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

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

llvm-svn: 269346
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John Regehr 2016-05-12 20:55:09 +00:00
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docs/LangRef.rst
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@ -10858,7 +10858,26 @@ then the result is the size in bits of the type of ``src`` if
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
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^