[mypyc] Fixes to float to int conversion (#14936)

Fix undefined behavior due to converting a negative float to an unsigned
integer type. Fix edge cases on 32-bit platforms.

The first issue was caught by the testI32BasicOps test case, but only in
some configurations, it looks like.

mypyc/lib-rt/int_ops.c

@@ -294,8 +294,8 @@ PyObject *CPyLong_FromStr(PyObject *o) {
 }
 
 CPyTagged CPyTagged_FromFloat(double f) {
-    if (f < (double)CPY_TAGGED_MAX && f > CPY_TAGGED_MIN) {
-        return (CPyTagged)f << 1;
+    if (f < ((double)CPY_TAGGED_MAX + 1.0) && f > (CPY_TAGGED_MIN - 1.0)) {
+        return (Py_ssize_t)f << 1;
     }
     PyObject *o = PyLong_FromDouble(f);
     if (o == NULL)

mypyc/test-data/run-floats.test

@@ -185,6 +185,16 @@ def test_explicit_conversion_to_int() -> None:
         else:
             assert repr(int(x)) == repr(int_any(x))
 
+    # Test some edge cases
+    assert 2**30 == int(2.0**30 + int())
+    assert 2**30 - 1 == int(1073741823.9999999 + int())  # math.nextafter(2.0**30, 0))
+    assert -2**30 - 1 == int(-2.0**30 - 1 + int())
+    assert -2**30 == int(-1073741824.9999998 + int())  # math.nextafter(-2.0**30 - 1, 0)
+    assert 2**62 == int(2.0**62 + int())
+    assert 2**62 == int(2.0**62 - 1 + int())
+    assert -2**62 == int(-2.0**62 + int())
+    assert -2**62 == int(-2.0**62 - 1 + int())
+
 def str_to_float(x: str) -> float:
     return float(x)
 

test-data/unit/lib-stub/math.pyi

@@ -13,3 +13,4 @@ def copysign(__x: float, __y: float) -> float: ...
 def isinf(__x: float) -> bool: ...
 def isnan(__x: float) -> bool: ...
 def isfinite(__x: float) -> bool: ...
+def nextafter(__x: float, __y: float) -> float: ...