Reland r296442 with modifications reverted in r296463.

Original commit message:

"Allow externally dlopen-ed libraries to be registered as permanent libraries.

This is also useful in cases when llvm is in a shared library. First we dlopen
the llvm shared library and then we register it as a permanent library in order
to keep the JIT and other services working.

Patch reviewed by Vedant Kumar (D29955)!"




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296774 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/DynamicLibrary.h

@@ -14,6 +14,8 @@
 #ifndef LLVM_SUPPORT_DYNAMICLIBRARY_H
 #define LLVM_SUPPORT_DYNAMICLIBRARY_H
 
+#include "llvm/Support/Mutex.h"
+
 #include <string>
 
 namespace llvm {
@@ -68,6 +70,15 @@ namespace sys {
     static DynamicLibrary getPermanentLibrary(const char *filename,
                                               std::string *errMsg = nullptr);
 
+    /// Registers an externally loaded library. The library will be unloaded
+    /// when the program terminates.
+    ///
+    /// It is safe to call this function multiple times for the same library.
+    ///
+    /// \returns An empty \p DynamicLibrary if the library was already loaded.
+    static DynamicLibrary addPermanentLibrary(void *handle,
+                                              std::string *errMsg = nullptr);
+
     /// This function permanently loads the dynamic library at the given path.
     /// Use this instead of getPermanentLibrary() when you won't need to get
     /// symbols from the library itself.

lib/Support/DynamicLibrary.cpp

@@ -76,6 +76,18 @@ DynamicLibrary DynamicLibrary::getPermanentLibrary(const char *filename,
   return DynamicLibrary(handle);
 }
 
+DynamicLibrary DynamicLibrary::addPermanentLibrary(void *handle,
+                                                   std::string *errMsg) {
+  SmartScopedLock<true> lock(*SymbolsMutex);
+  // If we've already loaded this library, tell the caller.
+  if (!OpenedHandles->insert(handle).second) {
+    if (errMsg) *errMsg = "Library already loaded";
+    return DynamicLibrary();
+  }
+
+  return DynamicLibrary(handle);
+}
+
 void *DynamicLibrary::getAddressOfSymbol(const char *symbolName) {
   if (!isValid())
     return nullptr;

lib/Support/Windows/DynamicLibrary.inc

@@ -92,6 +92,18 @@ DynamicLibrary DynamicLibrary::getPermanentLibrary(const char *filename,
   return DynamicLibrary(a_handle);
 }
 
+DynamicLibrary DynamicLibrary::addPermanentLibrary(void *handle,
+                                                   std::string *errMsg) {
+  SmartScopedLock<true> lock(*SymbolsMutex);
+  // If we've already loaded this library, tell the caller.
+  if (!OpenedHandles->insert(handle).second) {
+    MakeErrMsg(errMsg, "Library already loaded");
+    return DynamicLibrary();
+  }
+
+  return DynamicLibrary(handle);
+}
+
 // Stack probing routines are in the support library (e.g. libgcc), but we don't
 // have dynamic linking on windows. Provide a hook.
 #define EXPLICIT_SYMBOL(SYM)                    \