Add a cautionary note to this API. It was not at all obvious to me how

expensive the most useful interface to this analysis is.

Fun story -- it's also not correct. That's getting fixed in another
patch.

llvm-svn: 144523

include/llvm/CodeGen/MachineBranchProbabilityInfo.h

@@ -62,6 +62,9 @@ public:
   // Return a probability as a fraction between 0 (0% probability) and
   // 1 (100% probability), however the value is never equal to 0, and can be 1
   // only iff SRC block has only one successor.
+  // NB: This routine's complexity is linear on the number of successors of
+  // Src. Querying sequentially for each successor's probability is a quadratic
+  // query pattern.
   BranchProbability getEdgeProbability(MachineBasicBlock *Src,
                                        MachineBasicBlock *Dst) const;