[PATCH] Introduce bitset metadata format and bitset lowering pass.

[Kostya Serebryany]

================
Comment at: lib/Transforms/IPO/LowerBitSets.cpp:115
@@ +114,3 @@
+
+LowerBitSets::BitSetInfo LowerBitSets::buildBitSet(
+    Module &M,
----------------
I wonder if this thing is unit-testable separately? 
We are not doing these things frequently, but I think we should. 
E.g. see ASanStackFrameLayoutTest.cpp

================
Comment at: lib/Transforms/IPO/LowerBitSets.cpp:121
@@ +120,3 @@
+
+  std::vector<uint64_t> Offsets;
+  uint64_t Min = std::numeric_limits<uint64_t>::max(), Max = 0;
----------------
SmallVector, here and maybe in other places. 

================
Comment at: lib/Transforms/IPO/LowerBitSets.cpp:210
@@ +209,3 @@
+    // also conveniently gives us a bit offset to use during the load from
+    // the bitset.
+    Value *OffsetSHR =
----------------
Clever! 

================
Comment at: lib/Transforms/IPO/LowerBitSets.cpp:222
@@ +221,3 @@
+  TerminatorInst *Term = SplitBlockAndInsertIfThen(OffsetInRange, CI, false);
+  IRBuilder<> ThenB(Term);
+
----------------
Do we really want a separate BB here? 
On valid programs, i.e. in 99.9999% cases both checks will pass, so it might be better to do this w/o branches.

http://reviews.llvm.org/D7288

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