The attached patch moves to use the fenv functions on FreeBSD to control the floating-point environment. This will be needed as I don't expect FreeBSD will have these functions on arm64.

I would expect a similar change should be applied to any supported development branch.

It looks like fpsetmask() was deprecated since many years, and that fedisableexcept() is available since FreeBSD 5.3. Since we try to support FreeBSD 9, it's fine to drop support of FreeBSD < 5.3.

I'm concerned by the "CAVEATS" section below. Should we put "#pragma STDC FENV_ACCESS ON" in all .c file using a C double?

@skrah: I saw this pragma in Modules/_decimal/libmpdec/mpdecimal.c. Any idea if applying this patch is fine?

Maybe we can start by applying it to the default branch?

If this patch is required to support arm64, I think that it's ok to apply it to 2.7, 3.4 and 3.5 since we still accept changes to fix platform compatibility issues.

http://www.unix.com/man-page/freebsd/3/fpsetmask/
---
DESCRIPTION
     The routines described herein are deprecated.  New code should use the functionality provided by fenv(3).
---

http://www.unix.com/man-page/freebsd/3/fenv/
---
HISTORY
     The <fenv.h> header first appeared in FreeBSD 5.3.  It supersedes the non-standard routines defined in <ieeefp.h> and documented in fpgetround(3).

CAVEATS
     The FENV_ACCESS pragma can be enabled with
	   #pragma STDC FENV_ACCESS ON
---

In theory we should set FENV_ACCESS whenever the control word is
changed (C99):

"If part of a program tests floating-point status flags, sets
floating-point control modes, or runs under non-default mode
settings, but was translated with the state for the FENV_ACCESS
pragma "off", the behavior is undefined."


In practice gcc ignores the pragma, icc and cl.exe use it, not
sure about clang.

FYI in Python/pytime.c, I put a lot of "volatile double" when tests started to fail on some platforms. It's inefficient, but it's easier than teaching how to disable optimizations on each different compiler :-p

Ok, clang does not support it either:

https://llvm.org/bugs/show_bug.cgi?id=10409

Regarding volatile: gcc/clang seem to honor *some* changes to
the control word. At least in libmpdec both compilers have
always left the settings 80bit-prec/ROUND_CHOP intact, even
though it's not the default.

Let's rewrite Python in asm to avoid these issues. :)

Sorry, I don't understand if we should apply or not the patch :-/

Can this be applied?

This issue is becoming increasingly important as FreeBSD 11.0-RELEASE time nears.

What remains to be done/identified/answers here to make progress?

Andrew Turner: "Can this be applied?"

koobs: "What remains to be done/identified/answers here to make progress?"

*I* don't understand if replacing fpsetmask() with fedisableexcept() is enough, or if we also need to starting putting "#pragma STDC FENV_ACCESS ON" in all .c file using a C double? It's not a rhetorical question, I don't understand the consequence of the attached patch.

If two FreeBSD developers (Andrew Turner and koobs) are confident, maybe I should trust them and apply blindly the patch without trying to understand it :-D

1) Feedback whether the compiler used on FreeBSD recognizes FENV_ACCESS.

2) If not, an indication why the man page mentions it.

I think this patch is correct.

Clang, as shipped by FreeBSD, doesn't support FENV_ACCESS. It raises the following warning:

fenv_test.c:2:14: warning: pragma STDC FENV_ACCESS ON is not supported, ignoring pragma [-Wunknown-pragmas]
#pragma STDC FENV_ACCESS ON
             ^
1 warning generated.

I expect the man page mentions it because it is mentioned in the standard. In the bugs section it does say the pragma is unimplemented.

Thanks, then the patch looks good to me.

New changeset 76f35f35be50 by Victor Stinner in branch '3.5':
Replace fpgetmask() with fedisableexcept()
https://hg.python.org/cpython/rev/76f35f35be50

New changeset 6134d9ecab60 by Victor Stinner in branch 'default':
Merge 3.5 (issue #24520)
https://hg.python.org/cpython/rev/6134d9ecab60

New changeset 394ae9efc5c2 by Victor Stinner in branch '2.7':
Replace fpgetmask() with fedisableexcept()
https://hg.python.org/cpython/rev/394ae9efc5c2

"I expect the man page mentions it because it is mentioned in the standard. In the bugs section it does say the pragma is unimplemented."

Ah ah, funnny. Sorry, I was confused by the manual page.

I pushed your patch to Python 2.7, 3.5 and default (3.6) branches.

@koobs: sorry, 3.4 now only accept security fixes :-p

> @koobs: sorry, 3.4 now only accept security fixes :-p

I wrote a table giving the status of each Python branch to know which ones still accept bugfixes or not:
https://docs.python.org/devguide/#status-of-python-branches