TargetLowering interface to set/get minimum block entries for jump tables.

Provide interface in TargetLowering to set or get the minimum number of basic
blocks whereby jump tables are generated for switch statements rather than an
if sequence.

    getMinimumJumpTableEntries() defaults to 4.
    setMinimumJumpTableEntries() allows target configuration.

    This patch changes the default for the Hexagon architecture to 5
    as it improves performance on some benchmarks.

llvm-svn: 164628

4 files changed, 21 insertions, 2 deletions

diff --git a/llvm/include/llvm/Target/TargetLowering.h b/llvm/include/llvm/Target/TargetLowering.h
index ff510486b71..9e46a40e298 100644
--- a/llvm/include/llvm/Target/TargetLowering.h
+++ b/llvm/include/llvm/Target/TargetLowering.h
@@ -712,6 +712,12 @@ public:
     return SupportJumpTables;
   }
 
+  /// getMinimumJumpTableEntries - return integer threshold on number of
+  /// blocks to use jump tables rather than if sequence.
+  int getMinimumJumpTableEntries() const {
+    return MinimumJumpTableEntries;
+  }
+
   /// getStackPointerRegisterToSaveRestore - If a physical register, this
   /// specifies the register that llvm.savestack/llvm.restorestack should save
   /// and restore.
@@ -1032,6 +1038,12 @@ protected:
     SupportJumpTables = Val;
   }
 
+  /// setMinimumJumpTableEntries - Indicate the number of blocks to generate
+  /// jump tables rather than if sequence.
+  void setMinimumJumpTableEntries(int Val) {
+    MinimumJumpTableEntries = Val;
+  }
+
   /// setStackPointerRegisterToSaveRestore - If set to a physical register, this
   /// specifies the register that llvm.savestack/llvm.restorestack should save
   /// and restore.
@@ -1826,6 +1838,9 @@ private:
   /// If it's not true, then each jumptable must be lowered into if-then-else's.
   bool SupportJumpTables;
 
+  /// MinimumJumpTableEntries - Number of blocks threshold to use jump tables.
+  int MinimumJumpTableEntries;
+
   /// BooleanContents - Information about the contents of the high-bits in
   /// boolean values held in a type wider than i1.  See getBooleanContents.
   BooleanContent BooleanContents;
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 483b051bdcf..f71c8b81cb5 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -2093,7 +2093,7 @@ bool SelectionDAGBuilder::handleJTSwitchCase(CaseRec &CR,
   for (CaseItr I = CR.Range.first, E = CR.Range.second; I != E; ++I)
     TSize += I->size();
 
-  if (!areJTsAllowed(TLI) || TSize.ult(4))
+  if (!areJTsAllowed(TLI) || TSize.ult(TLI.getMinimumJumpTableEntries()))
     return false;
 
   APInt Range = ComputeRange(First, Last);
@@ -2565,9 +2565,10 @@ void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
     if (handleSmallSwitchRange(CR, WorkList, SV, Default, SwitchMBB))
       continue;
 
-    // If the switch has more than 5 blocks, and at least 40% dense, and the
+    // If the switch has more than N blocks, and is at least 40% dense, and the
     // target supports indirect branches, then emit a jump table rather than
     // lowering the switch to a binary tree of conditional branches.
+    // N defaults to 4 and is controlled via TLS.getMinimumJumpTableEntries().
     if (handleJTSwitchCase(CR, WorkList, SV, Default, SwitchMBB))
       continue;
 
diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 56f3a45c9a6..be3ecf34f75 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -613,6 +613,7 @@ TargetLowering::TargetLowering(const TargetMachine &tm,
   ShouldFoldAtomicFences = false;
   InsertFencesForAtomic = false;
   SupportJumpTables = true;
+  MinimumJumpTableEntries = 4;
 
   InitLibcallNames(LibcallRoutineNames);
   InitCmpLibcallCCs(CmpLibcallCCs);
diff --git a/llvm/lib/Target/Hexagon/HexagonISelLowering.cpp b/llvm/lib/Target/Hexagon/HexagonISelLowering.cpp
index 703a128ee02..1c891f14d8f 100644
--- a/llvm/lib/Target/Hexagon/HexagonISelLowering.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonISelLowering.cpp
@@ -1350,6 +1350,8 @@ HexagonTargetLowering::HexagonTargetLowering(HexagonTargetMachine
     } else {
       setOperationAction(ISD::BR_JT, MVT::Other, Expand);
     }
+    // Increase jump tables cutover to 5, was 4.
+    setMinimumJumpTableEntries(5);
 
     setOperationAction(ISD::BR_CC, MVT::i32, Expand);