https://github.com/python/cpython/commit/07824995a056b2894adac69813444307835cc245
commit: 07824995a056b2894adac69813444307835cc245
branch: main
author: Michael Droettboom <[email protected]>
committer: gvanrossum <[email protected]>
date: 2024-02-26T07:18:30-08:00
summary:
gh-113706: Update comment about long int representation (#113707)
files:
M Include/cpython/longintrepr.h
diff --git a/Include/cpython/longintrepr.h b/Include/cpython/longintrepr.h
index f5ccbb704e8bb8..f037c7bb90deda 100644
--- a/Include/cpython/longintrepr.h
+++ b/Include/cpython/longintrepr.h
@@ -62,21 +62,32 @@ typedef long stwodigits; /* signed variant of twodigits */
#define PyLong_MASK ((digit)(PyLong_BASE - 1))
/* Long integer representation.
+
+ Long integers are made up of a number of 30- or 15-bit digits, depending on
+ the platform. The number of digits (ndigits) is stored in the high bits of
+ the lv_tag field (lvtag >> _PyLong_NON_SIZE_BITS).
+
The absolute value of a number is equal to
- SUM(for i=0 through abs(ob_size)-1) ob_digit[i] * 2**(SHIFT*i)
- Negative numbers are represented with ob_size < 0;
- zero is represented by ob_size == 0.
- In a normalized number, ob_digit[abs(ob_size)-1] (the most significant
+ SUM(for i=0 through ndigits-1) ob_digit[i] * 2**(PyLong_SHIFT*i)
+
+ The sign of the value is stored in the lower 2 bits of lv_tag.
+
+ - 0: Positive
+ - 1: Zero
+ - 2: Negative
+
+ The third lowest bit of lv_tag is reserved for an immortality flag, but is
+ not currently used.
+
+ In a normalized number, ob_digit[ndigits-1] (the most significant
digit) is never zero. Also, in all cases, for all valid i,
- 0 <= ob_digit[i] <= MASK.
+ 0 <= ob_digit[i] <= PyLong_MASK.
+
The allocation function takes care of allocating extra memory
- so that ob_digit[0] ... ob_digit[abs(ob_size)-1] are actually available.
+ so that ob_digit[0] ... ob_digit[ndigits-1] are actually available.
We always allocate memory for at least one digit, so accessing ob_digit[0]
- is always safe. However, in the case ob_size == 0, the contents of
+ is always safe. However, in the case ndigits == 0, the contents of
ob_digit[0] may be undefined.
-
- CAUTION: Generic code manipulating subtypes of PyVarObject has to
- aware that ints abuse ob_size's sign bit.
*/
typedef struct _PyLongValue {
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