+/// processSwitch - Simplify a switch instruction by removing cases which can
+/// never fire. If the uselessness of a case could be determined locally then
+/// constant propagation would already have figured it out. Instead, walk the
+/// predecessors and statically evaluate cases based on information available
+/// on that edge. Cases that cannot fire no matter what the incoming edge can
+/// safely be removed. If a case fires on every incoming edge then the entire
+/// switch can be removed and replaced with a branch to the case destination.
+bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) {
+ Value *Cond = SI->getCondition();
+ BasicBlock *BB = SI->getParent();
+
+ // If the condition was defined in same block as the switch then LazyValueInfo
+ // currently won't say anything useful about it, though in theory it could.
+ if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB)
+ return false;
+
+ // If the switch is unreachable then trying to improve it is a waste of time.
+ pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
+ if (PB == PE) return false;
+
+ // Analyse each switch case in turn. This is done in reverse order so that
+ // removing a case doesn't cause trouble for the iteration.
+ bool Changed = false;
+ for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE;
+ ) {
+ ConstantInt *Case = CI.getCaseValue();
+
+ // Check to see if the switch condition is equal to/not equal to the case
+ // value on every incoming edge, equal/not equal being the same each time.
+ LazyValueInfo::Tristate State = LazyValueInfo::Unknown;
+ for (pred_iterator PI = PB; PI != PE; ++PI) {
+ // Is the switch condition equal to the case value?
+ LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ,
+ Cond, Case, *PI,
+ BB, SI);
+ // Give up on this case if nothing is known.
+ if (Value == LazyValueInfo::Unknown) {
+ State = LazyValueInfo::Unknown;
+ break;
+ }
+
+ // If this was the first edge to be visited, record that all other edges
+ // need to give the same result.
+ if (PI == PB) {
+ State = Value;
+ continue;
+ }
+
+ // If this case is known to fire for some edges and known not to fire for
+ // others then there is nothing we can do - give up.
+ if (Value != State) {
+ State = LazyValueInfo::Unknown;
+ break;
+ }
+ }
+
+ if (State == LazyValueInfo::False) {
+ // This case never fires - remove it.
+ CI.getCaseSuccessor()->removePredecessor(BB);
+ SI->removeCase(CI); // Does not invalidate the iterator.
+
+ // The condition can be modified by removePredecessor's PHI simplification
+ // logic.
+ Cond = SI->getCondition();
+
+ ++NumDeadCases;
+ Changed = true;
+ } else if (State == LazyValueInfo::True) {
+ // This case always fires. Arrange for the switch to be turned into an
+ // unconditional branch by replacing the switch condition with the case
+ // value.
+ SI->setCondition(Case);
+ NumDeadCases += SI->getNumCases();
+ Changed = true;
+ break;
+ }
+ }
+
+ if (Changed)
+ // If the switch has been simplified to the point where it can be replaced
+ // by a branch then do so now.
+ ConstantFoldTerminator(BB);
+
+ return Changed;
+}
+