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

[Mark Heffernan]

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Comment at: docs/LangRef.rst:2913
@@ +2912,3 @@
+vectorizer will only vectorize loops if it believes it is valid to do
+so.
+
----------------
hfinkel at anl.gov wrote:
> 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.
I'll add this.

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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
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hfinkel at anl.gov wrote:
> 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).
I think the fix here is to rename llvm.loop.vectorize.unroll to  llvm.loop.vectorize.interleave.  Then there is no chance of conflating the vectorize metadata with the unroll metadata.  I'll work on a patch for this.

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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:
+
----------------
hfinkel at anl.gov wrote:
> 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?
I added a bit more explanation about that case.

I'd imagine an underlying cause of confusion is that ``llvm.loop.unroll.enable 1`` doesn't exactly mean enable.  It means try to fully unroll the loop.  So we want the following possible hints:

don't unroll
unroll fully
unroll by N

This doesn't exactly map nicely to the two metadata we have (one with boolean operand, one with i32).  Maybe a cleaner way to have done this is with the following metadata:

llvm.loop.unroll.disable (no operand)
llvm.loop.unroll.fully_unroll (no operand)
llvm.loop.unroll.count i32

And only allow a single loop unroll metadata node per loop.  Worth making this change?

http://reviews.llvm.org/D4576