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handle undef values much more carefully: generalize the resolveundefbranches

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code to handle instructions as well, so that we properly fold things like
X & undef -> 0.
This fixes Transforms/SCCP/2006-12-19-UndefBug.ll


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@32715 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2006-12-20 06:21:33 +00:00
parent d407f41413
commit 3bad2533bb
1 changed files with 172 additions and 32 deletions
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194
lib/Transforms/Scalar/SCCP.cpp
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@ -47,19 +47,29 @@ STATISTIC(IPNumDeadBlocks , "Number of basic blocks unreachable by IPSCCP");
STATISTIC(IPNumArgsElimed ,"Number of arguments constant propagated by IPSCCP");
STATISTIC(IPNumGlobalConst, "Number of globals found to be constant by IPSCCP");
// LatticeVal class - This class represents the different lattice values that an
// instruction may occupy. It is a simple class with value semantics.
//
namespace {
/// LatticeVal class - This class represents the different lattice values that
/// an LLVM value may occupy. It is a simple class with value semantics.
///
class LatticeVal {
enum {
undefined, // This instruction has no known value
constant, // This instruction has a constant value
overdefined // This instruction has an unknown value
/// undefined - This LLVM Value has no known value yet.
undefined,
/// constant - This LLVM Value has a specific constant value.
constant,
/// forcedconstant - This LLVM Value was thought to be undef until
/// ResolvedUndefsIn. This is treated just like 'constant', but if merged
/// with another (different) constant, it goes to overdefined, instead of
/// asserting.
forcedconstant,
/// overdefined - This instruction is not known to be constant, and we know
/// it has a value.
overdefined
} LatticeValue; // The current lattice position
Constant *ConstantVal; // If Constant value, the current value
public:
inline LatticeVal() : LatticeValue(undefined), ConstantVal(0) {}
@ -73,11 +83,23 @@ public:
return false;
}
// markConstant - Return true if this is a new status for us...
// markConstant - Return true if this is a new status for us.
inline bool markConstant(Constant *V) {
if (LatticeValue != constant) {
if (LatticeValue == undefined) {
LatticeValue = constant;
ConstantVal = V;
} else {
assert(LatticeValue == forcedconstant &&
"Cannot move from overdefined to constant!");
// Stay at forcedconstant if the constant is the same.
if (V == ConstantVal) return false;
// Otherwise, we go to overdefined. Assumptions made based on the
// forced value are possibly wrong. Assuming this is another constant
// could expose a contradiction.
LatticeValue = overdefined;
}
return true;
} else {
assert(ConstantVal == V && "Marking constant with different value");
@ -85,8 +107,16 @@ public:
return false;
}
inline void markForcedConstant(Constant *V) {
assert(LatticeValue == undefined && "Can't force a defined value!");
LatticeValue = forcedconstant;
ConstantVal = V;
}
inline bool isUndefined() const { return LatticeValue == undefined; }
inline bool isConstant() const { return LatticeValue == constant; }
inline bool isConstant() const {
return LatticeValue == constant || LatticeValue == forcedconstant;
}
inline bool isOverdefined() const { return LatticeValue == overdefined; }
inline Constant *getConstant() const {
@ -174,12 +204,12 @@ public:
///
void Solve();
/// ResolveBranchesIn - While solving the dataflow for a function, we assume
/// ResolvedUndefsIn - While solving the dataflow for a function, we assume
/// that branches on undef values cannot reach any of their successors.
/// However, this is not a safe assumption. After we solve dataflow, this
/// method should be use to handle this. If this returns true, the solver
/// should be rerun.
bool ResolveBranchesIn(Function &F);
bool ResolvedUndefsIn(Function &F);
/// getExecutableBlocks - Once we have solved for constants, return the set of
/// blocks that is known to be executable.
@ -217,6 +247,13 @@ private:
InstWorkList.push_back(V);
}
}
inline void markForcedConstant(LatticeVal &IV, Value *V, Constant *C) {
IV.markForcedConstant(C);
DOUT << "markForcedConstant: " << *C << ": " << *V;
InstWorkList.push_back(V);
}
inline void markConstant(Value *V, Constant *C) {
markConstant(ValueState[V], V, C);
}
@ -266,11 +303,11 @@ private:
hash_map<Value*, LatticeVal>::iterator I = ValueState.find(V);
if (I != ValueState.end()) return I->second; // Common case, in the map
if (Constant *CPV = dyn_cast<Constant>(V)) {
if (Constant *C = dyn_cast<Constant>(V)) {
if (isa<UndefValue>(V)) {
// Nothing to do, remain undefined.
} else {
ValueState[CPV].markConstant(CPV); // Constants are constant
ValueState[C].markConstant(C); // Constants are constant
}
}
// All others are underdefined by default...
@ -1044,7 +1081,7 @@ void SCCPSolver::Solve() {
}
}
/// ResolveBranchesIn - While solving the dataflow for a function, we assume
/// ResolvedUndefsIn - While solving the dataflow for a function, we assume
/// that branches on undef values cannot reach any of their successors.
