8000 GH-90043: Handle NaNs in `COMPARE_OP_FLOAT_JUMP` by brandtbucher · Pull Request #100278 · python/cpython · GitHub
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GH-90043: Handle NaNs in COMPARE_OP_FLOAT_JUMP #100278

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Merged
merged 5 commits into from
Dec 16, 2022
Merged

GH-90043: Handle NaNs in COMPARE_OP_FLOAT_JUMP #100278

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f0728cf
Handle NaNs in COMPARE_OP_FLOAT_JUMP
brandtbucher Nov 28, 2022
2665c9a
Remove NaN checks!
brandtbucher Nov 30, 2022
dba5894
Catch up with main
brandtbucher Nov 30, 2022
66d2d63
Catch up with main
brandtbucher Dec 15, 2022
9dc68aa
blurb add
brandtbucher Dec 15, 2022
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Remove NaN checks!
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@brandtbucher
brandtbucher committed Nov 30, 2022
commit 2665c9a82699614cadc78529fe7ced82c90bc5e8
11 changes: 4 additions & 7 deletions Python/bytecodes.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2027,11 +2027,8 @@ dummy_func(
STAT_INC(COMPARE_OP, hit);
double dleft = PyFloat_AS_DOUBLE(left);
double dright = PyFloat_AS_DOUBLE(right);
// 1 if <, 2 if ==, 4 if >, 8 if unordered; this matches when_to_jump_mask
int sign_ish = (+ 6 * (Py_IS_NAN(dleft) | Py_IS_NAN(dright))
+ 2 * (dleft > dright)
- 1 * (dleft < dright)
+ 2);
// 1 if NaN, 2 if <, 4 if >, 8 if ==; this matches when_to_jump_mask
int sign_ish = 1 << (2 * (dleft >= dright) + (dleft <= dright));
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Clever -- roughly the same number of instructions too!

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@brandtbucher brandtbucher Dec 16, 2022 •
edited
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Credit to @markshannon for figuring this out. I had tried something similar (with >= and <=), but couldn't get the masks to work correctly without his help.

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@mdickinson mdickinson Dec 16, 2022 •
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This is potentially a bit dangerous: a comparison with NaN may raise FE_INVALID. It may be safer to use the isless macro instead.

From C99 (§7.12.14.3):

The isless macro determines whether its first argument is less than its second argument. The value of isless(x, y) is always equal to (x) < (y); however, unlike (x) < (y), isless(x, y) does not raise the ‘‘invalid’’ floating-point exception when x and y are unordered.

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Sorry, that should be islessequal of course, not isless.

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float_richcompare uses simple i < j comparisons. Why is that OK, but not here?

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@brandtbucher No, not specific to signalling NaNs, just to comparisons involving NaNs. And yes, float_richcompare should be updated to use the safer code, now that we're allowed to depend on C99 features.

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Okay. I'll merge this, and create a separate issue where we can discuss how/if we should avoid potential floating-point exceptions everywhere (since that seems like a much bigger change).

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And yes, it's platform-specific. I think we're in a place now where all current platforms do the "right" thing, but that wasn't always the case. There were a few related issues in the past - #81655 is the only one I'm finding right now, though. So yes, probably a non-issue at this point.

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create a separate issue

Thanks. Don't worry about this, though - I think I'm fussing about things that were historically an issue, but are no longer. It's not that long ago that we did protect all floating-point operations, e.g., here's float_richcompare in Python 2.7:

cpython/Objects/floatobject.c

Lines 600 to 621 in ca079a3

PyFPE_START_PROTECT("richcompare", return NULL)
switch (op) {
case Py_EQ:
r = i == j;
break;
case Py_NE:
r = i != j;
break;
case Py_LE:
r = i <= j;
break;
case Py_GE:
r = i >= j;
break;
case Py_LT:
r = i < j;
break;
case Py_GT:
r = i > j;
break;
}
PyFPE_END_PROTECT(r)

But it seems that all current platforms do follow the IEEE 754 standard's advice on "default" floating-point exception handling settings, so this is something we no longer need to worry about.

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@mdickinson mdickinson Dec 16, 2022 •
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(FTR, it looks like Python 3.9 is where we discarded the PyFPE_START_PROTECT / PyFPE_END_PROTECT guards everywhere. They're still present in 3.8.)

_Py_DECREF_SPECIALIZED(left, _PyFloat_ExactDealloc);
_Py_DECREF_SPECIALIZED(right, _PyFloat_ExactDealloc);
jump = sign_ish & when_to_jump_mask;
Expand All @@ -2058,8 +2055,8 @@ dummy_func(
assert(Py_ABS(Py_SIZE(left)) <= 1 && Py_ABS(Py_SIZE(right)) <= 1);
Py_ssize_t ileft = Py_SIZE(left) * ((PyLongObject *)left)->ob_digit[0];
Py_ssize_t iright = Py_SIZE(right) * ((PyLongObject *)right)->ob_digit[0];
// 1 if <, 2 if ==, 4 if >; this matches when _to_jump_mask
int sign_ish = 2*(ileft > iright) + 2 - (ileft < iright);
// 2 if <, 4 if >, 8 if ==; this matches when_to_jump_mask
int sign_ish = 1 << (2 * (ileft >= iright) + (ileft <= iright));
_Py_DECREF_SPECIALIZED(left, (destructor)PyObject_Free);
_Py_DECREF_SPECIALIZED(right, (destructor)PyObject_Free);
jump = sign_ish & when_to_jump_mask;
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11 changes: 4 additions & 7 deletions Python/generated_cases.c.h
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18 changes: 9 additions & 9 deletions Python/specialize.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1924,16 +1924,16 @@ compare_op_fail_kind(PyObject *lhs, PyObject *rhs)


static int compare_masks[] = {
// 1-bit: jump if less than
// 2-bit: jump if equal
// 1-bit: jump if unordered
// 2-bit: jump if less
// 4-bit: jump if greater
// 8-bit: jump if unordered
[Py_LT] = 1 | 0 | 0 | 0,
[Py_LE] = 1 | 2 | 0 | 0,
[Py_EQ] = 0 | 2 | 0 | 0,
[Py_NE] = 1 | 0 | 4 | 8,
// 8-bit: jump if equal
[Py_LT] = 0 | 2 | 0 | 0,
[Py_LE] = 0 | 2 | 0 | 8,
[Py_EQ] = 0 | 0 | 0 | 8,
[Py_NE] = 1 | 2 | 4 | 0,
[Py_GT] = 0 | 0 | 4 | 0,
[Py_GE] = 0 | 2 | 4 | 0,
[Py_GE] = 0 | 0 | 4 | 8,
};

void
Expand Down Expand Up @@ -1983,7 +1983,7 @@ _Py_Specialize_CompareOp(PyObject *lhs, PyObject *rhs, _Py_CODEUNIT *instr,
}
else {
_Py_SET_OPCODE(*instr, COMPARE_OP_STR_JUMP);
cache->mask = (when_to_jump_mask & 2) == 0;
cache->mask = (when_to_jump_mask & 8) == 0;
goto success;
}
}
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