Fix a bug which prevented tail merging of return instructions in

beneficial cases. See the changes in test/CodeGen/X86/tail-opts.ll and
test/CodeGen/ARM/ifcvt2.ll for details.

The fix is to change HashEndOfMBB to hash at most one instruction,
instead of trying to apply heuristics about when it will be profitable to
consider more than one instruction. The regular tail-merging heuristics
are already prepared to handle the same cases, and they're more precise.

Also, make test/CodeGen/ARM/ifcvt5.ll and
test/CodeGen/Thumb2/thumb2-branch.ll slightly more complex so that they
continue to test what they're intended to test.

And, this eliminates the problem in
test/CodeGen/Thumb2/2009-10-15-ITBlockBranch.ll, the testcase from
PR5204. Update it accordingly.

llvm-svn: 102907

1 files changed, 5 insertions, 24 deletions

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 8f519407ccd..9dec22ec78a 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -264,14 +264,8 @@ static unsigned HashMachineInstr(const MachineInstr *MI) {
   return Hash;
 }
 
-/// HashEndOfMBB - Hash the last few instructions in the MBB.  For blocks
-/// with no successors, we hash two instructions, because cross-jumping
-/// only saves code when at least two instructions are removed (since a
-/// branch must be inserted).  For blocks with a successor, one of the
-/// two blocks to be tail-merged will end with a branch already, so
-/// it gains to cross-jump even for one instruction.
-static unsigned HashEndOfMBB(const MachineBasicBlock *MBB,
-                             unsigned minCommonTailLength) {
+/// HashEndOfMBB - Hash the last instruction in the MBB.
+static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) {
   MachineBasicBlock::const_iterator I = MBB->end();
   if (I == MBB->begin())
     return 0;   // Empty MBB.
@@ -283,20 +277,8 @@ static unsigned HashEndOfMBB(const MachineBasicBlock *MBB,
       return 0;      // MBB empty except for debug info.
     --I;
   }
-  unsigned Hash = HashMachineInstr(I);
 
-  if (I == MBB->begin() || minCommonTailLength == 1)
-    return Hash;   // Single instr MBB.
-
-  --I;
-  while (I->isDebugValue()) {
-    if (I==MBB->begin())
-      return Hash;      // MBB with single non-debug instr.
-    --I;
-  }
-  // Hash in the second-to-last instruction.
-  Hash ^= HashMachineInstr(I) << 2;
-  return Hash;
+  return HashMachineInstr(I);
 }
 
 /// ComputeCommonTailLength - Given two machine basic blocks, compute the number
@@ -811,7 +793,7 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
   MergePotentials.clear();
   for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
     if (I->succ_empty())
-      MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I));
+      MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I), I));
   }
 
   // See if we can do any tail merging on those.
@@ -897,8 +879,7 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
               // reinsert conditional branch only, for now
               TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond);
           }
-          MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U),
-                                                       *P));
+          MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB), *P));
         }
       }
       if (MergePotentials.size() >= 2)