CodeGenPrep: turn lookup tables into switches for some targets.

This is a follow-up from r163302, which added a transformation to
SimplifyCFG that turns some switches into loads from lookup tables.

It was pointed out that some targets, such as GPUs and deeply embedded
targets, might not find this appropriate, but SimplifyCFG doesn't have
enough information about the target to decide this.

This patch adds the reverse transformation to CodeGenPrep: it turns
loads from lookup tables back into switches for targets where we do not
build jump tables (assuming these are also the targets where lookup
tables are inappropriate).

Hopefully we will eventually get to have target information in
SimplifyCFG, and then this CodeGenPrep transformation can be removed.

llvm-svn: 164206

1 files changed, 114 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp b/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp
index 65249153bd0..495cdc63214 100644
--- a/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp
+++ b/llvm/lib/Transforms/Scalar/CodeGenPrepare.cpp
@@ -18,6 +18,7 @@
 #include "llvm/Constants.h"
 #include "llvm/DerivedTypes.h"
 #include "llvm/Function.h"
+#include "llvm/GlobalVariable.h"
 #include "llvm/IRBuilder.h"
 #include "llvm/InlineAsm.h"
 #include "llvm/Instructions.h"
@@ -126,6 +127,7 @@ namespace {
     bool OptimizeSelectInst(SelectInst *SI);
     bool DupRetToEnableTailCallOpts(ReturnInst *RI);
     bool PlaceDbgValues(Function &F);
+    bool ConvertLoadToSwitch(LoadInst *LI);
   };
 }
 
@@ -169,7 +171,7 @@ bool CodeGenPrepare::runOnFunction(Function &F) {
   bool MadeChange = true;
   while (MadeChange) {
     MadeChange = false;
-    for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
+    for (Function::iterator I = F.begin(); I != F.end(); ) {
       BasicBlock *BB = I++;
       MadeChange |= OptimizeBlock(*BB);
     }
@@ -1283,9 +1285,11 @@ bool CodeGenPrepare::OptimizeInst(Instruction *I) {
     return OptimizeCmpExpression(CI);
 
   if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
+    bool Changed = false;
     if (TLI)
-      return OptimizeMemoryInst(I, I->getOperand(0), LI->getType());
-    return false;
+      Changed |= OptimizeMemoryInst(I, I->getOperand(0), LI->getType());
+    Changed |= ConvertLoadToSwitch(LI);
+    return Changed;
   }
 
   if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
@@ -1329,7 +1333,7 @@ bool CodeGenPrepare::OptimizeBlock(BasicBlock &BB) {
   bool MadeChange = false;
 
   CurInstIterator = BB.begin();
-  for (BasicBlock::iterator E = BB.end(); CurInstIterator != E; )
+  while (CurInstIterator != BB.end())
     MadeChange |= OptimizeInst(CurInstIterator++);
 
