_FENV_MINUS, expand_FENV_MULT,
         expand_FENV_DIV): New functions.
         * internal-fn.def (FENV_PLUS, FENV_MINUS, FENV_MULT, FENV_DIV): New
         internal functions.
         * passes.def (pass_lower_fenv): New pass.
         * tree-pass.h (make_pass_lower_fenv): Declare new function.

-- 
Marc Glisse