[embedded] Only treat findStringSwitchCaseWithCache as anchor in early DFE, let late DFE collect it if unused

Author: kubamracek

lib/SILOptimizer/IPO/DeadFunctionElimination.cpp

@@ -90,6 +90,7 @@ class DeadFunctionAndGlobalElimination {
   llvm::SmallPtrSet<void *, 32> AliveFunctionsAndTables;
 
   bool keepExternalWitnessTablesAlive;
+  bool keepStringSwitchIntrinsicAlive;
 
   /// Checks is a function is alive, e.g. because it is visible externally.
   bool isAnchorFunction(SILFunction *F) {
@@ -127,9 +128,10 @@ class DeadFunctionAndGlobalElimination {
     // To support ObjectOutliner's replacing of calls to findStringSwitchCase
     // with _findStringSwitchCaseWithCache. In Embedded Swift, we have to load
     // the body of this function early and specialize it, so that ObjectOutliner
-    // can reference it later. To make this work we have to avoid DFE'ing it.
-    // Linker's dead-stripping will eventually remove this if actually unused.
-    if (F->hasSemanticsAttr("findStringSwitchCaseWithCache"))
+    // can reference it later. To make this work we have to avoid DFE'ing it in
+    // the early DFE pass. Late DFE will take care of it if actually unused.
+    if (keepStringSwitchIntrinsicAlive &&
+        F->hasSemanticsAttr("findStringSwitchCaseWithCache"))
       return true;
 
     return false;
@@ -721,9 +723,11 @@ class DeadFunctionAndGlobalElimination {
 
 public:
   DeadFunctionAndGlobalElimination(SILModule *module,
-                                   bool keepExternalWitnessTablesAlive) :
+                                   bool keepExternalWitnessTablesAlive,
+                                   bool keepStringSwitchIntrinsicAlive) :
     Module(module),
-    keepExternalWitnessTablesAlive(keepExternalWitnessTablesAlive) {}
+    keepExternalWitnessTablesAlive(keepExternalWitnessTablesAlive),
+    keepStringSwitchIntrinsicAlive(keepStringSwitchIntrinsicAlive) {}
 
   /// The main entry point of the optimization.
   void eliminateFunctionsAndGlobals(SILModuleTransform *DFEPass) {
@@ -807,8 +811,10 @@ class DeadFunctionAndGlobalEliminationPass : public SILModuleTransform {
     // can eliminate such functions.
     getModule()->invalidateSILLoaderCaches();
 
-    DeadFunctionAndGlobalElimination deadFunctionElimination(getModule(),
-                                /*keepExternalWitnessTablesAlive*/ !isLateDFE);
+    DeadFunctionAndGlobalElimination deadFunctionElimination(
+        getModule(),
+        /*keepExternalWitnessTablesAlive*/ !isLateDFE,
+        /*keepStringSwitchIntrinsicAlive*/ !isLateDFE);
     deadFunctionElimination.eliminateFunctionsAndGlobals(this);
   }
 };