[sil-linker] Improve the implementation of the hasSILFunction API

It it now possible to check if a function with a given name and a given linkage exists in one of the modules,
even if the current module contains a function with this name but a difference linkage.
This is useful e.g. for performing a lookup of pre-specializations.

include/swift/Serialization/SerializedSILLoader.h

@@ -94,6 +94,7 @@ public:
   SILFunction *
   lookupSILFunction(StringRef Name, bool declarationOnly = false,
                     SILLinkage linkage = SILLinkage::Private);
+  bool hasSILFunction(StringRef Name, SILLinkage linkage = SILLinkage::Private);
   SILVTable *lookupVTable(Identifier Name);
   SILVTable *lookupVTable(const ClassDecl *C) {
     return lookupVTable(C->getName());

lib/SIL/Linker.cpp

@@ -139,6 +139,10 @@ SILFunction *SILLinkerVisitor::lookupFunction(StringRef Name,
   return NewFn;
 }
 
+/// Process Decl, recursively deserializing any thing Decl may reference.
+bool SILLinkerVisitor::hasFunction(StringRef Name, SILLinkage Linkage) {
+  return  Loader->hasSILFunction(Name, Linkage);
+}
 
 /// Deserialize the VTable mapped to C if it exists and all SIL the VTable
 /// transitively references.

lib/SIL/Linker.h

@@ -58,6 +58,10 @@ public:
   /// this Name.
   SILFunction *lookupFunction(StringRef Name, SILLinkage Linkage);
 
+  /// Process Name, try to check if there is a declaration of a function
+  /// with this Name.
+  bool hasFunction(StringRef Name, SILLinkage Linkage);
+
   /// Process Decl, recursively deserializing any thing that
   /// the SILFunction corresponding to Decl may reference.
   bool processDeclRef(SILDeclRef Decl);

lib/SIL/SILModule.cpp

@@ -484,12 +484,48 @@ bool SILModule::linkFunction(StringRef Name, SILModule::LinkingMode Mode) {
 }
 
 SILFunction *SILModule::hasFunction(StringRef Name, SILLinkage Linkage) {
-  assert(!lookUpFunction(Name) && "hasFunction should be only called for "
+  SILFunction *F = lookUpFunction(Name);
-                                  "functions that are not contained in the "
+
-                                  "SILModule yet");
+  assert((Linkage == SILLinkage::Public ||
-  return SILLinkerVisitor(*this, getSILLoader(),
+          Linkage == SILLinkage::PublicExternal) &&
-                          SILModule::LinkingMode::LinkNormal)
+         "Only a lookup of public functions is supported currently");
-      .lookupFunction(Name, Linkage);
+
+  // Nothing to do if the current module has a required function
+  // with a proper linkage.
+  if (F && F->getLinkage() == Linkage)
+    return F;
+
+  assert((!F || F->getLinkage() != Linkage) &&
+         "hasFunction should be only called for functions that are not "
+         "contained  in the SILModule yet or do not have a required linkage");
+  (void)F;
+
+  SILLinkerVisitor Visitor(*this, getSILLoader(),
+                          SILModule::LinkingMode::LinkNormal);
+
+  if (Visitor.hasFunction(Name, Linkage)) {
+    if (!F) {
+      // Load the function.
+      F = Visitor.lookupFunction(Name, Linkage);
+      assert(F && "Function should be present");
+      assert(F->getLinkage() == Linkage &&
+             "SILFunction has a wrong linkage");
+    }
+    // If a function exists already and it is a non-optimizing
+    // compilaiton, simply convert it into an external declaration,
+    // so that a compiled version from the shared library is used.
+    if (F->isDefinition() &&
+        F->getModule().getOptions().Optimization <
+            SILOptions::SILOptMode::Optimize) {
+      F->convertToDeclaration();
+    }
+    if (F->isExternalDeclaration())
+      F->setFragile(IsFragile_t::IsNotFragile);
+    F->setLinkage(Linkage);
+  } else {
+    F = nullptr;
+  }
+  return F;
 }
 
 void SILModule::linkAllWitnessTables() {

lib/Serialization/DeserializeSIL.cpp

@@ -1798,6 +1798,68 @@ SILFunction *SILDeserializer::lookupSILFunction(SILFunction *InFunc) {
   return Func;
 }
 
