[PATCH] Add loop unrolling metadata descriptions to LangRef.rst

[hfinkel at anl.gov]

Thanks for working on this!

================
Comment at: docs/LangRef.rst:2913
@@ +2912,3 @@
+vectorizer will only vectorize loops if it believes it is valid to do
+so.
+
----------------
You should add a note here that an IR-producer looking at affect this safety determination might find the llvm.mem.parallel_loop_access metadata useful.

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Comment at: docs/LangRef.rst:2957
@@ +2956,3 @@
+optimizer believes it is valid to do so.
+
+'``llvm.loop.unroll.enable``' Metadata
----------------
We should say something explaining how this is different from llvm.loop.vectorize.unroll (which actually does interleaving -- maybe we should rename it?). This is for concatenation unrolling (where the loop body is essentially replicated several times by the unroller, although other optimizations may intermix instructions from different unrolled iterations).

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Comment at: docs/LangRef.rst:2965
@@ +2964,3 @@
+bit operand value is 0 loop unrolling is disabled. A value of 1
+indicates that the loop should be fully unrolled. For example:
+
----------------
I find this confusing. Specifically, to get partial unrolling, what needs to happen? If I want partial unrolling with a specific count, do I need both the 'count' and 'enable' metadata?

http://reviews.llvm.org/D4576