/// However, this is not a safe assumption. After we solve dataflow, this
/// method should be use to handle this. If this returns true, the solver
@ -1054,14 +1091,117 @@ void SCCPSolver::Solve() {
/// of the edges from the block as being feasible, even though the condition
/// doesn't say it would otherwise be. This allows SCCP to find the rest of the
/// CFG and only slightly pessimizes the analysis results (by marking one,
/// potentially unfeasible, edge feasible). This cannot usefully modify the
/// potentially infeasible, edge feasible). This cannot usefully modify the
/// constraints on the condition of the branch, as that would impact other users
/// of the value.
bool SCCPSolver::ResolveBranchesIn(Function &F) {
///
/// This scan also checks for values that use undefs, whose results are actually
/// defined. For example, 'zext i8 undef to i32' should produce all zeros
/// conservatively, as "(zext i8 X -> i32) & 0xFF00" must always return zero,
/// even if X isn't defined.
bool SCCPSolver::ResolvedUndefsIn(Function &F) {
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
if (!BBExecutable.count(BB))
continue;
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
// Look for instructions which produce undef values.
if (I->getType() == Type::VoidTy) continue;
LatticeVal &LV = getValueState(I);
if (!LV.isUndefined()) continue;
// Get the lattice values of the first two operands for use below.
LatticeVal &Op0LV = getValueState(I->getOperand(0));
LatticeVal Op1LV;
if (I->getNumOperands() == 2) {
// If this is a two-operand instruction, and if both operands are
// undefs, the result stays undef.
Op1LV = getValueState(I->getOperand(1));
if (Op0LV.isUndefined() && Op1LV.isUndefined())
continue;
}
// If this is an instructions whose result is defined even if the input is
// not fully defined, propagate the information.
const Type *ITy = I->getType();
switch (I->getOpcode()) {
default: break; // Leave the instruction as an undef.
case Instruction::ZExt:
// After a zero extend, we know the top part is zero. SExt doesn't have
// to be handled here, because we don't know whether the top part is 1's
// or 0's.
assert(Op0LV.isUndefined());
markForcedConstant(LV, I, Constant::getNullValue(ITy));
return true;
case Instruction::Mul:
case Instruction::And:
// undef * X -> 0. X could be zero.
// undef & X -> 0. X could be zero.
markForcedConstant(LV, I, Constant::getNullValue(ITy));
return true;
case Instruction::Or:
// undef | X -> -1. X could be -1.
markForcedConstant(LV, I, ConstantInt::getAllOnesValue(ITy));
return true;
case Instruction::SDiv:
case Instruction::UDiv:
case Instruction::SRem:
case Instruction::URem:
// X / undef -> undef. No change.
// X % undef -> undef. No change.
if (Op1LV.isUndefined()) break;
// undef / X -> 0. X could be maxint.
// undef % X -> 0. X could be 1.
markForcedConstant(LV, I, Constant::getNullValue(ITy));
return true;
case Instruction::AShr:
// undef >>s X -> undef. No change.
if (Op0LV.isUndefined()) break;
// X >>s undef -> X. X could be 0, X could have the high-bit known set.
if (Op0LV.isConstant())
markForcedConstant(LV, I, Op0LV.getConstant());
else
markOverdefined(LV, I);
return true;
case Instruction::LShr:
case Instruction::Shl:
// undef >> X -> undef. No change.
// undef << X -> undef. No change.
if (Op0LV.isUndefined()) break;
// X >> undef -> 0. X could be 0.
// X << undef -> 0. X could be 0.
markForcedConstant(LV, I, Constant::getNullValue(ITy));
return true;
case Instruction::Select:
// undef ? X : Y -> X or Y. There could be commonality between X/Y.
if (Op0LV.isUndefined()) {
if (!Op1LV.isConstant()) // Pick the constant one if there is any.
Op1LV = getValueState(I->getOperand(2));
} else if (Op1LV.isUndefined()) {
// c ? undef : undef -> undef. No change.
Op1LV = getValueState(I->getOperand(2));
if (Op1LV.isUndefined())
break;
// Otherwise, c ? undef : x -> x.
} else {
// Leave Op1LV as Operand(1)'s LatticeValue.
}
if (Op1LV.isConstant())
markForcedConstant(LV, I, Op1LV.getConstant());
else
markOverdefined(LV, I);
return true;
}
}
TerminatorInst *TI = BB->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
if (!BI->isConditional()) continue;
@ -1133,11 +1273,11 @@ bool SCCP::runOnFunction(Function &F) {
Values[AI].markOverdefined();
// Solve for constants.
bool ResolvedBranches = true;
while (ResolvedBranches) {
bool ResolvedUndefs = true;
while (ResolvedUndefs) {
Solver.Solve();
DOUT << "RESOLVING UNDEF BRANCHES\n";
ResolvedBranches = Solver.ResolveBranchesIn(F);
DOUT << "RESOLVING UNDEFs\n";
ResolvedUndefs = Solver.ResolvedUndefsIn(F);
}
bool MadeChanges = false;
@ -1270,14 +1410,14 @@ bool IPSCCP::runOnModule(Module &M) {
Solver.TrackValueOfGlobalVariable(G);
// Solve for constants.
bool ResolvedBranches = true;
while (ResolvedBranches) {
bool ResolvedUndefs = true;
while (ResolvedUndefs) {
Solver.Solve();
DOUT << "RESOLVING UNDEF BRANCHES\n";
ResolvedBranches = false;
DOUT << "RESOLVING UNDEFS\n";
ResolvedUndefs = false;
for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F)
ResolvedBranches |= Solver.ResolveBranchesIn(*F);
ResolvedUndefs |= Solver.ResolvedUndefsIn(*F);
}
bool MadeChanges = false;