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[NFC] Fix formatting of README.md for better rendering 

Reviewed as https://reviews.llvm.org/D56051.

llvm-svn: 350539
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Louis Dionne 2019-01-07 16:54:02 +00:00
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Parallel STL is an implementation of the C++ standard library algorithms with support for execution policies,
as specified in ISO/IEC 14882:2017 standard, commonly called C++17. The implementation also supports the unsequenced
execution policy specified in Parallelism TS version 2 and proposed for the next version of the C++ standard in the
C++ working group paper P1001R1.
C++ working group paper [P1001](https://wg21.link/p1001).
Parallel STL offers efficient support for both parallel and vectorized execution of algorithms. For sequential
execution, it relies on an available implementation of the C++ standard library.
## Prerequisites
To use Parallel STL, you must have the following software installed:
* C++ compiler with:
* Support for C++11
* Support for OpenMP* 4.0 SIMD constructs
* Threading Building Blocks (TBB) which is available to download in the GitHub [repository](https://github.com/01org/tbb/)
* Threading Building Blocks (TBB) which is available for download at https://github.com/01org/tbb/
## Known Issues or limitations
unseq and par_unseq policies only have effect with compilers that
support '#pragma omp simd' or '#pragma simd'.
Parallel and vector execution is only supported for the algorithms
if random access iterators are provided, while for other iterator
types the execution will remain serial.
The following algorithms do not allow efficient SIMD execution:
includes, inplace_merge, merge, nth_element, partial_sort,
partial_sort_copy, set_difference, set_intersection,
set_symmetric_difference, set_union, sort, stable_partition,
stable_sort, unique.
The initial value type for exclusive_scan, inclusive_scan,
transform_exclusive_scan, transform_inclusive_scan shall satisfy
the DefaultConstructible requirements. A default constructed-instance
of the initial value type shall be the identity element for binary_op.
For max_element, min_element, minmax_element, partial_sort,
partial_sort_copy, sort, stable_sort the dereferenced value type of
the provided iterators shall be DefaultConstructible.
For remove, remove_if, unique the dereferenced value type of the provided
iterators shall be MoveConstructible.
The following algorithms require additional O(n) memory space for parallel
execution: copy_if, inplace_merge, partial_sort, partial_sort_copy,
partition_copy, remove, remove_if, rotate, sort, stable_sort, unique,
unique_copy.
## Known issues and limitations
* `unseq` and `par_unseq` policies only have effect with compilers that support `#pragma omp simd` or `#pragma simd`.
* Parallel and vector execution is only supported for the algorithms if random access iterators are provided,
while for other iterator types the execution will remain serial.
* The following algorithms do not allow efficient SIMD execution: `includes`, `inplace_merge`, `merge`, `nth_element`,
`partial_sort`, `partial_sort_copy`, `set_difference`, `set_intersection`, `set_symmetric_difference`, `set_union`,
`sort`, `stable_partition`, `stable_sort`, `unique`.
* The initial value type for `exclusive_scan`, `inclusive_scan`, `transform_exclusive_scan`, `transform_inclusive_scan`
shall be DefaultConstructible. A default constructed-instance of the initial value type shall be the identity element
for the specified binary operation.
* For `max_element`, `min_element`, `minmax_element`, `partial_sort`, `partial_sort_copy`, `sort`, `stable_sort`
the dereferenced value type of the provided iterators shall be DefaultConstructible.
* For `remove`, `remove_if`, `unique` the dereferenced value type of the provided iterators shall be MoveConstructible.
* The following algorithms require additional O(n) memory space for parallel execution: `copy_if`, `inplace_merge`,
`partial_sort`, `partial_sort_copy`, `partition_copy`, `remove`, `remove_if`, `rotate`, `sort`, `stable_sort`,
`unique`, `unique_copy`.