Build lookup tables for switches (PR884)
This adds a transformation to SimplifyCFG that attemps to turn switch
instructions into loads from lookup tables. It works on switches that
are only used to initialize one or more phi nodes in a common successor
basic block, for example:
int f(int x) {
switch (x) {
case 0: return 5;
case 1: return 4;
case 2: return -2;
case 5: return 7;
case 6: return 9;
default: return 42;
}
This speeds up the code by removing the hard-to-predict jump, and
reduces code size by removing the code for the jump targets.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163302 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 6cd3bbc..62b98cb 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -22,6 +22,7 @@
#include "llvm/LLVMContext.h"
#include "llvm/MDBuilder.h"
#include "llvm/Metadata.h"
+#include "llvm/Module.h"
#include "llvm/Operator.h"
#include "llvm/Type.h"
#include "llvm/ADT/DenseMap.h"
@@ -54,6 +55,7 @@
cl::desc("Duplicate return instructions into unconditional branches"));
STATISTIC(NumSpeculations, "Number of speculative executed instructions");
+STATISTIC(NumLookupTables, "Number of switch instructions turned into lookup tables");
namespace {
/// ValueEqualityComparisonCase - Represents a case of a switch.
@@ -2977,6 +2979,287 @@
return Changed;
}
+/// ValidLookupTableConstant - Return true if the backend will be able to handle
+/// initializing an array of constants like C.
+bool ValidLookupTableConstant(Constant *C) {
+ if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C))
+ return CE->isGEPWithNoNotionalOverIndexing();
+
+ return isa<ConstantFP>(C) ||
+ isa<ConstantInt>(C) ||
+ isa<ConstantPointerNull>(C) ||
+ isa<GlobalValue>(C) ||
+ isa<UndefValue>(C);
+}
+
+/// GetCaseResulsts - Try to determine the resulting constant values in phi
+/// nodes at the common destination basic block for one of the case
+/// destinations of a switch instruction.
+static bool GetCaseResults(SwitchInst *SI,
+ BasicBlock *CaseDest,
+ BasicBlock **CommonDest,
+ SmallVector<std::pair<PHINode*,Constant*>, 4> &Res) {
+ // The block from which we enter the common destination.
+ BasicBlock *Pred = SI->getParent();
+
+ // If CaseDest is empty, continue to its successor.
+ if (CaseDest->getFirstNonPHIOrDbg() == CaseDest->getTerminator() &&
+ !isa<PHINode>(CaseDest->begin())) {
+
+ TerminatorInst *Terminator = CaseDest->getTerminator();
+ if (Terminator->getNumSuccessors() != 1)
+ return false;
+
+ Pred = CaseDest;
+ CaseDest = Terminator->getSuccessor(0);
+ }
+
+ // If we did not have a CommonDest before, use the current one.
+ if (!*CommonDest)
+ *CommonDest = CaseDest;
+ // If the destination isn't the common one, abort.
+ if (CaseDest != *CommonDest)
+ return false;
+
+ // Get the values for this case from phi nodes in the destination block.
+ BasicBlock::iterator I = (*CommonDest)->begin();
+ while (PHINode *PHI = dyn_cast<PHINode>(I++)) {
+ int Idx = PHI->getBasicBlockIndex(Pred);
+ if (Idx == -1)
+ continue;
+
+ Constant *ConstVal = dyn_cast<Constant>(PHI->getIncomingValue(Idx));
+ if (!ConstVal)
+ return false;
+
+ // Be conservative about which kinds of constants we support.
+ if (!ValidLookupTableConstant(ConstVal))
+ return false;
+
+ Res.push_back(std::make_pair(PHI, ConstVal));
+ }
+
+ return true;
+}
+
+/// BuildLookupTable - Build a lookup table with the contents of Results, using
+/// DefaultResult to fill the holes in the table. If the table ends up
+/// containing the same result in each element, set *SingleResult to that value
+/// and return NULL.
+static GlobalVariable *BuildLookupTable(
+ Module &M,
+ uint64_t TableSize,
+ ConstantInt *Offset,
+ const std::vector<std::pair<ConstantInt*,Constant*> >& Results,
+ Constant *DefaultResult,
+ Constant **SingleResult) {
+ assert(Results.size() && "Need values to build lookup table");
+ assert(TableSize >= Results.size() && "Table needs to hold all values");
+
+ // If all values in the table are equal, this is that value.
+ Constant *SameResult = Results.begin()->second;
+
+ // Build up the table contents.
