InitializeStaticGlobals: fix a SIL verifier crash

When merging stores in a global initializer, it's possible that the merged store is inserted at the wrong location, causing a SIL verifier error.
This is hard to reproduce, but can happen.
The merged store must be inserted _after_ all other stores. Instead it's inserted after the store of the last property. Now, if properties are _not_ initialized in the order they are declared, this problem can show up.

rdar://117189962

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/InitializeStaticGlobals.swift

@@ -156,8 +156,8 @@ private extension Function {
 private func mergeStores(in function: Function, _ context: FunctionPassContext) {
   for inst in function.instructions {
     if let store = inst as? StoreInst {
-      if let elementStores = getSequenceOfElementStores(firstStore: store) {
-        merge(elementStores: elementStores, context)
+      if let (elementStores, lastStore) = getSequenceOfElementStores(firstStore: store) {
+        merge(elementStores: elementStores, lastStore: lastStore, context)
       }
     }
   }
@@ -171,7 +171,7 @@ private func mergeStores(in function: Function, _ context: FunctionPassContext)
 ///   %addr_n = struct_element_addr %structAddr, #field_n
 ///   store %element_n to %addr_n
 ///
-private func getSequenceOfElementStores(firstStore: StoreInst) -> [StoreInst]? {
+private func getSequenceOfElementStores(firstStore: StoreInst) -> ([StoreInst], lastStore: StoreInst)? {
   guard let elementAddr = firstStore.destination as? StructElementAddrInst else {
     return nil
   }
@@ -195,7 +195,7 @@ private func getSequenceOfElementStores(firstStore: StoreInst) -> [StoreInst]? {
       numStoresFound += 1
       if numStoresFound == numElements {
         // If we saw  `numElements` distinct stores, it implies that all elements in `elementStores` are not nil.
-        return elementStores.map { $0! }
+        return (elementStores.map { $0! }, lastStore: store)
       }
     default:
       if inst.mayReadOrWriteMemory {
@@ -206,8 +206,7 @@ private func getSequenceOfElementStores(firstStore: StoreInst) -> [StoreInst]? {
   return nil
 }
 
-private func merge(elementStores: [StoreInst], _ context: FunctionPassContext) {
-  let lastStore = elementStores.last!
+private func merge(elementStores: [StoreInst], lastStore: StoreInst, _ context: FunctionPassContext) {
   let builder = Builder(after: lastStore, context)
 
   let structAddr = (lastStore.destination as! StructElementAddrInst).struct

test/SILOptimizer/init_static_globals.sil

@@ -98,6 +98,10 @@ sil_global [let] @g7 : $TwoFields
 // CHECK-NEXT:  }
 sil_global [let] @g8 : $UnsafePointer<Int32>
 
+sil_global [let] @g9 : $TwoFields
+
+sil @unknownfunc : $@convention(thin) () -> ()
+
 // CHECK-LABEL: sil [global_init_once_fn] [ossa] @globalinit_trivialglobal_func :
 // CHECK-NOT:     alloc_global
 // CHECK-NOT:     store
@@ -233,6 +237,34 @@ bb0:
   return %10 : $()
 }
 
+// CHECK-LABEL: sil [global_init_once_fn] [ossa] @merge_stores_but_no_global_init :
+// CHECK:         [[GA:%.*]] = global_addr @g9 : $*TwoFields
+// CHECK:         [[L10:%.*]] = integer_literal $Builtin.Int32, 10
+// CHECK:         [[I10:%.*]] = struct $Int32 ([[L10]] : $Builtin.Int32)
+// CHECK:         [[L11:%.*]] = integer_literal $Builtin.Int32, 11
+// CHECK:         [[I11:%.*]] = struct $Int32 ([[L11]] : $Builtin.Int32)
+// CHECK:         [[TF:%.*]] = struct $TwoFields ([[I11]] : $Int32, [[I10]] : $Int32)
+// CHECK:         store [[TF]] to [trivial] [[GA]]
+// CHECK:         function_ref
+// CHECK:       } // end sil function 'merge_stores_but_no_global_init'
+sil [global_init_once_fn] [ossa] @merge_stores_but_no_global_init : $@convention(c) () -> () {
+bb0:
+  alloc_global @g9
+  %1 = global_addr @g9 : $*TwoFields
+  %2 = integer_literal $Builtin.Int32, 10
+  %3 = struct $Int32 (%2 : $Builtin.Int32)
+  %4 = struct_element_addr %1 : $*TwoFields, #TwoFields.b
+  store %3 to [trivial] %4 : $*Int32
+  %6 = integer_literal $Builtin.Int32, 11
+  %7 = struct $Int32 (%6 : $Builtin.Int32)
+  %8 = struct_element_addr %1 : $*TwoFields, #TwoFields.a
+  store %7 to [trivial] %8 : $*Int32
+  %f = function_ref @unknownfunc : $@convention(thin) () -> ()
+  %a = apply %f() : $@convention(thin) () -> ()
+  %10 = tuple ()
+  return %10 : $()
+}
+
 // CHECK-LABEL: sil [global_init_once_fn] [ossa] @globalinit_wrong_separate_stores :
 // CHECK:         alloc_global
 // CHECK:         store