[LangRef] fptosi and fptoui return poison on overflow.

I think we assume poison, not undef, for certain transforms we
currently do. In any case, we should clarify the language here.

(This sort of conversion is undefined behavior according to the C
and C++ standards. And in practice, hardware implementations handle
overflow inconsistently, so it would be difficult to define the
result here.)

Differential Revision: https://reviews.llvm.org/D47851

llvm-svn: 334326

1 files changed, 8 insertions, 6 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 12490e91ecd..3d252e39f17 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -3275,13 +3275,15 @@ The following is the syntax for constant expressions:
     integer constant. TYPE must be a scalar or vector integer type. CST
     must be of scalar or vector floating-point type. Both CST and TYPE
     must be scalars, or vectors of the same number of elements. If the
-    value won't fit in the integer type, the results are undefined.
+    value won't fit in the integer type, the result is a
+    :ref:`poison value <poisonvalues>`.
 ``fptosi (CST to TYPE)``
     Convert a floating-point constant to the corresponding signed
     integer constant. TYPE must be a scalar or vector integer type. CST
     must be of scalar or vector floating-point type. Both CST and TYPE
     must be scalars, or vectors of the same number of elements. If the
-    value won't fit in the integer type, the results are undefined.
+    value won't fit in the integer type, the result is a
+    :ref:`poison value <poisonvalues>`.
 ``uitofp (CST to TYPE)``
     Convert an unsigned integer constant to the corresponding 
     floating-point constant. TYPE must be a scalar or vector floating-point
@@ -8765,8 +8767,8 @@ Semantics:
 
 The '``fptoui``' instruction converts its :ref:`floating-point
 <t_floating>` operand into the nearest (rounding towards zero)
-unsigned integer value. If the value cannot fit in ``ty2``, the results
-are undefined.
+unsigned integer value. If the value cannot fit in ``ty2``, the result
+is a :ref:`poison value <poisonvalues>`.
 
 Example:
 """"""""
@@ -8807,8 +8809,8 @@ Semantics:
 
 The '``fptosi``' instruction converts its :ref:`floating-point
 <t_floating>` operand into the nearest (rounding towards zero)
-signed integer value. If the value cannot fit in ``ty2``, the results
-are undefined.
+signed integer value. If the value cannot fit in ``ty2``, the result
+is a :ref:`poison value <poisonvalues>`.
 
 Example:
 """"""""