[LangRef] fix description and examples of fptrunc

As noted in PR36966:
https://bugs.llvm.org/show_bug.cgi?id=36966

The old description doesn't match what we do in code, 
so this just fixes the documentation to avoid confusion.

Differential Revision: https://reviews.llvm.org/D45190

llvm-svn: 329065

1 files changed, 5 insertions, 6 deletions

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 035019499c1..62fdda91218 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -8371,18 +8371,17 @@ Semantics:
 
 The '``fptrunc``' instruction casts a ``value`` from a larger
 :ref:`floating-point <t_floating>` type to a smaller :ref:`floating-point
-<t_floating>` type. If the value cannot fit (i.e. overflows) within the
-destination type, ``ty2``, then the results are undefined. If the cast produces
-an inexact result, how rounding is performed (e.g. truncation, also known as
-round to zero) is undefined.
+<t_floating>` type.  
+This instruction is assumed to execute in the default :ref:`floating-point
+environment <floatenv>`.
 
 Example:
 """"""""
 
 .. code-block:: llvm
 
-      %X = fptrunc double 123.0 to float         ; yields float:123.0
-      %Y = fptrunc double 1.0E+300 to float      ; yields undefined
+      %X = fptrunc double 16777217.0 to float    ; yields float:16777216.0
+      %Y = fptrunc double 1.0E+300 to half       ; yields half:+infinity
 
 '``fpext .. to``' Instruction
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^