https://github.com/python/cpython/commit/eb30e3d9d4b523e3f8de2cd1485c9ae9a996ab03
commit: eb30e3d9d4b523e3f8de2cd1485c9ae9a996ab03
branch: main
author: Victor Stinner <[email protected]>
committer: vstinner <[email protected]>
date: 2026-10-01T12:25:38Z
summary:
Optimize PyFloat_Pack*() functions (#158483)
Use _Py_bswap16/32/64() functions to swap bytes. PyFloat_Pack4() and
PyFloat_Pack8() avoid temporary buffer copy in the native byte order.
files:
M Objects/floatobject.c
diff --git a/Objects/floatobject.c b/Objects/floatobject.c
index 1bb5311ac678a5..d3ae337236a693 100644
--- a/Objects/floatobject.c
+++ b/Objects/floatobject.c
@@ -5,6 +5,7 @@
#include "Python.h"
#include "pycore_abstract.h" // _PyNumber_Index()
+#include "pycore_bitutils.h" // _Py_bswap16()
#include "pycore_dtoa.h" // _Py_dg_dtoa()
#include "pycore_floatobject.h" // _PyFloat_FormatAdvancedWriter()
#include "pycore_freelist.h" // _Py_FREELIST_FREE(), _Py_FREELIST_POP()
@@ -1894,7 +1895,6 @@ _PyFloat_DebugMallocStats(FILE *out)
int
PyFloat_Pack2(double x, char *data, int le)
{
- unsigned char *p = (unsigned char *)data;
#if _Py_HAVE_FLOAT16
/* Conversion can change NaNs type or alter payload. Here we
just fallback to the generic code, instead of providing
@@ -1906,16 +1906,14 @@ PyFloat_Pack2(double x, char *data, int le)
goto Overflow;
}
- unsigned char s[sizeof(_Float16)];
-
- memcpy(s, &y, sizeof(_Float16));
if ((_PY_FLOAT_LITTLE_ENDIAN && !le) || (_PY_FLOAT_BIG_ENDIAN && le)) {
- p[1] = s[0];
- p[0] = s[1];
+ uint16_t word;
+ memcpy(&word, &y, 2);
+ word = _Py_bswap16(word);
+ memcpy(data, &word, 2);
}
else {
- p[0] = s[0];
- p[1] = s[1];
+ memcpy(data, &y, sizeof(_Float16));
}
return 0;
}
@@ -1923,8 +1921,7 @@ PyFloat_Pack2(double x, char *data, int le)
unsigned char sign;
int e;
double f;
- unsigned short bits;
- int incr = 1;
+ uint16_t bits;
if (x == 0.0) {
sign = (copysign(1.0, x) == -1.0);
@@ -2004,18 +2001,10 @@ PyFloat_Pack2(double x, char *data, int le)
bits |= (e << 10) | (sign << 15);
/* Write out result. */
- if (le) {
- p += 1;
- incr = -1;
+ if ((_PY_FLOAT_LITTLE_ENDIAN && !le) || (_PY_FLOAT_BIG_ENDIAN && le)) {
+ bits = _Py_bswap16(bits);
}
-
- /* First byte */
- *p = (unsigned char)((bits >> 8) & 0xFF);
- p += incr;
-
- /* Second byte */
- *p = (unsigned char)(bits & 0xFF);
-
+ memcpy(data, &bits, 2);
return 0;
Overflow:
@@ -2027,10 +2016,7 @@ PyFloat_Pack2(double x, char *data, int le)
int
PyFloat_Pack4(double x, char *data, int le)
{
- unsigned char *p = (unsigned char *)data;
float y = (float)x;
- int i, incr = 1;
-
if (isinf(y) && !isinf(x)) {
PyErr_SetString(PyExc_OverflowError,
"float too large to pack with f format");
@@ -2054,8 +2040,8 @@ PyFloat_Pack4(double x, char *data, int le)
}
#else
uint32_t u32;
-
memcpy(&u32, &y, 4);
+
/* Workaround RISC-V: "If a NaN value is converted to a
* different floating-point type, the result is the
* canonical NaN of the new type". The canonical NaN here
@@ -2075,17 +2061,14 @@ PyFloat_Pack4(double x, char *data, int le)
#endif
}
- unsigned char s[sizeof(float)];
- memcpy(s, &y, sizeof(float));
-
if ((_PY_FLOAT_LITTLE_ENDIAN && !le) || (_PY_FLOAT_BIG_ENDIAN && le)) {
- p += 3;
- incr = -1;
+ uint32_t word;
+ memcpy(&word, &y, 4);
+ word = _Py_bswap32(word);
+ memcpy(data, &word, 4);
}
-
- for (i = 0; i < 4; i++) {
- *p = s[i];
- p += incr;
+ else {
+ memcpy(data, &y, sizeof(float));
}
return 0;
}
@@ -2093,20 +2076,14 @@ PyFloat_Pack4(double x, char *data, int le)
int
PyFloat_Pack8(double x, char *data, int le)
{
- unsigned char *p = (unsigned char *)data;
- unsigned char as_bytes[8];
- memcpy(as_bytes, &x, 8);
- const unsigned char *s = as_bytes;
- int i, incr = 1;
-
if ((_PY_FLOAT_LITTLE_ENDIAN && !le) || (_PY_FLOAT_BIG_ENDIAN && le)) {
- p += 7;
- incr = -1;
+ uint64_t word;
+ memcpy(&word, &x, 8);
+ word = _Py_bswap64(word);
+ memcpy(data, &word, 8);
}
-
- for (i = 0; i < 8; i++) {
- *p = *s++;
- p += incr;
+ else {
+ memcpy(data, &x, 8);
}
return 0;
}
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