[issue37474] Should Py_Initialize() control the floating point mode?

[STINNER Victor]

STINNER Victor  added the comment:


New changeset 5e0ea7540f577c9684e272000fdfc80d29bb78a2 by Victor Stinner in 
branch 'master':
bpo-37474: Don't call fedisableexcept() on FreeBSD (GH-16515)
https://github.com/python/cpython/commit/5e0ea7540f577c9684e272000fdfc80d29bb78a2


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[issue37474] Should Py_Initialize() control the floating point mode?

[STINNER Victor]

STINNER Victor  added the comment:

Sorry, I failed to find time to convert attached tests to tests in the Python 
test suite. But I understood that existing tests should be enough to control 
the rounding mode.

I removed the call. If something goes wrong, we can add it back.

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue37474] Should Py_Initialize() control the floating point mode?

[STINNER Victor]

Change by STINNER Victor :


--
keywords: +patch
pull_requests: +16105
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/16515

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[issue37474] Should Py_Initialize() control the floating point mode?

[Mark Dickinson]

Mark Dickinson  added the comment:

> Mark: Do you think that it's worth it to convert attached fp_except.c into
> tests run by test_float, to check floating point exceptions in Python?

Sure, it wouldn't harm. I'd expect that all these cases are already being 
tested indirectly by some part of test_math, but direct tests are better.

Note that Python still can't/won't assume IEEE 754 floating-point, so if the 
tests use special values like infinities and nans, or other IEEE 754 
assumptions, then they should be guarded by a suitable "skipIf" decorator.

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[issue37474] Should Py_Initialize() control the floating point mode?

[STINNER Victor]

STINNER Victor  added the comment:

Mark: Do you think that it's worth it to convert attached fp_except.c into 
tests run by test_float, to check floating point exceptions in Python?

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[issue37474] Should Py_Initialize() control the floating point mode?

[Mark Dickinson]

Mark Dickinson  added the comment:

> And since it was only introduced 

Sorry, that was unclear. "it" refers to "fedisableexcept" in the above.

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[issue37474] Should Py_Initialize() control the floating point mode?

[Mark Dickinson]

Mark Dickinson  added the comment:

> Should I understand that fedisableexcept(FE_OVERFLOW) is useless since 
> FreeBSD 6?

That's my understanding, yes. And since it was only introduced in FreeBSD 6, 
it's been useless forever. IOW, I think it's true that this line of code, in 
its current form, hasn't ever had any effect.

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[issue37474] Should Py_Initialize() control the floating point mode?

[STINNER Victor]

STINNER Victor  added the comment:

fp_except.c: C program to test for float point exceptions:

* FE_OVERFLOW
* FE_UNDERFLOW
* FE_INVALID

I prefer to avoid testing FE_INEXACT which a test might be too specific to an 
implementation of the libm.

I also chose to avoid testing FE_DIVBYZERO, since the default behavior is to 
kill the process with SIGFPE.

--
Added file: https://bugs.python.org/file48511/fp_except.c

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[issue37474] Should Py_Initialize() control the floating point mode?

[STINNER Victor]

STINNER Victor  added the comment:

> The current FreeBSD documentation for fedisableexcept says: All exceptions 
> are masked by default.

The fedisableexcept manual page says that since this manual page was added to 
FreeBSD 6.0 which was released in 2005.

Python started to tune FPU control on FreeBSD in 2002.

Should I understand that fedisableexcept(FE_OVERFLOW) is useless since FreeBSD 
6?

I'm also surprised that I never saw any complain about that on other BSD: 
OpenBSD, NetBSD, macOS, etc. It seems like the fedisableexcept(FE_OVERFLOW) 
call is useless since a long time.

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[issue37474] Should Py_Initialize() control the floating point mode?

[Mark Dickinson]

Mark Dickinson  added the comment:

The current FreeBSD documentation for fedisableexcept says:

> All exceptions are masked by default.

Source: 
https://www.freebsd.org/cgi/man.cgi?query=fedisableexcept=0=0=FreeBSD+12.0-RELEASE+and+Ports=default=html

So it looks as though it may be safe to remove this.

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[issue37474] Should Py_Initialize() control the floating point mode?

[Mark Dickinson]

Change by Mark Dickinson :


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nosy: +tim.peters

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[issue37474] Should Py_Initialize() control the floating point mode?

[Mark Dickinson]

Mark Dickinson  added the comment:

> What is the behavior on FreeBSD when fedisableexcept(FE_OVERFLOW) is not 
> called?

That's an excellent question, that I'd love to have an answer to for currently 
supported FreeBSD versions.

I think the old behaviour is that anything that you'd expect to raise an FPE 
(e.g., `math.sqrt(-1.0)` would segfault the interpreter). It may be that that's 
no longer an issue with FreeBSD versions that we care about.

That code comment dates from over 16 years ago (see 
https://github.com/python/cpython/commit/4643bd9a9cf43332992d10982c52debf3f0d980c),
 so there's some hope that the situation's changed since then.

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[issue37474] Should Py_Initialize() control the floating point mode?

[STINNER Victor]

New submission from STINNER Victor :

Just after calling _PyRuntime_Initialize(), Py_Main() calls:

/* 754 requires that FP exceptions run in "no stop" mode by default,
 * and until C vendors implement C99's ways to control FP exceptions,
 * Python requires non-stop mode.  Alas, some platforms enable FP
 * exceptions by default.  Here we disable them.
 */
#ifdef __FreeBSD__
fedisableexcept(FE_OVERFLOW);
#endif

It's done before calling Py_Initialize(). So applications embedding Python 
don't call it. But "customized Python" which call the new Py_RunMain() with 
Py_InititializeFromConfig() will not call it neither.

I would like to know if this setup code should be moved into Py_Initialize()? 
Especially into the new PyConfig API:
https://docs.python.org/dev/c-api/init_config.html

PyConfig got 2 flags to control parameters which affect the whole process:

* configure_c_stdio
* configure_locale

What is the behavior on FreeBSD when fedisableexcept(FE_OVERFLOW) is not called?

Note: I'm not sure why this call is only needed on FreeBSD, but not on macOS, 
OpenBSD or NetBSD (operating systems close to FreeBSD).

--
components: Interpreter Core
messages: 347048
nosy: lemburg, mark.dickinson, stutzbach, vstinner
priority: normal
severity: normal
status: open
title: Should Py_Initialize() control the floating point mode?
versions: Python 3.9

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