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SimplifyCFG: Use ComputeMaskedBits to prune dead cases from switch instructions.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131345 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer
2011-05-14 15:57:25 +00:00
parent 67f071e697
commit 10fcfb53fd
2 changed files with 74 additions and 1 deletions
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37
lib/Transforms/Utils/SimplifyCFG.cpp
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@@ -20,6 +20,7 @@
#include "llvm/DerivedTypes.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/ADT/DenseMap.h"
@@ -2383,6 +2384,36 @@ static bool TurnSwitchRangeIntoICmp(SwitchInst *SI) {
return true;
}
/// EliminateDeadSwitchCases - Compute masked bits for the condition of a switch
/// and use it to remove dead cases.
static bool EliminateDeadSwitchCases(SwitchInst *SI) {
Value *Cond = SI->getCondition();
unsigned Bits = cast<IntegerType>(Cond->getType())->getBitWidth();
APInt KnownZero(Bits, 0), KnownOne(Bits, 0);
ComputeMaskedBits(Cond, APInt::getAllOnesValue(Bits), KnownZero, KnownOne);
// Gather dead cases.
SmallVector<ConstantInt*, 8> DeadCases;
for (unsigned I = 1, E = SI->getNumCases(); I != E; ++I) {
if ((SI->getCaseValue(I)->getValue() & KnownZero) != 0 ||
(SI->getCaseValue(I)->getValue() & KnownOne) != KnownOne) {
DeadCases.push_back(SI->getCaseValue(I));
DEBUG(dbgs() << "SimplifyCFG: switch case '"
<< SI->getCaseValue(I)->getValue() << "' is dead.\n");
}
}
// Remove dead cases from the switch.
for (unsigned I = 0, E = DeadCases.size(); I != E; ++I) {
unsigned Case = SI->findCaseValue(DeadCases[I]);
// Prune unused values from PHI nodes.
SI->getSuccessor(Case)->removePredecessor(SI->getParent());
SI->removeCase(Case);
}
return !DeadCases.empty();
}
bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI) {
// If this switch is too complex to want to look at, ignore it.
if (!isValueEqualityComparison(SI))
@@ -2414,7 +2445,11 @@ bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI) {
// Try to transform the switch into an icmp and a branch.
if (TurnSwitchRangeIntoICmp(SI))
return SimplifyCFG(BB) | true;
// Remove unreachable cases.
if (EliminateDeadSwitchCases(SI))
return SimplifyCFG(BB) | true;
return false;
}