+ std::vector<Constant*> TableContents(TableSize);
+ for (size_t I = 0, E = Results.size(); I != E; ++I) {
+ ConstantInt *CaseVal = Results[I].first;
+ Constant *CaseRes = Results[I].second;
+
+ uint64_t Idx = (CaseVal->getValue() - Offset->getValue()).getLimitedValue();
+ TableContents[Idx] = CaseRes;
+
+ if (CaseRes != SameResult)
+ SameResult = NULL;
+ }
+
+ // Fill in any holes in the table with the default result.
+ if (Results.size() < TableSize) {
+ for (unsigned i = 0; i < TableSize; ++i) {
+ if (!TableContents[i])
+ TableContents[i] = DefaultResult;
+ }
+
+ if (DefaultResult != SameResult)
+ SameResult = NULL;
+ }
+
+ // Same result was used in the entire table; just return that.
+ if (SameResult) {
+ *SingleResult = SameResult;
+ return NULL;
+ }
+
+ ArrayType *ArrayTy = ArrayType::get(DefaultResult->getType(), TableSize);
+ Constant *Initializer = ConstantArray::get(ArrayTy, TableContents);
+
+ GlobalVariable *GV = new GlobalVariable(M, ArrayTy, /*constant=*/ true,
+ GlobalVariable::PrivateLinkage,
+ Initializer,
+ "switch.table");
+ GV->setUnnamedAddr(true);
+ return GV;
+}
+
+/// SwitchToLookupTable - If the switch is only used to initialize one or more
+/// phi nodes in a common successor block with different constant values,
+/// replace the switch with lookup tables.
+static bool SwitchToLookupTable(SwitchInst *SI,
+ IRBuilder<> &Builder) {
+ assert(SI->getNumCases() > 1 && "Degenerate switch?");
+ // FIXME: Handle unreachable cases.
+
+ // FIXME: If the switch is too sparse for a lookup table, perhaps we could
+ // split off a dense part and build a lookup table for that.
+
+ // FIXME: If the results are all integers and the lookup table would fit in a
+ // target-legal register, we should store them as a bitmap and use shift/mask
+ // to look up the result.
+
+ // FIXME: This creates arrays of GEPs to constant strings, which means each
+ // GEP needs a runtime relocation in PIC code. We should just build one big
+ // string and lookup indices into that.
+
+ // Ignore the switch if the number of cases are too small.
+ // This is similar to the check when building jump tables in
+ // SelectionDAGBuilder::handleJTSwitchCase.
+ // FIXME: Determine the best cut-off.
+ if (SI->getNumCases() < 4)
+ return false;
+
+ // Figure out the corresponding result for each case value and phi node in the
+ // common destination, as well as the the min and max case values.
+ assert(SI->case_begin() != SI->case_end());
+ SwitchInst::CaseIt CI = SI->case_begin();
+ ConstantInt *MinCaseVal = CI.getCaseValue();
+ ConstantInt *MaxCaseVal = CI.getCaseValue();
+
+ BasicBlock *CommonDest = NULL;
+ typedef std::vector<std::pair<ConstantInt*, Constant*> > ResultListTy;
+ SmallDenseMap<PHINode*, ResultListTy> ResultLists;
+ SmallDenseMap<PHINode*, Constant*> DefaultResults;
+ SmallDenseMap<PHINode*, Type*> ResultTypes;
+ SmallVector<PHINode*, 4> PHIs;
+
+ for (SwitchInst::CaseIt E = SI->case_end(); CI != E; ++CI) {
+ ConstantInt *CaseVal = CI.getCaseValue();
+ if (CaseVal->getValue().slt(MinCaseVal->getValue()))
+ MinCaseVal = CaseVal;
+ if (CaseVal->getValue().sgt(MaxCaseVal->getValue()))
+ MaxCaseVal = CaseVal;
+
+ // Resulting value at phi nodes for this case value.
+ typedef SmallVector<std::pair<PHINode*, Constant*>, 4> ResultsTy;
+ ResultsTy Results;
+ if (!GetCaseResults(SI, CI.getCaseSuccessor(), &CommonDest, Results))
+ return false;
+
+ // Append the result from this case to the list for each phi.
+ for (ResultsTy::iterator I = Results.begin(), E = Results.end(); I!=E; ++I) {
+ if (!ResultLists.count(I->first))
+ PHIs.push_back(I->first);
+ ResultLists[I->first].push_back(std::make_pair(CaseVal, I->second));
+ }
+ }
+
+ // Get the resulting values for the default case.
