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Use TargetTransformInfo to control switch-to-lookup table transformation

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When the switch-to-lookup tables transform landed in SimplifyCFG, it
was pointed out that this could be inappropriate for some targets.
Since there was no way at the time for the pass to know anything about
the target, an awkward reverse-transform was added in CodeGenPrepare
that turned lookup tables back into switches for some targets.

This patch uses the new TargetTransformInfo to determine if a
switch should be transformed, and removes
CodeGenPrepare::ConvertLoadToSwitch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167011 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hans Wennborg
2012-10-30 11:23:25 +00:00
parent c588e0e162
commit 04d7d13d30
10 changed files with 79 additions and 208 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/Transforms/Scalar/CodeGenPrepare.cpp
+2 -112
View File
@@ -18,7 +18,6 @@
#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"
@@ -128,7 +127,6 @@ namespace {
bool OptimizeSelectInst(SelectInst *SI);
bool DupRetToEnableTailCallOpts(ReturnInst *RI);
bool PlaceDbgValues(Function &F);
bool ConvertLoadToSwitch(LoadInst *LI);
};
}
@@ -1292,11 +1290,9 @@ bool CodeGenPrepare::OptimizeInst(Instruction *I) {
return OptimizeCmpExpression(CI);
if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
bool Changed = false;
if (TLI)
Changed |= OptimizeMemoryInst(I, I->getOperand(0), LI->getType());
Changed |= ConvertLoadToSwitch(LI);
return Changed;
return OptimizeMemoryInst(I, I->getOperand(0), LI->getType());
return false;
}
if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
@@ -1376,109 +1372,3 @@ 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;
}