[llvm] r269346 - [Docs] clarify semantics of x.with.overflow intrinsics

[John Regehr via llvm-commits]

Author: regehr
Date: Thu May 12 15:55:09 2016
New Revision: 269346

URL: http://llvm.org/viewvc/llvm-project?rev=269346&view=rev
Log:

[Docs] clarify semantics of x.with.overflow intrinsics

Differential Revision: http://reviews.llvm.org/D20151


Modified:
    llvm/trunk/docs/LangRef.rst

Modified: llvm/trunk/docs/LangRef.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/LangRef.rst?rev=269346&r1=269345&r2=269346&view=diff
==============================================================================
--- llvm/trunk/docs/LangRef.rst (original)
+++ llvm/trunk/docs/LangRef.rst Thu May 12 15:55:09 2016
@@ -10858,7 +10858,26 @@ then the result is the size in bits of t
 Arithmetic with Overflow Intrinsics
 -----------------------------------
 
-LLVM provides intrinsics for some arithmetic with overflow operations.
+LLVM provides intrinsics for fast arithmetic overflow checking.
+
+Each of these intrinsics returns a two-element struct. The first
+element of this struct contains the result of the corresponding
+arithmetic operation modulo 2\ :sup:`n`\ , where n is the bit width of
+the result. Therefore, for example, the first element of the struct
+returned by ``llvm.sadd.with.overflow.i32`` is always the same as the
+result of a 32-bit ``add`` instruction with the same operands, where
+the ``add`` is *not* modified by an ``nsw`` or ``nuw`` flag.
+
+The second element of the result is an ``i1`` that is 1 if the
+arithmetic operation overflowed and 0 otherwise. An operation
+overflows if, for any values of its operands ``A`` and ``B`` and for
+any ``N`` larger than the operands' width, ``ext(A op B) to iN`` is
+not equal to ``(ext(A) to iN) op (ext(B) to iN)`` where ``ext`` is
+``sext`` for signed overflow and ``zext`` for unsigned overflow, and
+``op`` is the underlying arithmetic operation.
+
+The behavior of these intrinsics is well-defined for all argument
+values.
 
 '``llvm.sadd.with.overflow.*``' Intrinsics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^