ObjectOutliner: try outline alloc_refs in multiple iterations to handle multi-dimensional arrays

This allows to completely statically allocate multi-dimensional global arrays, like
```
var x = [[1, 2], [3, 4], [5, 6]]
```

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/ObjectOutliner.swift

@@ -59,16 +59,25 @@ let objectOutliner = FunctionPass(name: "object-outliner") {
     return
   }
 
-  for inst in function.instructions {
-    if let ari = inst as? AllocRefInstBase {
-      if !context.continueWithNextSubpassRun(for: inst) {
+  var allocRefs = Stack<AllocRefInstBase>(context)
+  defer { allocRefs.deinitialize() }
+
+  allocRefs.append(contentsOf: function.instructions.lazy.compactMap { $0 as? AllocRefInstBase })
+
+  // Try multiple iterations to handle multi-dimensional arrays.
+  var changed: Bool
+  repeat {
+    changed = false
+    for ari in allocRefs where !ari.isDeleted {
+      if !context.continueWithNextSubpassRun(for: ari) {
         return
       }
       if let globalValue = optimizeObjectAllocation(allocRef: ari, context) {
         optimizeFindStringCall(stringArray: globalValue, context)
+        changed = true
       }
     }
-  }
+  } while changed
 }
 
 private func optimizeObjectAllocation(allocRef: AllocRefInstBase, _ context: FunctionPassContext) -> GlobalValueInst? {

test/SILOptimizer/readonly_arrays.swift

@@ -17,7 +17,7 @@
 // CHECK-DAG: @"$s4test3StrV9staticLet_WZTv_r" = {{.*}} constant {{.*}} @"$ss20__StaticArrayStorageCN", {{.*}} @_swiftImmortalRefCount
 // CHECK-DAG: @"$s4test3StrV9staticVar_WZTv_r" = {{.*}} constant {{.*}} @"$ss20__StaticArrayStorageCN", {{.*}} @_swiftImmortalRefCount
 // CHECK-DAG: @"$s4test3StrV9staticVarSaySiGvpZ" = global %TSa <{ %Ts12_ArrayBufferV <{ %Ts14_BridgeStorageV <{ ptr @"$s4test3StrV9staticVar_WZTv_r" }> }> }>
-// CHECK-NOT: swift_initStaticObject
+// CHECK-DAG: @"$s4test3StrV14twoDimensionalSaySaySiGGvpZ" = global %TSa <{ %Ts12_ArrayBufferV <{ %Ts14_BridgeStorageV <{ ptr @"$s4test3StrV14twoDimensional_WZTv{{[0-9]*}}_r" }> }> }>, align 8
 
 // UNSUPPORTED: use_os_stdlib
 
@@ -28,6 +28,7 @@
 public struct Str {
   public static let staticLet = [ 200, 201, 202 ]
   public static var staticVar = [ 300, 301, 302 ]
+  public static var twoDimensional = [[1, 2], [3, 4], [5, 6]]
 }
 
 @inline(never)
@@ -72,6 +73,9 @@ print(Str.staticLet)
 // CHECK-OUTPUT:      [300, 301, 302]
 print(Str.staticVar)
 
+// CHECK-OUTPUT{LITERAL}: [[1, 2], [3, 4], [5, 6]]
+print(Str.twoDimensional)
+
 // CHECK-OUTPUT-NEXT: 11
 print(arrayLookup(1))