From c69517267f5ac46677d64f5fad898f0cfbf37b3a Mon Sep 17 00:00:00 2001
From: Ivan Levkivskyi <levkivskyi@gmail.com>
Date: Fri, 31 Oct 2025 02:16:12 +0000
Subject: [PATCH] More robust packing of flats in FF cache

---
 mypyc/lib-rt/librt_internal.c    | 20 ++++++++++++++------
 mypyc/test-data/run-classes.test |  2 ++
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/mypyc/lib-rt/librt_internal.c b/mypyc/lib-rt/librt_internal.c
index 8599017c31a8..cbd447964203 100644
--- a/mypyc/lib-rt/librt_internal.c
+++ b/mypyc/lib-rt/librt_internal.c
@@ -448,14 +448,18 @@ write_bytes(PyObject *self, PyObject *const *args, size_t nargs, PyObject *kwnam
 
 /*
 float format:
-    stored as a C double
+    stored using PyFloat helpers in little-endian format.
 */
 
 static double
 read_float_internal(PyObject *data) {
     _CHECK_BUFFER(data, CPY_FLOAT_ERROR)
-    _CHECK_READ(data, sizeof(double), CPY_FLOAT_ERROR)
-    double res = _READ(data, double);
+    _CHECK_READ(data, 8, CPY_FLOAT_ERROR)
+    char *buf = ((BufferObject *)data)->buf;
+    double res = PyFloat_Unpack8(buf + ((BufferObject *)data)->pos, 1);
+    if (unlikely((res == -1.0) && PyErr_Occurred()))
+        return CPY_FLOAT_ERROR;
+    ((BufferObject *)data)->pos += 8;
     return res;
 }
 
@@ -477,9 +481,13 @@ read_float(PyObject *self, PyObject *const *args, size_t nargs, PyObject *kwname
 static char
 write_float_internal(PyObject *data, double value) {
     _CHECK_BUFFER(data, CPY_NONE_ERROR)
-    _CHECK_SIZE(data, sizeof(double))
-    _WRITE(data, double, value)
-    ((BufferObject *)data)->end += sizeof(double);
+    _CHECK_SIZE(data, 8)
+    char *buf = ((BufferObject *)data)->buf;
+    int res = PyFloat_Pack8(value, buf + ((BufferObject *)data)->pos, 1);
+    if (unlikely(res == -1))
+        return CPY_NONE_ERROR;
+    ((BufferObject *)data)->pos += 8;
+    ((BufferObject *)data)->end += 8;
     return CPY_NONE;
 }
 
diff --git a/mypyc/test-data/run-classes.test b/mypyc/test-data/run-classes.test
index e08f2fd7007d..e44569630fec 100644
--- a/mypyc/test-data/run-classes.test
+++ b/mypyc/test-data/run-classes.test
@@ -2741,6 +2741,7 @@ def test_buffer_roundtrip() -> None:
     write_bytes(b, b"a" * 127)
     write_bytes(b, b"a" * 128)
     write_float(b, 0.1)
+    write_float(b, -1.0)
     write_float(b, -113.0)
     write_int(b, 0)
     write_int(b, 1)
@@ -2766,6 +2767,7 @@ def test_buffer_roundtrip() -> None:
     assert read_bytes(b) == b"a" * 127
     assert read_bytes(b) == b"a" * 128
     assert read_float(b) == 0.1
+    assert read_float(b) == -1.0
     assert read_float(b) == -113.0
     assert read_int(b) == 0
     assert read_int(b) == 1