DeadFunctionElimination: remove externally available witness tables at the end of the pipeline

include/swift/SILOptimizer/PassManager/Passes.def

@@ -200,6 +200,8 @@ PASS(LICM, "licm",
      "Loop Invariant Code Motion")
 PASS(LateCodeMotion, "late-codemotion",
      "Late Code Motion with Release Hoisting")
+PASS(LateDeadFunctionElimination, "late-deadfuncelim",
+     "Late Dead Function Elimination")
 PASS(LateInliner, "late-inline",
      "Late Function Inlining")
 PASS(LoopCanonicalizer, "loop-canonicalizer",

lib/SILOptimizer/IPO/DeadFunctionElimination.cpp

@@ -90,6 +90,8 @@ class FunctionLivenessComputation {
 
   llvm::SmallPtrSet<void *, 32> AliveFunctionsAndTables;
 
+  bool keepExternalWitnessTablesAlive;
+
   /// Checks is a function is alive, e.g. because it is visible externally.
   bool isAnchorFunction(SILFunction *F) {
 
@@ -148,6 +150,11 @@ class FunctionLivenessComputation {
   /// Marks all contained functions and witness tables of a witness table as
   /// alive.
   void makeAlive(SILWitnessTable *WT) {
+    if (isAvailableExternally(WT->getLinkage()) &&
+        !keepExternalWitnessTablesAlive) {
+      return;
+    }
+
     LLVM_DEBUG(llvm::dbgs() << "    scan witness table " << WT->getName()
                             << '\n');
 
@@ -393,8 +400,10 @@ class FunctionLivenessComputation {
   }
 
 public:
-  FunctionLivenessComputation(SILModule *module) :
-    Module(module) {}
+  FunctionLivenessComputation(SILModule *module,
+                              bool keepExternalWitnessTablesAlive) :
+    Module(module),
+    keepExternalWitnessTablesAlive(keepExternalWitnessTablesAlive) {}
 
   /// The main entry point of the optimization.
   bool findAliveFunctions() {
@@ -635,8 +644,8 @@ class DeadFunctionElimination : FunctionLivenessComputation {
   }
 
 public:
-  DeadFunctionElimination(SILModule *module)
-      : FunctionLivenessComputation(module) {}
+  DeadFunctionElimination(SILModule *module, bool keepExternalWitnessTablesAlive)
+      : FunctionLivenessComputation(module, keepExternalWitnessTablesAlive) {}
 
   /// The main entry point of the optimization.
   void eliminateFunctions(SILModuleTransform *DFEPass) {
@@ -693,6 +702,13 @@ class DeadFunctionElimination : FunctionLivenessComputation {
 namespace {
 
 class SILDeadFuncElimination : public SILModuleTransform {
+
+private:
+  bool isLateDFE;
+
+public:
+  SILDeadFuncElimination(bool isLateDFE) : isLateDFE(isLateDFE) { }
+
   void run() override {
     LLVM_DEBUG(llvm::dbgs() << "Running DeadFuncElimination\n");
 
@@ -703,15 +719,20 @@ class SILDeadFuncElimination : public SILModuleTransform {
     // can eliminate such functions.
     getModule()->invalidateSILLoaderCaches();
 
-    DeadFunctionElimination deadFunctionElimination(getModule());
+    DeadFunctionElimination deadFunctionElimination(getModule(),
+                                /*keepExternalWitnessTablesAlive*/ !isLateDFE);
     deadFunctionElimination.eliminateFunctions(this);
   }
 };
 
 } // end anonymous namespace
 
 SILTransform *swift::createDeadFunctionElimination() {
-  return new SILDeadFuncElimination();
+  return new SILDeadFuncElimination(/*isLateDFE*/ false);
+}
+
+SILTransform *swift::createLateDeadFunctionElimination() {
+  return new SILDeadFuncElimination(/*isLateDFE*/ true);
 }
 
 void swift::performSILDeadFunctionElimination(SILModule *M) {

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -522,7 +522,9 @@ static void addLateLoopOptPassPipeline(SILPassPipelinePlan &P) {
   P.startPipeline("LateLoopOpt");
 
   // Delete dead code and drop the bodies of shared functions.
-  P.addDeadFunctionElimination();
+  // Also, remove externally available witness tables. They are not needed
+  // anymore after the last devirtualizer run.
+  P.addLateDeadFunctionElimination();
 
   // Perform the final lowering transformations.
   P.addCodeSinking();

test/SIL/Serialization/init_existential_inst_deserializes_witness_tables.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-swift-frontend -sil-inline-threshold 0 %S/Inputs/init_existential_inst_deserializes_witness_tables_input.swift -o %t/Swift.swiftmodule -emit-module -parse-as-library -parse-stdlib -module-link-name swiftCore -module-name Swift -O
-// RUN: %target-swift-frontend -I %t -O %s -emit-sil -o - | %FileCheck %s
+// RUN: %target-swift-frontend -I %t -O %s -Xllvm -sil-disable-pass=late-deadfuncelim -emit-sil -o - | %FileCheck %s
 
 // CHECK: sil_witness_table public_external X: P module Swift {
 

test/SILOptimizer/devirt_opaque_witness.swift

@@ -1,6 +1,6 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-swift-frontend -emit-module -emit-module-path %t/opaque_conformance.swiftmodule -primary-file %S/Inputs/opaque_conformance.swift
-// RUN: %target-swift-frontend -O -emit-sil -primary-file %s -I %t | %FileCheck %s
+// RUN: %target-swift-frontend -O -emit-sil -primary-file %s -I %t -Xllvm -sil-disable-pass=late-deadfuncelim | %FileCheck %s
 
 import opaque_conformance
 

test/SILOptimizer/sil_witness_tables_external_witnesstable.swift

@@ -1,13 +1,19 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-swift-frontend -emit-module %S/Inputs/sil_witness_tables_external_input.swift -o %t/Swift.swiftmodule -parse-stdlib -parse-as-library -module-name Swift -module-link-name swiftCore
-// RUN: %target-swift-frontend -O -I %t %s -emit-sil | %FileCheck %s
+// RUN: %target-swift-frontend -O -I %t %s -Xllvm -sil-disable-pass=late-deadfuncelim -emit-sil | %FileCheck %s
+// RUN: %target-swift-frontend -O -I %t %s -emit-sil | %FileCheck -check-prefix=CHECK-DFE %s
 
 import Swift
 
 // Make sure the specializer produces an external witness table.
 //
 // CHECK: sil_witness_table public_external X: P module Swift {
 
+// Also check that late dead-function-elimination is removing externally
+// available witness tables.
+//
+// CHECK-DFE-NOT: sil_witness_table public_external
+
 func doSomething<T : P>(_ t : T) -> Y {
   return t.doSomething()
 }