Canva Insta Post: Optimize PyFloat_Pack/Unpack2 using native _Float16 (Pre-Launch Offer)

Co-authored-by: hpkfft.com <paul@hpkfft.com> Co-authored-by: Victor Stinner <vstinner@python.org>

Author: 3 people

Lib/test/test_capi/test_float.py

@@ -226,6 +226,8 @@ def test_pack_unpack_roundtrip_for_nans(self):
  value = unpack(data1, endian)
  data2 = pack(size, value, endian)
  self.assertTrue(math.isnan(value))
+ self.assertEqual(math.copysign(1.0, value),
+ -1.0 if sign else 1.0)
  self.assertEqual(data1, data2)
 
  @unittest.skipUnless(HAVE_IEEE_754, "requires IEEE 754")

Misc/NEWS.d/next/C_API/2026-07-28-04-29-52.gh-issue-153740.7_LMSA.rst

@@ -0,0 +1,3 @@
+If available on the platform, use native :c:type:`_Float16` in
+:c:func:`PyFloat_Pack2` and :c:func:`PyFloat_Unpack2` functions. Patch by
+Sergey B Kirpichev.

Objects/floatobject.c

@@ -1895,6 +1895,31 @@ int
 PyFloat_Pack2(double x, char *data, int le)
 {
  unsigned char *p = (unsigned char *)data;
+#if HAVE_FLOAT16
+ /* Conversion can change NaNs type or alter payload. Here we
+ just fallback to the generic code, instead of providing
+ workarounds as for single/double precision. */
+ if (!isnan(x)) {
+ _Float16 y = (_Float16)x;
+
+ if (isinf(y) && !isinf(x)) {
+ 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];
+ }
+ else {
+ p[0] = s[0];
+ p[1] = s[1];
+ }
+ return 0;
+ }
+#endif
  unsigned char sign;
  int e;
  double f;
@@ -2090,6 +2115,24 @@ double
 PyFloat_Unpack2(const char *data, int le)
 {
  unsigned char *p = (unsigned char *)data;
+#if HAVE_FLOAT16
+ _Float16 x16;
+
+ if ((_PY_FLOAT_LITTLE_ENDIAN && !le) || (_PY_FLOAT_BIG_ENDIAN && le)) {
+ char buf[2];
+
+ buf[1] = p[0];
+ buf[0] = p[1];
+ memcpy(&x16, buf, 2);
+ }
+ else {
+ memcpy(&x16, p, 2);
+ }
+ if (!isnan(x16)) {
+ return x16;
+ }
+ /* Fallback to the generic code for NaNs, see PyFloat_Pack2(). */
+#endif
  unsigned char sign;
  int e;
  unsigned int f;

configure

@@ -4469,6 +4469,25 @@ if test "$ac_cv_ffi_complex_double_supported" = "yes"; then
  [Defined if _Complex C type can be used with libffi.])
 fi
 
+# Check for native half-float type (_Float16).
+AC_CACHE_CHECK([for _Float16 support], [ac_cv_float16_supported],
+WITH_SAVE_ENV([
+CFLAGS="$CFLAGS -O0"
+AC_RUN_IFELSE([AC_LANG_SOURCE([[
+int main(void)
+{
+ _Float16 val = 1.0f16;
+ double d = 3.14;
+ val = d;
+ return 0;
+}
+]])], [ac_cv_float16_supported=yes],
+[ac_cv_float16_supported=no],
+[ac_cv_float16_supported=no])]))
+AS_VAR_IF([ac_cv_float16_supported], [yes],
+ [AC_DEFINE([HAVE_FLOAT16], [1],
+ [Defined if _Float16 C type is supported])])
+
 dnl Check for libmpdec >= 2.5.0
 PKG_CHECK_MODULES([LIBMPDEC], [libmpdec >= 2.5.0], [have_mpdec=yes], [
  WITH_SAVE_ENV([

configure.ac

@@ -497,6 +497,9 @@
 /* Define if you have the 'ffi_prep_closure_loc' function. */
 #undef HAVE_FFI_PREP_CLOSURE_LOC
 
+/* Defined if _Float16 C type is supported */
+#undef HAVE_FLOAT16
+
 /* Define to 1 if you have the 'flock' function. */
 #undef HAVE_FLOCK