[sil-opened-archetype-tracker] Fix a memory leak

Placeholder global_addr instructions were never destroyed.
Now they are inserted in the entry basic block and later removed from there by the end of SIL deserialization.

Author: swiftix

include/swift/SIL/SILBuilder.h

@@ -423,7 +423,10 @@ class SILBuilder {
     return insert(new (F.getModule())
                       GlobalAddrInst(getSILDebugLocation(Loc), g));
   }
-
+  GlobalAddrInst *createGlobalAddr(SILLocation Loc, SILType Ty) {
+    return insert(new (F.getModule())
+                  GlobalAddrInst(getSILDebugLocation(Loc), Ty));
+  }
   IntegerLiteralInst *createIntegerLiteral(IntegerLiteralExpr *E);
 
   IntegerLiteralInst *createIntegerLiteral(SILLocation Loc, SILType Ty,

lib/SIL/SILOpenedArchetypesTracker.cpp

@@ -10,18 +10,23 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "swift/SIL/SILBuilder.h"
 #include "swift/SIL/SILOpenedArchetypesTracker.h"
 
 using namespace swift;
 
 void SILOpenedArchetypesTracker::addOpenedArchetypeDef(CanArchetypeType archetype,
                                                        SILValue Def) {
   auto OldDef = getOpenedArchetypeDef(archetype);
-  if (OldDef && isa<GlobalAddrInst>(OldDef)) {
-    // It is a forward definition created during deserialization.
-    // Replace it with the real definition now.
-    OldDef->replaceAllUsesWith(Def);
-    OldDef = SILValue();
+  if (OldDef) {
+    if (auto *OldI = dyn_cast<GlobalAddrInst>(OldDef)) {
+      // It is a forward definition created during deserialization.
+      // Replace it with the real definition now.
+      OldDef->replaceAllUsesWith(Def);
+      // Remove the placeholder instruction.
+      OldI->eraseFromParent();
+      OldDef = SILValue();
+    }
   }
   assert(!OldDef &&
          "There can be only one definition of an opened archetype");
@@ -56,11 +61,22 @@ bool SILOpenedArchetypesTracker::registerUsedOpenedArchetypes(CanType Ty) {
     if (getOpenedArchetypeDef(archetypeTy))
       return;
 
-    auto &SILMod = this->getFunction().getModule();
+    auto *CurF = const_cast<SILFunction *>(&this->getFunction());
+    auto &SILMod = CurF->getModule();
     // Create a placeholder representing a forward definition.
-    auto Placeholder = new (SILMod)
-        GlobalAddrInst(SILDebugLocation(),
-                       SILMod.Types.getLoweredType(archetypeTy));
+    // Add the placeholder at the beginning of the entry block.
+    SILValue Placeholder;
+    if (!CurF->getEntryBlock()->empty()) {
+      SILBuilder B(CurF->getEntryBlock()->begin());
+      Placeholder =
+          B.createGlobalAddr(ArtificialUnreachableLocation(),
+                             SILMod.Types.getLoweredType(archetypeTy));
+    } else {
+      SILBuilder B(CurF->getEntryBlock());
+      Placeholder =
+          B.createGlobalAddr(ArtificialUnreachableLocation(),
+                             SILMod.Types.getLoweredType(archetypeTy));
+    }
     // Make it available to SILBuilder, so that instructions using this
     // archetype can be constructed.
     addOpenedArchetypeDef(archetypeTy, Placeholder);