[llvm-commits] [llvm] r93600 - /llvm/trunk/lib/CodeGen/TailDuplication.cpp

[Bob Wilson]

Author: bwilson
Date: Fri Jan 15 18:42:25 2010
New Revision: 93600

URL: http://llvm.org/viewvc/llvm-project?rev=93600&view=rev
Log:
Treat indirect branches specially only during pre-regalloc tail duplication,
not during the later post-alloc tail duplication.

Modified:
    llvm/trunk/lib/CodeGen/TailDuplication.cpp

Modified: llvm/trunk/lib/CodeGen/TailDuplication.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/TailDuplication.cpp?rev=93600&r1=93599&r2=93600&view=diff

==============================================================================
--- llvm/trunk/lib/CodeGen/TailDuplication.cpp (original)
+++ llvm/trunk/lib/CodeGen/TailDuplication.cpp Fri Jan 15 18:42:25 2010
@@ -433,28 +433,28 @@
 TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
                                  SmallVector<MachineBasicBlock*, 8> &TDBBs,
                                  SmallVector<MachineInstr*, 16> &Copies) {
-  // Pre-regalloc tail duplication hurts compile time and doesn't help
-  // much except for indirect branches.
-  bool hasIndirectBranch = (!TailBB->empty() &&
-                            TailBB->back().getDesc().isIndirectBranch());
-  if (PreRegAlloc && !hasIndirectBranch)
-    return false;
-
   // Set the limit on the number of instructions to duplicate, with a default
   // of one less than the tail-merge threshold. When optimizing for size,
   // duplicate only one, because one branch instruction can be eliminated to
   // compensate for the duplication.
   unsigned MaxDuplicateCount;
-  if (hasIndirectBranch)
+  if (MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
+    MaxDuplicateCount = 1;
+  else
+    MaxDuplicateCount = TailDuplicateSize;
+
+  if (PreRegAlloc) {
+      // Pre-regalloc tail duplication hurts compile time and doesn't help
+      // much except for indirect branches.
+    if (TailBB->empty() || !TailBB->back().getDesc().isIndirectBranch())
+      return false;
     // If the target has hardware branch prediction that can handle indirect
     // branches, duplicating them can often make them predictable when there
     // are common paths through the code.  The limit needs to be high enough
-    // to allow undoing the effects of tail merging.
+    // to allow undoing the effects of tail merging and other optimizations
+    // that rearrange the predecessors of the indirect branch.
     MaxDuplicateCount = 20;
-  else if (MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
-    MaxDuplicateCount = 1;
-  else
-    MaxDuplicateCount = TailDuplicateSize;
+  }
 
   // Don't try to tail-duplicate single-block loops.
   if (TailBB->isSuccessor(TailBB))