libswift: implement SILCombine's global_value optimization as instruction pass in libswift.

But keeping the old visit function as legacy

Author: eeckstein

include/swift/SILOptimizer/PassManager/Passes.def

@@ -421,6 +421,8 @@ PASS(PruneVTables, "prune-vtables",
      "Mark class methods that do not require vtable dispatch")
 PASS_RANGE(AllPasses, AADumper, PruneVTables)
 
+SWIFT_INSTRUCTION_PASS_WITH_LEGACY(GlobalValueInst, "simplify-global_value")
+
 #undef IRGEN_PASS
 #undef SWIFT_FUNCTION_PASS
 #undef SWIFT_FUNCTION_PASS_WITH_LEGACY

lib/SILOptimizer/SILCombiner/SILCombiner.h

@@ -326,11 +326,12 @@ class SILCombiner :
 
   SILInstruction *visitMarkDependenceInst(MarkDependenceInst *MDI);
   SILInstruction *visitClassifyBridgeObjectInst(ClassifyBridgeObjectInst *CBOI);
-  SILInstruction *visitGlobalValueInst(GlobalValueInst *globalValue);
   SILInstruction *visitConvertFunctionInst(ConvertFunctionInst *CFI);
   SILInstruction *
   visitConvertEscapeToNoEscapeInst(ConvertEscapeToNoEscapeInst *Cvt);
 
+  SILInstruction *legacyVisitGlobalValueInst(GlobalValueInst *globalValue);
+
 #define PASS(ID, TAG, DESCRIPTION)
 #define SWIFT_FUNCTION_PASS(ID, TAG, DESCRIPTION)
 #define SWIFT_INSTRUCTION_PASS(INST, TAG) \

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -2312,7 +2312,7 @@ static bool checkGlobalValueUsers(SILValue val,
 }
 
 SILInstruction *
-SILCombiner::visitGlobalValueInst(GlobalValueInst *globalValue) {
+SILCombiner::legacyVisitGlobalValueInst(GlobalValueInst *globalValue) {
   // Delete all reference count instructions on a global_value if the only other
   // users are projections (ref_element_addr and ref_tail_addr).
   SmallVector<SILInstruction *, 8> toDelete;

libswift/Sources/Optimizer/CMakeLists.txt

@@ -9,5 +9,6 @@
 add_libswift_module(Optimizer
     DEPENDS SIL)
 
+add_subdirectory(InstructionPasses)
 add_subdirectory(PassManager)
 add_subdirectory(FunctionPasses)

libswift/Sources/Optimizer/InstructionPasses/CMakeLists.txt

@@ -0,0 +1,10 @@
+# This source file is part of the Swift.org open source project
+#
+# Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors
+# Licensed under Apache License v2.0 with Runtime Library Exception
+#
+# See http://swift.org/LICENSE.txt for license information
+# See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+
+libswift_sources(Optimizer
+  SimplifyGlobalValue.swift)

libswift/Sources/Optimizer/InstructionPasses/SimplifyGlobalValue.swift

@@ -0,0 +1,52 @@
+//===--- SimplifyGlobalValue.swift - Simplify global_value instruction ----===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2021 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
+
+let simplifyGlobalValuePass = InstructionPass<GlobalValueInst>(
+  name: "simplify-global_value", {
+  (globalValue: GlobalValueInst, context: InstructionPassContext) in
+
+  var users = StackList<Instruction>(context)
+  if checkUsers(of: globalValue, users: &users) {
+    while let inst = users.pop() {
+      context.erase(instruction: inst)
+    }
+  } else {
+    users.removeAll()
+  }
+})
+  
+/// Returns true if reference counting and debug_value users of a global_value
+/// can be deleted.
+private func checkUsers(of val: Value, users: inout StackList<Instruction>) -> Bool {
+  for use in val.uses {
+    let user = use.instruction
+    if user is RefCountingInst || user is DebugValueInst {
+      users.push(user)
+      continue
+    }
+    if let upCast = user as? UpcastInst {
+      if !checkUsers(of: upCast, users: &users) {
+        return false
+      }
+      continue
+    }
+    // Projection instructions don't access the object header, so they don't
+    // prevent deleting reference counting instructions.
+    if user is RefElementAddrInst || user is RefTailAddrInst {
+      continue
+    }
+    return false
+  }
+  return true
+}

libswift/Sources/Optimizer/PassManager/PassRegistration.swift

@@ -38,4 +38,5 @@ private func registerPass<InstType: Instruction>(
 private func registerSwiftPasses() {
   registerPass(silPrinterPass, { silPrinterPass.run($0) })
   registerPass(mergeCondFailsPass, { mergeCondFailsPass.run($0) })
+  registerPass(simplifyGlobalValuePass, { simplifyGlobalValuePass.run($0) })
 }