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Differential D10857 Diff 29425 docs/LanguageExtensions.rst

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docs/LanguageExtensions.rst

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	Unrolling a loop reduces the loop control overhead and exposes more			Unrolling a loop reduces the loop control overhead and exposes more
	opportunities for ILP. Loops can be fully or partially unrolled. Full unrolling			opportunities for ILP. Loops can be fully or partially unrolled. Full unrolling
	eliminates the loop and replaces it with an enumerated sequence of loop			eliminates the loop and replaces it with an enumerated sequence of loop
	iterations. Full unrolling is only possible if the loop trip count is known at			iterations. Full unrolling is only possible if the loop trip count is known at
	compile time. Partial unrolling replicates the loop body within the loop and			compile time. Partial unrolling replicates the loop body within the loop and
	reduces the trip count.			reduces the trip count.

	If ``unroll(full)`` is specified the unroller will attempt to fully unroll the			If ``unroll(full)`` is specified the unroller will attempt to fully unroll the
	loop if the trip count is known at compile time. If the loop count is not known			loop if the trip count is known at compile time. If the fully unrolled code size
	or the fully unrolled code size is greater than the limit specified by the			is greater than an internal limit the loop will be partially unrolled up to this
				hfinkelUnsubmitted Not Done Reply Inline Actions I know this is a pre-existing issue, but -pragma-unroll-threshold is an LLVM option, not a Clang option, and I don't think we should mention it directly here. If I did not know how LLVM worked, and what -mllvm did, I'd find this documentation confusing. Maybe just not mentioning this is best. What do you think? hfinkel: I know this is a pre-existing issue, but -pragma-unroll-threshold is an LLVM option, not a…
	`-pragma-unroll-threshold` command line option the loop will be partially			limit. If the loop count is not known at compile time the loop will not be
	unrolled subject to the same limit.			unrolled.

	.. code-block:: c++			.. code-block:: c++

	#pragma clang loop unroll(full)			#pragma clang loop unroll(full)
	for(...) {			for(...) {
	...			...
	}			}

	The unroll count can be specified explicitly with ``unroll_count(_value_)`` where			The unroll count can be specified explicitly with ``unroll_count(_value_)`` where
	_value_ is a positive integer. If this value is greater than the trip count the			_value_ is a positive integer. If this value is greater than the trip count the
	loop will be fully unrolled. Otherwise the loop is partially unrolled subject			loop will be fully unrolled. Otherwise the loop is partially unrolled subject
	to the `-pragma-unroll-threshold` limit.			to the same code size limit as with ``unroll(full)``.

	.. code-block:: c++			.. code-block:: c++

	#pragma clang loop unroll_count(8)			#pragma clang loop unroll_count(8)
	for(...) {			for(...) {
	...			...
	}			}

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