Stop trying to merge identical jump tables. This had been inadvertently

disabled for several months (since svn r88806) and no one noticed.  My fix
for pr6543 yesterday reenabled it, but broke the ARM port's code for using
TBB/TBH.  Rather than adding a target hook to disable merging for Thumb2 only,
I'm just taking this out.  It is not common to have identical jump tables,
the code we used to merge them was O(N^2), and it only helps code size, not
performance.

llvm-svn: 98977

1 files changed, 6 insertions, 28 deletions

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index b45abc7b..151e9cd 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -192,7 +192,7 @@ bool BranchFolder::OptimizeFunction(MachineFunction &MF,
     MadeChange |= MadeChangeThisIteration;
   }
 
-  // See if any jump tables have become mergable or dead as the code generator
+  // See if any jump tables have become dead as the code generator
   // did its thing.
   MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
   if (JTI == 0) {
@@ -200,27 +200,8 @@ bool BranchFolder::OptimizeFunction(MachineFunction &MF,
     return MadeChange;
   }
   
-  const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables();
-  // Figure out how these jump tables should be merged.
-  std::vector<unsigned> JTMapping;
-  JTMapping.reserve(JTs.size());
-
-  // We always keep the 0th jump table.
-  JTMapping.push_back(0);
-
-  // Scan the jump tables, seeing if there are any duplicates.  Note that this
-  // is N^2, which should be fixed someday.
-  for (unsigned i = 1, e = JTs.size(); i != e; ++i) {
-    if (JTs[i].MBBs.empty())
-      JTMapping.push_back(i);
-    else
-      JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs));
-  }
-
-  // If a jump table was merge with another one, walk the function rewriting
-  // references to jump tables to reference the new JT ID's.  Keep track of
-  // whether we see a jump table idx, if not, we can delete the JT.
-  BitVector JTIsLive(JTs.size());
+  // Walk the function to find jump tables that are live.
+  BitVector JTIsLive(JTI->getJumpTables().size());
   for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
        BB != E; ++BB) {
     for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
@@ -228,17 +209,14 @@ bool BranchFolder::OptimizeFunction(MachineFunction &MF,
       for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
         MachineOperand &Op = I->getOperand(op);
         if (!Op.isJTI()) continue;
-        unsigned NewIdx = JTMapping[Op.getIndex()];
-        Op.setIndex(NewIdx);
 
         // Remember that this JT is live.
-        JTIsLive.set(NewIdx);
+        JTIsLive.set(Op.getIndex());
       }
   }
 
-  // Finally, remove dead jump tables.  This happens either because the
-  // indirect jump was unreachable (and thus deleted) or because the jump
-  // table was merged with some other one.
+  // Finally, remove dead jump tables.  This happens when the
+  // indirect jump was unreachable (and thus deleted).
   for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
     if (!JTIsLive.test(i)) {
       JTI->RemoveJumpTable(i);