DeadStoreElimination: don't require values stored back to memory if the control flow ends in an unreachable

This makes dead store elimination less conservative, e.g. if error checks are in the control flow.

Author: eeckstein

lib/SILOptimizer/Transforms/DeadStoreElimination.cpp

@@ -717,6 +717,10 @@ void DSEContext::mergeSuccessorLiveIns(SILBasicBlock *BB) {
   // dead for block with no successor.
   BlockState *C = getBlockState(BB);
   if (BB->succ_empty()) {
+    if (isa<UnreachableInst>(BB->getTerminator())) {
+      C->BBWriteSetOut.set();
+      return;
+    }
     C->BBWriteSetOut |= C->BBDeallocateLocation;
     return;
   }

test/SILOptimizer/dead_store_elim.sil

@@ -291,6 +291,29 @@ bb3:
   return %9999 : $()
 }
 
+// CHECK-LABEL: sil @handle_unreachable : $@convention(thin) (@inout Builtin.Int32) -> () {
+// CHECK: bb0
+// CHECK-NEXT: integer_literal
+// CHECK-NEXT: cond_br
+// CHECK: return
+sil @handle_unreachable : $@convention(thin) (@inout Builtin.Int32) -> () {
+bb0(%0 : $*Builtin.Int32):
+  %1 = integer_literal $Builtin.Int32, 0
+  store %1 to %0 : $*Builtin.Int32
+  cond_br undef, bb1, bb2
+
+bb1:
+  unreachable
+
+bb2:
+  br bb3
+
+bb3:
+  store %1 to %0 : $*Builtin.Int32
+  %9999 = tuple()
+  return %9999 : $()
+}
+
 // CHECK-LABEL: sil @post_dominating_dead_store_partial : $@convention(thin) (@inout Builtin.Int32) -> () {
 // CHECK: bb0(
 // CHECK-NOT: {{ store}}