add TLI support indicating that jumps are more expensive than logical operations

and use this to disable a specific optimization.  Patch by Micah Villmow!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120435 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 23 insertions, 2 deletions

diff --git a/include/llvm/Target/TargetLowering.h b/include/llvm/Target/TargetLowering.h
index 75e5325524..b450f42752 100644
--- a/include/llvm/Target/TargetLowering.h
+++ b/include/llvm/Target/TargetLowering.h
@@ -125,6 +125,10 @@ public:
   /// srl/add/sra.
   bool isPow2DivCheap() const { return Pow2DivIsCheap; }
 
+  /// isJumpExpensive() - Return true if Flow Control is an expensive operation
+  /// that should be avoided.
+  bool isJumpExpensive() const { return JumpIsExpensive; }
+
   /// getSetCCResultType - Return the ValueType of the result of SETCC
   /// operations.  Also used to obtain the target's preferred type for
   /// the condition operand of SELECT and BRCOND nodes.  In the case of
@@ -1013,7 +1017,16 @@ protected:
 
   /// SelectIsExpensive - Tells the code generator not to expand operations
   /// into sequences that use the select operations if possible.
-  void setSelectIsExpensive() { SelectIsExpensive = true; }
+  void setSelectIsExpensive(bool isExpensive = true) { 
+    SelectIsExpensive = isExpensive; 
+  }
+
+  /// JumpIsExpensive - Tells the code generator not to expand sequence of 
+  /// operations into a seperate sequences that increases the amount of 
+  /// flow control.
+  void setJumpIsExpensive(bool isExpensive = true) {
+    JumpIsExpensive = isExpensive;
+  }
 
   /// setIntDivIsCheap - Tells the code generator that integer divide is
   /// expensive, and if possible, should be replaced by an alternate sequence
@@ -1597,6 +1610,11 @@ private:
   /// it.
   bool Pow2DivIsCheap;
 
+  /// JumpIsExpensive - Tells the code generator that it shouldn't generate
+  /// extra flow control instructions and should attempt to combine flow
+  /// control instructions via predication.
+  bool JumpIsExpensive;
+
   /// UseUnderscoreSetJmp - This target prefers to use _setjmp to implement
   /// llvm.setjmp.  Defaults to false.
   bool UseUnderscoreSetJmp;
diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index d00643f1f1..6dfc6b53c7 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -1403,6 +1403,7 @@ void SelectionDAGBuilder::visitBr(const BranchInst &I) {
 
   // If this is a series of conditions that are or'd or and'd together, emit
   // this as a sequence of branches instead of setcc's with and/or operations.
+  // As long as jumps are not expensive, this should improve performance.
   // For example, instead of something like:
   //     cmp A, B
   //     C = seteq
@@ -1417,7 +1418,8 @@ void SelectionDAGBuilder::visitBr(const BranchInst &I) {
   //     jle foo
   //
   if (const BinaryOperator *BOp = dyn_cast<BinaryOperator>(CondVal)) {
-    if (BOp->hasOneUse() &&
+    if (!TLI.isJumpExpensive() && 
+        BOp->hasOneUse() &&
         (BOp->getOpcode() == Instruction::And ||
          BOp->getOpcode() == Instruction::Or)) {
       FindMergedConditions(BOp, Succ0MBB, Succ1MBB, BrMBB, BrMBB,
diff --git a/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 12ca9291d9..b2c2dbb927 100644
--- a/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -572,6 +572,7 @@ TargetLowering::TargetLowering(const TargetMachine &tm,
   SelectIsExpensive = false;
   IntDivIsCheap = false;
   Pow2DivIsCheap = false;
+  JumpIsExpensive = false;
   StackPointerRegisterToSaveRestore = 0;
   ExceptionPointerRegister = 0;
   ExceptionSelectorRegister = 0;