Compute the loop nest depth of each CfgNode and weight Variables by it.

src/IceOperand.cpp

Index: src/IceOperand.cpp
diff --git a/src/IceOperand.cpp b/src/IceOperand.cpp
index 32316ba6ffda6d4f1e8bcab3e1a1ddd589960695..f0c5c73f805ec0085f60348d5b0461b8115594aa 100644
--- a/src/IceOperand.cpp
+++ b/src/IceOperand.cpp
@@ -157,8 +157,16 @@ void VariableTracking::markUse(MetadataKind TrackingKind, const Inst *Instr,
                                CfgNode *Node, bool IsImplicit) {
   (void)TrackingKind;
 
-  // TODO(ascull): get the loop nest depth from CfgNode
-  UseWeight += 1;
+  // Increment the use weight depending on the loop nest depth. The weight is
+  // exponential in the nest depth as inner loops are expected to be executed
+  // an exponentially greater number of times.
+  constexpr uint32_t LogTypicalLoopTripCount = 3; // 2^3 = 8

[Jim Stichnoth, 2015/09/01 22:17:37]

nit: "Typical" implies knowledge we don't have.  I think
LogLoopTripCountEstimate would be better.

[ascull, 2015/09/03 19:52:38]

Done.

+  uint32_t ThisUseWeight =

[Jim Stichnoth, 2015/09/01 22:17:37]

Maybe SizeT instead of uint32_t?

[ascull, 2015/09/03 19:52:38]

Done.

+      1 << std::min(Node->getLoopNestDepth() * LogTypicalLoopTripCount, 31u);

[Jim Stichnoth, 2015/09/01 22:17:37]

No magic constants please. :)

CHAR_BIT * sizeof(ThisUseWeight) - 1

[ascull, 2015/09/03 19:52:38]

Done.

+  uint32_t NewWeight = UseWeight + ThisUseWeight;
+  // Make sure the weight doesn't overflow or become infinite.
+  UseWeight = NewWeight < UseWeight ? RegWeight::Inf - 1

[Jim Stichnoth, 2015/09/01 22:17:38]

How about something defining something like RegWeight::Max which is denoted as
the maximum non-infinite weight?  RegWeight::addWeight() could also be tightened
up using that.

+                                    : std::min(NewWeight, RegWeight::Inf - 1);
 
   if (MultiBlock == MBS_MultiBlock)
     return;