+ {
+ SmallVector<std::pair<PHINode*, Constant*>, 4> DefaultResultsList;
+ if (!GetCaseResults(SI, SI->getDefaultDest(), &CommonDest, DefaultResultsList))
+ return false;
+ for (size_t I = 0, E = DefaultResultsList.size(); I != E; ++I) {
+ PHINode *PHI = DefaultResultsList[I].first;
+ Constant *Result = DefaultResultsList[I].second;
+ DefaultResults[PHI] = Result;
+ ResultTypes[PHI] = Result->getType();
+ }
+ }
+
+ APInt RangeSpread = MaxCaseVal->getValue() - MinCaseVal->getValue();
+ // The table density should be at lest 40%. This is the same criterion as for
+ // jump tables, see SelectionDAGBuilder::handleJTSwitchCase.
+ // FIXME: Find the best cut-off.
+ // Be careful to avoid overlow in the density computation.
+ if (RangeSpread.zextOrSelf(64).ugt(UINT64_MAX / 4 - 1))
+ return false;
+ uint64_t TableSize = RangeSpread.getLimitedValue() + 1;
+ if (SI->getNumCases() * 10 < TableSize * 4)
+ return false;
+
+ // Build the lookup tables.
+ SmallDenseMap<PHINode*, GlobalVariable*> LookupTables;
+ SmallDenseMap<PHINode*, Constant*> SingleResults;
+
+ Module &Mod = *CommonDest->getParent()->getParent();
+ for (SmallDenseMap<PHINode*, ResultListTy>::iterator I = ResultLists.begin(),
+ E = ResultLists.end(); I != E; ++I) {
+ PHINode *PHI = I->first;
+
+ Constant *SingleResult = NULL;
+ LookupTables[PHI] = BuildLookupTable(Mod, TableSize, MinCaseVal, I->second,
+ DefaultResults[PHI], &SingleResult);
+ SingleResults[PHI] = SingleResult;
+ }
+
+ // Create the BB that does the lookups.
+ BasicBlock *LookupBB = BasicBlock::Create(Mod.getContext(),
+ "switch.lookup",
+ CommonDest->getParent(),
+ CommonDest);
+
+ // Check whether the condition value is within the case range, and branch to
+ // the new BB.
+ Builder.SetInsertPoint(SI);
+ Value *TableIndex = Builder.CreateSub(SI->getCondition(), MinCaseVal,
+ "switch.tableidx");
+ Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get(
+ MinCaseVal->getType(), TableSize));
+ Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
+
+ // Populate the BB that does the lookups.
+ Builder.SetInsertPoint(LookupBB);
+ bool ReturnedEarly = false;
+ for (SmallVector<PHINode*, 4>::iterator I = PHIs.begin(), E = PHIs.end();
+ I != E; ++I) {
+ PHINode *PHI = *I;
+ // There was a single result for this phi; just use that.
+ if (Constant *SingleResult = SingleResults[PHI]) {
+ PHI->addIncoming(SingleResult, LookupBB);
+ continue;
+ }
+
+ Value *GEPIndices[] = { Builder.getInt32(0), TableIndex };
+ Value *GEP = Builder.CreateInBoundsGEP(LookupTables[PHI], GEPIndices,
+ "switch.gep");
+ Value *Result = Builder.CreateLoad(GEP, "switch.load");
+
+ // If the result is only going to be used to return from the function,
+ // we want to do that right here.
+ if (PHI->hasOneUse() && isa<ReturnInst>(*PHI->use_begin())) {
+ if (CommonDest->getFirstNonPHIOrDbg() == CommonDest->getTerminator()) {
+ Builder.CreateRet(Result);
+ ReturnedEarly = true;
+ }
+ }
+
+ if (!ReturnedEarly)
+ PHI->addIncoming(Result, LookupBB);
+ }
+
+ if (!ReturnedEarly)
+ Builder.CreateBr(CommonDest);
+
+ // Remove the switch.
+ for (unsigned i = 0; i < SI->getNumSuccessors(); ++i) {
+ BasicBlock *Succ = SI->getSuccessor(i);
+ if (Succ == SI->getDefaultDest()) continue;
+ Succ->removePredecessor(SI->getParent());
+ }
+ SI->eraseFromParent();
+
+ ++NumLookupTables;
+ return true;
+}
+
bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
// If this switch is too complex to want to look at, ignore it.
if (!isValueEqualityComparison(SI))
@@ -3016,6 +3299,9 @@
if (ForwardSwitchConditionToPHI(SI))
return SimplifyCFG(BB) | true;
+ if (SwitchToLookupTable(SI, Builder))
+ return SimplifyCFG(BB) | true;
+
return false;
}