gh-133304: workaround for RISC-V in PyFloat_Pack4()

Author: skirpichev

Lib/test/test_capi/test_float.py

@@ -199,13 +199,13 @@ def test_pack_unpack_roundtrip_for_nans(self):
                     signaling = 0
                 quiet = int(not signaling)
                 if size == 8:
-                    payload = random.randint(signaling, 1 << 50)
+                    payload = random.randint(signaling, 0x7ffffffffffff)
                     i = (sign << 63) + (0x7ff << 52) + (quiet << 51) + payload
                 elif size == 4:
-                    payload = random.randint(signaling, 1 << 21)
+                    payload = random.randint(signaling, 0x3fffff)
                     i = (sign << 31) + (0xff << 23) + (quiet << 22) + payload
                 elif size == 2:
-                    payload = random.randint(signaling, 1 << 8)
+                    payload = random.randint(signaling, 0x1ff)
                     i = (sign << 15) + (0x1f << 10) + (quiet << 9) + payload
                 data = bytes.fromhex(f'{i:x}')
                 for endian in (BIG_ENDIAN, LITTLE_ENDIAN):

Misc/NEWS.d/next/Core_and_Builtins/2025-05-03-07-41-21.gh-issue-133304.YMuSne.rst

@@ -0,0 +1 @@
+Workaround NaN's "canonicalization" in :c:func:`PyFloat_Pack4` on RISC-V.

Objects/floatobject.c

@@ -2194,14 +2194,27 @@ PyFloat_Pack4(double x, char *data, int le)
         /* correct y if x was a sNaN, transformed to qNaN by conversion */
         if (isnan(x)) {
             uint64_t v;
+            uint32_t u32;
 
             memcpy(&v, &x, 8);
+            memcpy(&u32, &y, 4);
+
             if ((v & (1ULL << 51)) == 0) {
-                uint32_t u32;
-                memcpy(&u32, &y, 4);
                 u32 &= ~(1 << 22); /* make sNaN */
-                memcpy(&y, &u32, 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
+             * is a positive qNaN with zero payload. */
+            if (v & (1ULL << 63)) {
+                u32 |= (1 << 31); /* set sign */
+            }
+            /* add payload */
+            u32 -= (u32 & 0x3fffff);
+            u32 += (uint32_t)((v & 0x7ffffffffffffULL) >> 29);
+
+            memcpy(&y, &u32, 4);
         }
 
         unsigned char s[sizeof(float)];