+/// Check for existence of a function with a given name and required linkage.
+/// This function is modelled after readSILFunction. But it does not
+/// create a SILFunction object.
+bool SILDeserializer::hasSILFunction(StringRef Name,
+                                     SILLinkage Linkage) {
+  if (!FuncTable)
+    return false;
+  auto iter = FuncTable->find(Name);
+  if (iter == FuncTable->end())
+    return false;
+
+  // There is a function with the required name.
+  // Find out which linkage it has.
+  auto FID = *iter;
+  auto &cacheEntry = Funcs[FID-1];
+  if (cacheEntry.isFullyDeserialized() ||
+      (cacheEntry.isDeserialized()))
+    return cacheEntry.get()->getLinkage() == Linkage ||
+           Linkage == SILLinkage::Private;
+
+  BCOffsetRAII restoreOffset(SILCursor);
+  SILCursor.JumpToBit(cacheEntry.getOffset());
+
+  auto entry = SILCursor.advance(AF_DontPopBlockAtEnd);
+  if (entry.Kind == llvm::BitstreamEntry::Error) {
+    DEBUG(llvm::dbgs() << "Cursor advance error in hasSILFunction.\n");
+    MF->error();
+    return false;
+  }
+
+  SmallVector<uint64_t, 64> scratch;
+  StringRef blobData;
+  unsigned kind = SILCursor.readRecord(entry.ID, scratch, &blobData);
+  assert(kind == SIL_FUNCTION && "expect a sil function");
+  (void)kind;
+
+  // Read function properties only, e.g. its linkage and other attributes.
+  // TODO: If this results in any noticable performance problems, Cache the
+  // linkage to avoid re-reading it from the bitcode each time?
+  TypeID funcTyID;
+  unsigned rawLinkage, isTransparent, isFragile, isThunk, isGlobal,
+    inlineStrategy, effect, numSpecAttrs;
+  ArrayRef<uint64_t> SemanticsIDs;
+  SILFunctionLayout::readRecord(scratch, rawLinkage, isTransparent, isFragile,
+                                isThunk, isGlobal, inlineStrategy, effect,
+                                numSpecAttrs, funcTyID, SemanticsIDs);
+  auto linkage = fromStableSILLinkage(rawLinkage);
+  if (!linkage) {
+    DEBUG(llvm::dbgs() << "invalid linkage code " << rawLinkage
+                       << " for SIL function " << Name << "\n");
+    return false;
+  }
+
+  // Bail if it is not a required linkage.
+  if (linkage.getValue() != Linkage && Linkage != SILLinkage::Private)
+    return false;
+
+  DEBUG(llvm::dbgs() << "Found SIL Function: " << Name << "\n");
+  return true;
+}
+
+
 SILFunction *SILDeserializer::lookupSILFunction(StringRef name,
                                                 bool declarationOnly) {
   if (!FuncTable)

lib/Serialization/DeserializeSIL.h

@@ -117,6 +117,7 @@ public:
     SILFunction *lookupSILFunction(SILFunction *InFunc);
     SILFunction *lookupSILFunction(StringRef Name,
                                    bool declarationOnly = false);
+    bool hasSILFunction(StringRef Name, SILLinkage Linkage);
     SILVTable *lookupVTable(Identifier Name);
     SILWitnessTable *lookupWitnessTable(SILWitnessTable *wt);
     SILDefaultWitnessTable *

lib/Serialization/SerializedSILLoader.cpp

@@ -104,6 +104,18 @@ SILFunction *SerializedSILLoader::lookupSILFunction(StringRef Name,
   return retVal;
 }
 
+bool SerializedSILLoader::hasSILFunction(StringRef Name, SILLinkage Linkage) {
+  // It is possible that one module has a declaration of a SILFunction, while
+  // another has the full definition.
+  SILFunction *retVal = nullptr;
+  for (auto &Des : LoadedSILSections) {
+    if (Des->hasSILFunction(Name, Linkage))
+      return true;
+  }
+  return retVal;
+}
+
+
 SILVTable *SerializedSILLoader::lookupVTable(Identifier Name) {
   for (auto &Des : LoadedSILSections) {
     if (auto VT = Des->lookupVTable(Name))