[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

Benjamin Peterson added the comment:


New changeset cb356c2ecc0528d47fee2b9f4b32da4fcfb48b3a by Benjamin Peterson in 
branch '3.6':
[3.6] bpo-31373: remove overly strict float range checks (GH-3486) (#3495)
https://github.com/python/cpython/commit/cb356c2ecc0528d47fee2b9f4b32da4fcfb48b3a


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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

Changes by Benjamin Peterson :


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pull_requests: +3488

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

Benjamin Peterson added the comment:


New changeset 2bb69a5b4e7f96cb35d1b28aa7b7b3974b351f59 by Benjamin Peterson in 
branch 'master':
bpo-31373: remove overly strict float range checks (#3486)
https://github.com/python/cpython/commit/2bb69a5b4e7f96cb35d1b28aa7b7b3974b351f59


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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Mark Dickinson]

Mark Dickinson added the comment:

> It's hard to win here I think.

Agreed.

> It seems like the undefined behavior sanitizer is being overzealous when the 
> target supports IEEE754.

Also agreed.

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

Benjamin Peterson added the comment:

I'm going to undo the changes to getargs.c and floatobject.c. I think the 
pytime.c change is still correct because the doubles are explicitly rounded 
before conversion (and the old code checked for error > 1.0).

It's hard to win here I think. The clang undefined behavior sanitizer uses 
FLT_MAX to make its determination:
runtime error: value 3.40282e+38 is outside the range of representable values 
of type 'float'

So to satisfy it and preserve the old behavior, we would presumably have to 
implement the rounding ourselves, which doesn't seem like fun.

Annex F of C99, which defines the requirements for C implementations using 
IEE754, says, "the conversions for floating types provide  the IEC 60559 
conversions between floating-point precisions.". Those are, of course, fully 
defined. It seems like the undefined behavior sanitizer is being overzealous 
when the target supports IEEE754.

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

Changes by Benjamin Peterson :


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pull_requests: +3476

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Mark Dickinson]

Mark Dickinson added the comment:

> Do you know a way to get the IEEE 754 rounding behavior without invoking C 
> undefined behavior?

One option is to hard-code the actual boundary, which is 2**128 * (1 - 2**-25) 
(as opposed to FLT_MAX, which is 2**128 * (1 - 2**-24)): values equal to or 
larger than 2**128 * (1 - 2**-25) in absolute value should raise. But that 
means assuming IEEE 754 and round-ties-to-even, which isn't an outrageous 
assumption but does make the solution feel a bit fragile.

An alternative would be to scale values in the range (FLT_MAX, 2.0 * FLT_MAX] 
by 0.5 before doing the conversion, something like this:

if (fabs(x) > FLT_MAX && !Py_IS_INFINITY(x)) {
  double half_x = 0.5 * x;
  if (half_x > FLT_MAX) {
goto Overflow;
  }
  float half_y = (float)half_x;
  if (half_y > 0.5 * FLT_MAX) {
goto Overflow;
  }
}

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

Benjamin Peterson added the comment:

I agree that's bad. Do you know a way to get the IEEE 754 rounding behavior 
without invoking C undefined behavior?

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Mark Dickinson]

Mark Dickinson added the comment:

There's a (to my mind) unfortunate change in behaviour here. Under normal IEEE 
754 rules, some C double values larger than FLT_MAX still round to FLT_MAX 
under conversion to float.

Python 3.6:

>>> import struct
>>> x = 3.40282356e38
>>> struct.pack(">> import struct
>>> x = 3.40282356e38
>>> struct.pack("

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

Benjamin Peterson added the comment:


New changeset b03623227ed1264e3cac4e6bb4878d96b91aa484 by Benjamin Peterson 
(Miss Islington (bot)) in branch '3.6':
[3.6] fixes bpo-31373: fix undefined floating-point demotions (GH-3396) (#3424)
https://github.com/python/cpython/commit/b03623227ed1264e3cac4e6bb4878d96b91aa484


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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

Changes by Benjamin Peterson :


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

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Roundup Robot]

Changes by Roundup Robot :


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pull_requests: +3421

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

Benjamin Peterson added the comment:


New changeset a853a8ba7850381d49b284295dd6f0dc491dbe44 by Benjamin Peterson in 
branch 'master':
bpo-31373: fix undefined floating-point demotions (#3396)
https://github.com/python/cpython/commit/a853a8ba7850381d49b284295dd6f0dc491dbe44


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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Raymond Hettinger]

Changes by Raymond Hettinger :


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nosy: +mark.dickinson

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[STINNER Victor]

Changes by STINNER Victor :


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nosy: +haypo

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

Changes by Benjamin Peterson :


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pull_requests: +3402

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[issue31373] demoting floating float values to unrepresentable types is undefined behavior

[Benjamin Peterson]

New submission from Benjamin Peterson:

According to C99, "When a finite value of real floating type is converted to an 
integer type other than _Bool, the  fractional  part  is  discarded  (i.e.,  
the  value  is  truncated  toward  zero). If the value of the integral part 
cannot be represented by the integer type,the behavior is undefined." So, e.g., 
(long)x, where x is a double is in general undefined behavior without a range 
check. We have several cases in CPython where we need to fix this.

--
components: Interpreter Core
messages: 301528
nosy: benjamin.peterson
priority: normal
severity: normal
status: open
title: demoting floating float values to unrepresentable types is undefined 
behavior
type: behavior
versions: Python 3.6, Python 3.7

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