   return MadeChange;
@@ -1365,3 +1369,109 @@ bool CodeGenPrepare::PlaceDbgValues(Function &F) {
   }
   return MadeChange;
 }
+
+static bool TargetSupportsJumpTables(const TargetLowering &TLI) {
+  return TLI.supportJumpTables() &&
+          (TLI.isOperationLegalOrCustom(ISD::BR_JT, MVT::Other) ||
+           TLI.isOperationLegalOrCustom(ISD::BRIND, MVT::Other));
+}
+
+/// ConvertLoadToSwitch - Convert loads from constant lookup tables into
+/// switches. This undos the switch-to-lookup table transformation in
+/// SimplifyCFG for targets where that is inprofitable.
+bool CodeGenPrepare::ConvertLoadToSwitch(LoadInst *LI) {
+  // This only applies to targets that don't support jump tables.
+  if (!TLI || TargetSupportsJumpTables(*TLI))
+    return false;
+
+  // FIXME: In the future, it would be desirable to have enough target
+  // information in SimplifyCFG, so we could decide at that stage whether to
+  // transform the switch to a lookup table or not, and this
+  // reverse-transformation could be removed.
+
+  GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(LI->getPointerOperand());
+  if (!GEP || !GEP->isInBounds() || GEP->getPointerAddressSpace())
+    return false;
+  if (GEP->getNumIndices() != 2)
+    return false;
+  Value *FirstIndex = GEP->idx_begin()[0];
+  ConstantInt *FirstIndexInt = dyn_cast<ConstantInt>(FirstIndex);
+  if (!FirstIndexInt || !FirstIndexInt->isZero())
+    return false;
+
+  Value *TableIndex = GEP->idx_begin()[1];
+  IntegerType *TableIndexTy = cast<IntegerType>(TableIndex->getType());
+
+  GlobalVariable *GV = dyn_cast<GlobalVariable>(GEP->getPointerOperand());
+  if (!GV || !GV->isConstant() || !GV->hasDefinitiveInitializer())
+    return false;
+
+  Constant *Arr = GV->getInitializer();
+  uint64_t NumElements;
+  if (ConstantArray *CA = dyn_cast<ConstantArray>(Arr))
+    NumElements = CA->getType()->getNumElements();
+  else if (ConstantDataArray *CDA = dyn_cast<ConstantDataArray>(Arr))
+    NumElements = CDA->getNumElements();
+  else
+    return false;
+  if (NumElements < 2)
+    return false;
+
+  // Split the block.
+  BasicBlock *OriginalBB = LI->getParent();
+  BasicBlock *PostSwitchBB = OriginalBB->splitBasicBlock(LI);
+
+  // Replace OriginalBB's terminator with a switch.
+  IRBuilder<> Builder(OriginalBB->getTerminator());
+  SwitchInst *Switch = Builder.CreateSwitch(TableIndex, PostSwitchBB,
+                                            NumElements - 1);
+  OriginalBB->getTerminator()->eraseFromParent();
+
+  // Count the frequency of each value to decide which to use as default.
+  SmallDenseMap<Constant*, uint64_t> ValueFreq;
+  for (uint64_t I = 0; I < NumElements; ++I)
+    ++ValueFreq[Arr->getAggregateElement(I)];
+  uint64_t MaxCount = 0;
+  Constant *DefaultValue = NULL;
+  for (SmallDenseMap<Constant*, uint64_t>::iterator I = ValueFreq.begin(),
+       E = ValueFreq.end(); I != E; ++I) {
+    if (I->second > MaxCount) {
+      MaxCount = I->second;
+      DefaultValue = I->first;
+    }
+  }
+  assert(DefaultValue && "No values in the array?");
+
+  // Create the phi node in PostSwitchBB, which will replace the load.
+  Builder.SetInsertPoint(PostSwitchBB->begin());
+  PHINode *PHI = Builder.CreatePHI(LI->getType(), NumElements);
+  PHI->addIncoming(DefaultValue, OriginalBB);
+
+  // Build basic blocks to target with the switch.
+  for (uint64_t I = 0; I < NumElements; ++I) {
+    Constant *C = Arr->getAggregateElement(I);
+    if (C == DefaultValue) continue; // Already covered by the default case.
+
+    BasicBlock *BB = BasicBlock::Create(PostSwitchBB->getContext(),
+                                        "lookup.bb",
+                                        PostSwitchBB->getParent(),
+                                        PostSwitchBB);
+    Switch->addCase(ConstantInt::get(TableIndexTy, I), BB);
+    Builder.SetInsertPoint(BB);
+    Builder.CreateBr(PostSwitchBB);
+    PHI->addIncoming(C, BB);
+  }
+
+  // Remove the load.
+  LI->replaceAllUsesWith(PHI);
+  LI->eraseFromParent();
+
+  // Clean up.
+  if (GEP->use_empty())
+    GEP->eraseFromParent();
+  if (GV->hasUnnamedAddr() && GV->hasPrivateLinkage() && GV->use_empty())
+    GV->eraseFromParent();
+
+  CurInstIterator = Switch;
+  return true;
+}