Add loop unrolling metadata descriptions to docs/LangRef.rst.
llvm-svn: 213397
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Mark Heffernan
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f27ae6cdcf
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893752af3a
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@ -2900,17 +2900,109 @@ constructs:
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!0 = metadata !{ metadata !0 }
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!1 = metadata !{ metadata !1 }
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The loop identifier metadata can be used to specify additional per-loop
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metadata. Any operands after the first operand can be treated as user-defined
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metadata. For example the ``llvm.loop.vectorize.unroll`` metadata is understood
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by the loop vectorizer to indicate how many times to unroll the loop:
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The loop identifier metadata can be used to specify additional
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per-loop metadata. Any operands after the first operand can be treated
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as user-defined metadata. For example the ``llvm.loop.unroll.count``
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suggests an unroll factor to the loop unroller:
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.. code-block:: llvm
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br i1 %exitcond, label %._crit_edge, label %.lr.ph, !llvm.loop !0
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...
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!0 = metadata !{ metadata !0, metadata !1 }
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!1 = metadata !{ metadata !"llvm.loop.vectorize.unroll", i32 2 }
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!1 = metadata !{ metadata !"llvm.loop.vectorize.width", i32 4 }
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'``llvm.loop.vectorize``'
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^^^^^^^^^^^^^^^^^^^^^^^^^
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Metadata prefixed with ``llvm.loop.vectorize`` is used to control
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per-loop vectorization parameters such as vectorization width and
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interleave count. ``llvm.loop.vectorize`` metadata should be used in
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conjunction with ``llvm.loop`` loop identification metadata. The
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``llvm.loop.vectorize`` metadata are only optimization hints and the
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vectorizer will only vectorize loops if it believes it is safe to do
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so. The ``llvm.mem.parallel_loop_access`` metadata which contains
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information about loop-carried memory dependencies can be helpful in
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determining the safety of loop vectorization.
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'``llvm.loop.vectorize.unroll``' Metadata
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This metadata suggests an interleave count to the loop vectorizer.
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The first operand is the string ``llvm.loop.vectorize.unroll`` and the
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second operand is an integer specifying the interleave count. For
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example:
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.. code-block:: llvm
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!0 = metadata !{ metadata !"llvm.loop.vectorize.unroll", i32 4 }
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Note that setting ``llvm.loop.vectorize.unroll`` to 1 disables
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interleaving multiple iterations of the loop. If
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``llvm.loop.vectorize.unroll`` is set to 0 then the interleave count
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will be determined automatically.
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'``llvm.loop.vectorize.width``' Metadata
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This metadata sets the target width of the vectorizer. The first
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operand is the string ``llvm.loop.vectorize.width`` and the second
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operand is an integer specifying the width. For example:
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.. code-block:: llvm
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!0 = metadata !{ metadata !"llvm.loop.vectorize.width", i32 4 }
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Note that setting ``llvm.loop.vectorize.width`` to 1 disables
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vectorization of the loop. If ``llvm.loop.vectorize.width`` is set to
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0 or if the loop does not have this metadata the width will be
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determined automatically.
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'``llvm.loop.unroll``'
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^^^^^^^^^^^^^^^^^^^^^^
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Metadata prefixed with ``llvm.loop.unroll`` are loop unrolling
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optimization hints such as the unroll factor. ``llvm.loop.unroll``
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metadata should be used in conjunction with ``llvm.loop`` loop
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identification metadata. The ``llvm.loop.unroll`` metadata are only
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optimization hints and the unrolling will only be performed if the
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optimizer believes it is safe to do so.
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'``llvm.loop.unroll.enable``' Metadata
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This metadata either disables loop unrolling or suggests that the loop
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be unrolled fully. The first operand is the string
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``llvm.loop.unroll.enable`` and the second operand is a bit. If the
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bit operand value is 0 loop unrolling is disabled. A value of 1
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indicates that the loop should be fully unrolled. For example:
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.. code-block:: llvm
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!0 = metadata !{ metadata !"llvm.loop.unroll.enable", i1 0 }
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!1 = metadata !{ metadata !"llvm.loop.unroll.enable", i1 1 }
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'``llvm.loop.unroll.count``' Metadata
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This metadata suggests an unroll factor to the loop unroller. The
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first operand is the string ``llvm.loop.unroll.count`` and the second
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operand is a positive integer specifying the unroll factor. For
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example:
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.. code-block:: llvm
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!0 = metadata !{ metadata !"llvm.loop.unroll.count", i32 4 }
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If the trip count of the loop is less than the unroll count the loop
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will be partially unrolled.
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If a loop has both a ``llvm.loop.unroll.enable`` metadata and
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``llvm.loop.unroll.count`` metadata the behavior depends upon the
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value of the ``llvm.loop.unroll.enable`` operand. If the value is 0,
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the loop will not be unrolled. If the value is 1, the loop will be
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unrolled with a factor determined by the ``llvm.loop.unroll.count``
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operand effectively ignoring the ``llvm.loop.unroll.enable`` metadata.
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'``llvm.mem``'
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^^^^^^^^^^^^^^^
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@ -2995,55 +3087,6 @@ the loop identifier metadata node directly:
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!1 = metadata !{ metadata !1 } ; an identifier for the inner loop
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!2 = metadata !{ metadata !2 } ; an identifier for the outer loop
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'``llvm.loop.vectorize``'
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^^^^^^^^^^^^^^^^^^^^^^^^^
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Metadata prefixed with ``llvm.loop.vectorize`` is used to control per-loop
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vectorization parameters such as vectorization factor and unroll factor.
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``llvm.loop.vectorize`` metadata should be used in conjunction with
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``llvm.loop`` loop identification metadata.
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'``llvm.loop.vectorize.unroll``' Metadata
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This metadata instructs the loop vectorizer to unroll the specified
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loop exactly ``N`` times.
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The first operand is the string ``llvm.loop.vectorize.unroll`` and the second
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operand is an integer specifying the unroll factor. For example:
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.. code-block:: llvm
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!0 = metadata !{ metadata !"llvm.loop.vectorize.unroll", i32 4 }
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Note that setting ``llvm.loop.vectorize.unroll`` to 1 disables
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unrolling of the loop.
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If ``llvm.loop.vectorize.unroll`` is set to 0 then the amount of
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unrolling will be determined automatically.
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'``llvm.loop.vectorize.width``' Metadata
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This metadata sets the target width of the vectorizer to ``N``. Without
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this metadata, the vectorizer will choose a width automatically.
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Regardless of this metadata, the vectorizer will only vectorize loops if
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it believes it is valid to do so.
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The first operand is the string ``llvm.loop.vectorize.width`` and the
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second operand is an integer specifying the width. For example:
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.. code-block:: llvm
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!0 = metadata !{ metadata !"llvm.loop.vectorize.width", i32 4 }
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Note that setting ``llvm.loop.vectorize.width`` to 1 disables
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vectorization of the loop.
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If ``llvm.loop.vectorize.width`` is set to 0 then the width will be
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determined automatically.
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Module Flags Metadata
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=====================
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