float-to-datetime conversion fixes

Avoid assuming that NaN casts to NaT (= fails on riscv64/hppa ?)
Don't round to int for the bounds check when we don't for the real
conversion (wrong near the bounds, and maybe also a waste of time)

Author: Rebecca N. Palmer <[email protected]>
Forwarded: pandas-dev/pandas#50183


Gbp-Pq: Name float_to_datetime.patch

Author: Debian Science Team

pandas/_libs/tslib.pyx

@@ -296,12 +296,18 @@ def array_with_unit_to_datetime(
         # if we have nulls that are not type-compat
         # then need to iterate
 
-        if values.dtype.kind in ["i", "f", "u"]:
+        if values.dtype.kind in ["i", "u"]:
             iresult = values.astype("i8", copy=False)
             # fill missing values by comparing to NPY_NAT
             mask = iresult == NPY_NAT
             iresult[mask] = 0
-            fvalues = iresult.astype("f8") * mult
+            fvalues = iresult.astype("f8") * mult # TODO as this is only used for bounds checking, would it be more efficient to divide the bounds by mult? (don't just use integer iresult * mult, we need arithmetic overflow to be an error not a wraparound)
+            need_to_iterate = False
+            
+        if values.dtype.kind in ["f",]:
+            mask = (values != values) | (values == NPY_NAT) # first is NaNs
+            fvalues = (values * mult).astype("f8") # TODO would values.astype('f8')*mult have less rounding error? (or does this need to support longer-than-double floats where it's worse?) would copy=False be faster?
+            fvalues[mask] = 0
             need_to_iterate = False
 
         if not need_to_iterate:
@@ -315,11 +321,9 @@ def array_with_unit_to_datetime(
                 result = (iresult * mult).astype("M8[ns]")
 
             elif values.dtype.kind == "f":
-                fresult = (values * mult).astype("f8")
-                fresult[mask] = 0
                 if prec:
-                    fresult = round(fresult, prec)
-                result = fresult.astype("M8[ns]", copy=False)
+                    fvalues = round(fvalues, prec) # TODO why does this exist, it looks like prec's always 0 (was p from precision_to_unit meant to be prec?)
+                result = fvalues.astype("M8[ns]", copy=False)
 
             iresult = result.view("i8")
             iresult[mask] = NPY_NAT