bool fixSSA(Procedure& proc)
{
PhaseScope phaseScope(proc, "fixSSA");
// Collect the stack "variables". If there aren't any, then we don't have anything to do.
// That's a fairly common case.
HashMap<StackSlotValue*, Type> stackVariable;
for (Value* value : proc.values()) {
if (StackSlotValue* stack = value->as<StackSlotValue>()) {
if (stack->kind() == StackSlotKind::Anonymous)
stackVariable.add(stack, Void);
}
}
if (stackVariable.isEmpty())
return false;
// Make sure that we know how to optimize all of these. We only know how to handle Load and
// Store on anonymous variables.
for (Value* value : proc.values()) {
auto reject = [&] (Value* value) {
if (StackSlotValue* stack = value->as<StackSlotValue>())
stackVariable.remove(stack);
};
auto handleAccess = [&] (Value* access, Type type) {
StackSlotValue* stack = access->lastChild()->as<StackSlotValue>();
if (!stack)
return;
if (value->as<MemoryValue>()->offset()) {
stackVariable.remove(stack);
return;
}
auto result = stackVariable.find(stack);
if (result == stackVariable.end())
return;
if (result->value == Void) {
result->value = type;
return;
}
if (result->value == type)
return;
stackVariable.remove(result);
};
switch (value->opcode()) {
case Load:
// We're OK with loads from stack variables at an offset of zero.
handleAccess(value, value->type());
break;
case Store:
// We're OK with stores to stack variables, but not storing stack variables.
reject(value->child(0));
handleAccess(value, value->child(0)->type());
break;
default:
for (Value* child : value->children())
reject(child);
break;
}
}
Vector<StackSlotValue*> deadValues;
for (auto& entry : stackVariable) {
if (entry.value == Void)
deadValues.append(entry.key);
}
for (StackSlotValue* deadValue : deadValues) {
deadValue->replaceWithNop();
stackVariable.remove(deadValue);
}
if (stackVariable.isEmpty())
return false;
// We know that we have variables to optimize, so do that now.
breakCriticalEdges(proc);
SSACalculator ssa(proc);
// Create a SSACalculator::Variable for every stack variable.
Vector<StackSlotValue*> variableToStack;
HashMap<StackSlotValue*, SSACalculator::Variable*> stackToVariable;
for (auto& entry : stackVariable) {
StackSlotValue* stack = entry.key;
SSACalculator::Variable* variable = ssa.newVariable();
RELEASE_ASSERT(variable->index() == variableToStack.size());
variableToStack.append(stack);
stackToVariable.add(stack, variable);
}
// Create Defs for all of the stores to the stack variable.
for (BasicBlock* block : proc) {
for (Value* value : *block) {
if (value->opcode() != Store)
continue;
StackSlotValue* stack = value->child(1)->as<StackSlotValue>();
if (!stack)
continue;
if (SSACalculator::Variable* variable = stackToVariable.get(stack))
ssa.newDef(variable, block, value->child(0));
}
}
// Decide where Phis are to be inserted. This creates them but does not insert them.
ssa.computePhis(
[&] (SSACalculator::Variable* variable, BasicBlock* block) -> Value* {
StackSlotValue* stack = variableToStack[variable->index()];
Value* phi = proc.add<Value>(Phi, stackVariable.get(stack), stack->origin());
if (verbose) {
dataLog(
"Adding Phi for ", pointerDump(stack), " at ", *block, ": ",
deepDump(proc, phi), "\n");
}
return phi;
});
// Now perform the conversion.
InsertionSet insertionSet(proc);
HashMap<StackSlotValue*, Value*> mapping;
for (BasicBlock* block : proc.blocksInPreOrder()) {
mapping.clear();
for (auto& entry : stackToVariable) {
StackSlotValue* stack = entry.key;
SSACalculator::Variable* variable = entry.value;
SSACalculator::Def* def = ssa.reachingDefAtHead(block, variable);
if (def)
mapping.set(stack, def->value());
}
for (SSACalculator::Def* phiDef : ssa.phisForBlock(block)) {
StackSlotValue* stack = variableToStack[phiDef->variable()->index()];
insertionSet.insertValue(0, phiDef->value());
mapping.set(stack, phiDef->value());
}
for (unsigned valueIndex = 0; valueIndex < block->size(); ++valueIndex) {
Value* value = block->at(valueIndex);
value->performSubstitution();
switch (value->opcode()) {
case Load: {
if (StackSlotValue* stack = value->child(0)->as<StackSlotValue>()) {
if (Value* replacement = mapping.get(stack))
value->replaceWithIdentity(replacement);
}
break;
}
case Store: {
if (StackSlotValue* stack = value->child(1)->as<StackSlotValue>()) {
if (stackToVariable.contains(stack)) {
mapping.set(stack, value->child(0));
value->replaceWithNop();
}
}
break;
}
default:
break;
}
}
unsigned upsilonInsertionPoint = block->size() - 1;
Origin upsilonOrigin = block->last()->origin();
for (BasicBlock* successorBlock : block->successorBlocks()) {
for (SSACalculator::Def* phiDef : ssa.phisForBlock(successorBlock)) {
Value* phi = phiDef->value();
SSACalculator::Variable* variable = phiDef->variable();
StackSlotValue* stack = variableToStack[variable->index()];
Value* mappedValue = mapping.get(stack);
if (verbose) {
dataLog(
"Mapped value for ", *stack, " with successor Phi ", *phi, " at end of ",
*block, ": ", pointerDump(mappedValue), "\n");
}
if (!mappedValue)
mappedValue = insertionSet.insertBottom(upsilonInsertionPoint, phi);
insertionSet.insert<UpsilonValue>(
upsilonInsertionPoint, upsilonOrigin, mappedValue, phi);
}
}
insertionSet.execute(block);
}
// Finally, kill the stack slots.
for (StackSlotValue* stack : variableToStack)
stack->replaceWithNop();
if (verbose) {
dataLog("B3 after SSA conversion:\n");
dataLog(proc);
}
return true;
}
