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