Merge pull request #3239 from Zac-HD/error-on-fast-math

Zac-HD · web-flow · commit e0f93a88ccc9 · 2022-02-26T02:07:09.000-08:00
Error if denormals-are-zero
diff --git a/hypothesis-python/RELEASE.rst b/hypothesis-python/RELEASE.rst
@@ -0,0 +1,11 @@
+RELEASE_TYPE: minor
+
+This release makes :func:`~hypothesis.strategies.floats` error *consistently* when
+your floating-point hardware has been configured to violate IEEE-754 for
+:wikipedia:`subnormal numbers <Subnormal_number>`, instead of
+only when an internal assertion was tripped (:issue:`3092`).
+
+If this happens to you, passing ``allow_subnormal=False`` will suppress the explicit
+error.  However, we strongly recommend fixing the root cause by disabling global-effect
+unsafe-math compiler options instead, or at least consulting e.g. Simon Byrne's
+`Beware of fast-math <https://simonbyrne.github.io/notes/fastmath/>`__ explainer first.
diff --git a/hypothesis-python/src/hypothesis/strategies/_internal/numbers.py b/hypothesis-python/src/hypothesis/strategies/_internal/numbers.py
@@ -342,6 +342,22 @@ def floats(
     check_valid_bound(min_value, "min_value")
     check_valid_bound(max_value, "max_value")
 
+    if allow_subnormal and next_up(0.0, width=width) == 0:  # pragma: no cover
+        # Not worth having separate CI envs and dependencies just to cover this branch;
+        # discussion in https://github.com/HypothesisWorks/hypothesis/issues/3092
+        #
+        # Erroring out here ensures that the database contents are interpreted
+        # consistently - which matters for such a foundational strategy, even if it's
+        # not always true for all user-composed strategies further up the stack.
+        raise FloatingPointError(
+            f"Got allow_subnormal={allow_subnormal!r}, but we can't represent "
+            f"subnormal floats right now, in violation of the IEEE-754 floating-point "
+            f"specification.  This is usually because something was compiled with "
+            f"-ffast-math or a similar option, which sets global processor state.  "
+            f"See https://simonbyrne.github.io/notes/fastmath/ for a more detailed "
+            f"writeup - and good luck!"
+        )
+
     min_arg, max_arg = min_value, max_value
     if min_value is not None:
         min_value = float_of(min_value, width)
