SelectionDAG switch lowering: Replace unreachable default with most popular case.

This can significantly reduce the size of the switch, allowing for more
efficient lowering.

I also worked with the idea of exploiting unreachable defaults by
omitting the range check for jump tables, but always ended up with a
non-neglible binary size increase. It might be worth looking into some more.

SimplifyCFG currently does this transformation, but I'm working towards changing
that so we can optimize harder based on unreachable defaults.

Differential Revision: http://reviews.llvm.org/D6510

llvm-svn: 223566

1 files changed, 40 insertions, 17 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index c3b197e97eb..a5fdfc55487 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -2698,32 +2698,55 @@ void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
   if (SwitchMBB + 1 != FuncInfo.MF->end())
     NextBlock = SwitchMBB + 1;
 
+
+  // Create a vector of Cases, sorted so that we can efficiently create a binary
+  // search tree from them.
+  CaseVector Cases;
+  Clusterify(Cases, SI);
+
+  // Get the default destination MBB.
   MachineBasicBlock *Default = FuncInfo.MBBMap[SI.getDefaultDest()];
 
-  // If there is only the default destination, branch to it if it is not the
-  // next basic block.  Otherwise, just fall through.
-  if (!SI.getNumCases()) {
+  if (isa<UnreachableInst>(SI.getDefaultDest()->getFirstNonPHIOrDbg()) &&
+      !Cases.empty()) {
+    // Replace an unreachable default destination with the most popular case
+    // destination.
+    DenseMap<const BasicBlock *, uint64_t> Popularity;
+    uint64_t MaxPop = 0;
+    const BasicBlock *MaxBB = nullptr;
+    for (auto I : SI.cases()) {
+      const BasicBlock *BB = I.getCaseSuccessor();
+      if (++Popularity[BB] > MaxPop) {
+        MaxPop = Popularity[BB];
+        MaxBB = BB;
+      }
+    }
+
+    // Set new default.
+    assert(MaxPop > 0);
+    assert(MaxBB);
+    Default = FuncInfo.MBBMap[MaxBB];
+
+    // Remove cases that were pointing to the destination that is now the default.
+    Cases.erase(std::remove_if(Cases.begin(), Cases.end(),
+                               [&](const Case &C) { return C.BB == Default; }),
+                Cases.end());
+  }
+
+  // If there is only the default destination, go there directly.
+  if (Cases.empty()) {
     // Update machine-CFG edges.
     SwitchMBB->addSuccessor(Default);
 
     // If this is not a fall-through branch, emit the branch.
-    if (Default != NextBlock)
-      DAG.setRoot(DAG.getNode(ISD::BR, getCurSDLoc(),
-                              MVT::Other, getControlRoot(),
-                              DAG.getBasicBlock(Default)));
-
+    if (Default != NextBlock) {
+      DAG.setRoot(DAG.getNode(ISD::BR, getCurSDLoc(), MVT::Other,
+                              getControlRoot(), DAG.getBasicBlock(Default)));
+    }
     return;
   }
 
-  // If there are any non-default case statements, create a vector of Cases
-  // representing each one, and sort the vector so that we can efficiently
-  // create a binary search tree from them.
-  CaseVector Cases;
-  Clusterify(Cases, SI);
-
-  // Get the Value to be switched on and default basic blocks, which will be
-  // inserted into CaseBlock records, representing basic blocks in the binary
-  // search tree.
+  // Get the Value to be switched on.
   const Value *SV = SI.getCondition();
 
   // Push the initial CaseRec onto the worklist