Swift Optimizer: add the module pass ReadOnlyGlobalVariables

It marks global `var` variables as `let` if they are never written.

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/ModulePasses/CMakeLists.txt

@@ -7,5 +7,6 @@
 # See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 
 swift_compiler_sources(Optimizer
+  ReadOnlyGlobalVariables.swift
   StackProtection.swift
 )

SwiftCompilerSources/Sources/Optimizer/ModulePasses/ReadOnlyGlobalVariables.swift

@@ -0,0 +1,75 @@
+//===--- ReadOnlyGlobalVariables.swift ------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SIL
+
+/// Marks global `var` variables as `let` if they are never written.
+///
+/// Note that this pass relies on the initialize-static-globals pass which converts lazily
+/// initialized globals to statically initialized globals.
+/// This pass does not mark lazily initialized globals as `let`, because such globals _are_
+/// written: in their initializers.
+///
+let readOnlyGlobalVariablesPass = ModulePass(name: "read-only-global-variables") {
+  (moduleContext: ModulePassContext) in
+
+  var writtenGlobals = Set<GlobalVariable>()
+
+  for f in moduleContext.functions {
+    for inst in f.instructions {
+      if let gAddr = inst as? GlobalAddrInst {
+        if gAddr.addressHasWrites {
+          writtenGlobals.insert(gAddr.global)
+        }
+      }
+    }
+  }
+
+  for g in moduleContext.globalVariables {
+    if !g.isPossiblyUsedExternally,
+       !g.isLet,
+       !writtenGlobals.contains(g) {
+      g.setIsLet(to: true, moduleContext)
+    }
+  }
+}
+
+private extension Value {
+  var addressHasWrites: Bool {
+    var walker = FindWrites()
+    return walker.walkDownUses(ofAddress: self, path: UnusedWalkingPath()) == .abortWalk
+  }
+}
+
+private struct FindWrites : AddressDefUseWalker {
+  mutating func leafUse(address: Operand, path: UnusedWalkingPath) -> WalkResult {
+    switch address.instruction {
+    case is LoadInst, is LoadBorrowInst:
+      return .continueWalk
+
+    case let ca as CopyAddrInst:
+      return address == ca.sourceOperand ? .continueWalk : .abortWalk
+
+    case let apply as FullApplySite:
+      if let callerArgIdx = apply.argumentIndex(of: address) {
+        let calleeArgIdx = apply.calleeArgIndex(callerArgIndex: callerArgIdx)
+        let convention = apply.getArgumentConvention(calleeArgIndex: calleeArgIdx)
+        if convention.isIndirectIn {
+          return .continueWalk
+        }
+      }
+      return .abortWalk
+    default:
+      return .abortWalk
+    }
+  }
+}

SwiftCompilerSources/Sources/Optimizer/PassManager/PassRegistration.swift

@@ -61,6 +61,7 @@ private func registerForSILCombine<InstType: SILCombineSimplifyable>(
 
 private func registerSwiftPasses() {
   // Module passes
+  registerPass(readOnlyGlobalVariablesPass, { readOnlyGlobalVariablesPass.run($0) })
   registerPass(stackProtection, { stackProtection.run($0) })
 
   // Function passes

include/swift/SILOptimizer/PassManager/Passes.def

@@ -384,6 +384,8 @@ SWIFT_FUNCTION_PASS(SILPrinter, "sil-printer",
      "Test pass which prints the SIL of a function")
 SWIFT_MODULE_PASS(FunctionUsesDumper, "dump-function-uses",
      "Dump the results of FunctionUses")
+SWIFT_MODULE_PASS(ReadOnlyGlobalVariablesPass, "read-only-global-variables",
+     "Converts read-only var-globals to let-globals")
 SWIFT_MODULE_PASS(StackProtection, "stack-protection",
      "Decides which functions need stack protectors")
 SWIFT_FUNCTION_PASS(FunctionStackProtection, "function-stack-protection",

test/SILOptimizer/read_only_global_vars.sil

@@ -0,0 +1,45 @@
+// RUN: %target-sil-opt -enable-sil-verify-all %s -read-only-global-variables | %FileCheck %s
+
+// REQUIRES: swift_in_compiler
+
+import Builtin
+import Swift
+
+// CHECK-LABEL: sil_global private [let] @read_only_var : $Int32 = {
+sil_global private @read_only_var : $Int32 = {
+  %0 = integer_literal $Builtin.Int32, 27
+  %initval = struct $Int32 (%0 : $Builtin.Int32)
+}
+
+// CHECK-LABEL: sil_global private @written_var : $Int32 = {
+sil_global private @written_var : $Int32 = {
+  %0 = integer_literal $Builtin.Int32, 27
+  %initval = struct $Int32 (%0 : $Builtin.Int32)
+}
+
+// CHECK-LABEL: sil_global @public_var : $Int32 = {
+sil_global @public_var : $Int32 = {
+  %0 = integer_literal $Builtin.Int32, 27
+  %initval = struct $Int32 (%0 : $Builtin.Int32)
+}
+
+sil @unknown_read_func : $@convention(thin) (@in Int32) -> ()
+
+sil @read_var : $@convention(thin) (@inout Int32) -> Int32 {
+bb0(%0 : $*Int32):
+  %1 = global_addr @read_only_var : $*Int32
+  %2 = load %1 : $*Int32
+  copy_addr %1 to %0 : $*Int32
+  %4 = function_ref @unknown_read_func : $@convention(thin) (@in Int32) -> ()
+  %5 = apply %4(%1) : $@convention(thin) (@in Int32) -> ()
+  return %2 : $Int32
+}
+
+sil @write_var : $@convention(thin) (Int32) -> () {
+bb0(%0 : $Int32):
+  %1 = global_addr @written_var : $*Int32
+  store %0 to %1 : $*Int32
+  %r = tuple ()
+  return %r : $()
+}
+