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Add loop unrolling metadata descriptions to docs/LangRef.rst.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213397 91177308-0d34-0410-b5e6-96231b3b80d8
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Mark Heffernan 2014-07-18 19:24:51 +00:00
parent d94715e273
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@ -2900,17 +2900,109 @@ constructs:
!0 = metadata !{ metadata !0 }
!1 = metadata !{ metadata !1 }
The loop identifier metadata can be used to specify additional per-loop
metadata. Any operands after the first operand can be treated as user-defined
metadata. For example the ``llvm.loop.vectorize.unroll`` metadata is understood
by the loop vectorizer to indicate how many times to unroll the loop:
The loop identifier metadata can be used to specify additional
per-loop metadata. Any operands after the first operand can be treated
as user-defined metadata. For example the ``llvm.loop.unroll.count``
suggests an unroll factor to the loop unroller:
.. code-block:: llvm
br i1 %exitcond, label %._crit_edge, label %.lr.ph, !llvm.loop !0
...
!0 = metadata !{ metadata !0, metadata !1 }
!1 = metadata !{ metadata !"llvm.loop.vectorize.unroll", i32 2 }
!1 = metadata !{ metadata !"llvm.loop.vectorize.width", i32 4 }
'``llvm.loop.vectorize``'
^^^^^^^^^^^^^^^^^^^^^^^^^
Metadata prefixed with ``llvm.loop.vectorize`` is used to control
per-loop vectorization parameters such as vectorization width and
interleave count. ``llvm.loop.vectorize`` metadata should be used in
conjunction with ``llvm.loop`` loop identification metadata. The
``llvm.loop.vectorize`` metadata are only optimization hints and the
vectorizer will only vectorize loops if it believes it is safe to do
so. The ``llvm.mem.parallel_loop_access`` metadata which contains
information about loop-carried memory dependencies can be helpful in
determining the safety of loop vectorization.
'``llvm.loop.vectorize.unroll``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata suggests an interleave count to the loop vectorizer.
The first operand is the string ``llvm.loop.vectorize.unroll`` and the
second operand is an integer specifying the interleave count. For
example:
.. code-block:: llvm
!0 = metadata !{ metadata !"llvm.loop.vectorize.unroll", i32 4 }
Note that setting ``llvm.loop.vectorize.unroll`` to 1 disables
interleaving multiple iterations of the loop. If
``llvm.loop.vectorize.unroll`` is set to 0 then the interleave count
will be determined automatically.
'``llvm.loop.vectorize.width``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata sets the target width of the vectorizer. The first
operand is the string ``llvm.loop.vectorize.width`` and the second
operand is an integer specifying the width. For example:
.. code-block:: llvm
!0 = metadata !{ metadata !"llvm.loop.vectorize.width", i32 4 }
Note that setting ``llvm.loop.vectorize.width`` to 1 disables
vectorization of the loop. If ``llvm.loop.vectorize.width`` is set to
0 or if the loop does not have this metadata the width will be
determined automatically.
'``llvm.loop.unroll``'
^^^^^^^^^^^^^^^^^^^^^^
Metadata prefixed with ``llvm.loop.unroll`` are loop unrolling
optimization hints such as the unroll factor. ``llvm.loop.unroll``
metadata should be used in conjunction with ``llvm.loop`` loop
identification metadata. The ``llvm.loop.unroll`` metadata are only
optimization hints and the unrolling will only be performed if the
optimizer believes it is safe to do so.
'``llvm.loop.unroll.enable``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata either disables loop unrolling or suggests that the loop
be unrolled fully. The first operand is the string
``llvm.loop.unroll.enable`` and the second operand is a bit. If the
bit operand value is 0 loop unrolling is disabled. A value of 1
indicates that the loop should be fully unrolled. For example:
.. code-block:: llvm
!0 = metadata !{ metadata !"llvm.loop.unroll.enable", i1 0 }
!1 = metadata !{ metadata !"llvm.loop.unroll.enable", i1 1 }
'``llvm.loop.unroll.count``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata suggests an unroll factor to the loop unroller. The
first operand is the string ``llvm.loop.unroll.count`` and the second
operand is a positive integer specifying the unroll factor. For
example:
.. code-block:: llvm
!0 = metadata !{ metadata !"llvm.loop.unroll.count", i32 4 }
If the trip count of the loop is less than the unroll count the loop
will be partially unrolled.
If a loop has both a ``llvm.loop.unroll.enable`` metadata and
``llvm.loop.unroll.count`` metadata the behavior depends upon the
value of the ``llvm.loop.unroll.enable`` operand. If the value is 0,
the loop will not be unrolled. If the value is 1, the loop will be
unrolled with a factor determined by the ``llvm.loop.unroll.count``
operand effectively ignoring the ``llvm.loop.unroll.enable`` metadata.
'``llvm.mem``'
^^^^^^^^^^^^^^^
@ -2995,55 +3087,6 @@ the loop identifier metadata node directly:
!1 = metadata !{ metadata !1 } ; an identifier for the inner loop
!2 = metadata !{ metadata !2 } ; an identifier for the outer loop
'``llvm.loop.vectorize``'
^^^^^^^^^^^^^^^^^^^^^^^^^
Metadata prefixed with ``llvm.loop.vectorize`` is used to control per-loop
vectorization parameters such as vectorization factor and unroll factor.
``llvm.loop.vectorize`` metadata should be used in conjunction with
``llvm.loop`` loop identification metadata.
'``llvm.loop.vectorize.unroll``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata instructs the loop vectorizer to unroll the specified
loop exactly ``N`` times.
The first operand is the string ``llvm.loop.vectorize.unroll`` and the second
operand is an integer specifying the unroll factor. For example:
.. code-block:: llvm
!0 = metadata !{ metadata !"llvm.loop.vectorize.unroll", i32 4 }
Note that setting ``llvm.loop.vectorize.unroll`` to 1 disables
unrolling of the loop.
If ``llvm.loop.vectorize.unroll`` is set to 0 then the amount of
unrolling will be determined automatically.
'``llvm.loop.vectorize.width``' Metadata
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This metadata sets the target width of the vectorizer to ``N``. Without
this metadata, the vectorizer will choose a width automatically.
Regardless of this metadata, the vectorizer will only vectorize loops if
it believes it is valid to do so.
The first operand is the string ``llvm.loop.vectorize.width`` and the
second operand is an integer specifying the width. For example:
.. code-block:: llvm
!0 = metadata !{ metadata !"llvm.loop.vectorize.width", i32 4 }
Note that setting ``llvm.loop.vectorize.width`` to 1 disables
vectorization of the loop.
If ``llvm.loop.vectorize.width`` is set to 0 then the width will be
determined automatically.
Module Flags Metadata
=====================