[LTO][Legacy] Add new C inferface to query libcall functions

Summary:
This is needed to implemented the same approach as lld (implemented in r338434)
for how to handling symbols that can be generated by LTO code generator
but not present in the symbol table for linker that uses legacy C APIs.

libLTO is in charge of providing the list of symbols. Linker is in
charge of implementing the eager loading from static libraries using
the list of symbols.

rdar://problem/52853974

Reviewers: tejohnson, bd1976llvm, deadalnix, espindola

Reviewed By: tejohnson

Subscribers: emaste, arichardson, hiraditya, MaskRay, dang, kledzik, mehdi_amini, inglorion, jkorous, dexonsmith, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67568

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@372021 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm-c/lto.h

@@ -44,7 +44,7 @@ typedef bool lto_bool_t;
  * @{
  */
 
-#define LTO_API_VERSION 24
+#define LTO_API_VERSION 25
 
 /**
  * \since prior to LTO_API_VERSION=3
@@ -592,6 +592,14 @@ extern const char * lto_input_get_dependent_library(lto_input_t input,
                                                     size_t index,
                                                     size_t *size);
 
+/**
+ * Returns the list of libcall symbols that can be generated by LTO
+ * that might not be visible from the symbol table of bitcode files.
+ *
+ * \since prior to LTO_API_VERSION=25
+ */
+extern const char *const *lto_runtime_lib_symbols_list(size_t *size);
+
 /**
  * @} // endgoup LLVMCLTO
  * @defgroup LLVMCTLTO ThinLTO

include/llvm/LTO/LTO.h

@@ -298,6 +298,10 @@ class LTO {
   /// Cache) for each task identifier.
   Error run(AddStreamFn AddStream, NativeObjectCache Cache = nullptr);
 
+  /// Static method that returns a list of libcall symbols that can be generated
+  /// by LTO but might not be visible from bitcode symbol table.
+  static ArrayRef<const char*> getRuntimeLibcallSymbols();
+
 private:
   Config Conf;
 

lib/LTO/LTO.cpp

@@ -1011,6 +1011,16 @@ Error LTO::runRegularLTO(AddStreamFn AddStream) {
                  std::move(RegularLTO.CombinedModule), ThinLTO.CombinedIndex);
 }
 
+static const char *libcallRoutineNames[] = {
+#define HANDLE_LIBCALL(code, name) name,
+#include "llvm/IR/RuntimeLibcalls.def"
+#undef HANDLE_LIBCALL
+};
+
+ArrayRef<const char*> LTO::getRuntimeLibcallSymbols() {
+  return makeArrayRef(libcallRoutineNames);
+}
+
 /// This class defines the interface to the ThinLTO backend.
 class lto::ThinBackendProc {
 protected:

tools/lto/lto.cpp

@@ -652,3 +652,9 @@ extern const char *lto_input_get_dependent_library(lto_input_t input,
                                                    size_t *size) {
   return LTOModule::getDependentLibrary(unwrap(input), index, size);
 }
+
+extern const char *const *lto_runtime_lib_symbols_list(size_t *size) {
+  auto symbols = lto::LTO::getRuntimeLibcallSymbols();
+  *size = symbols.size();
+  return symbols.data();
+}

tools/lto/lto.exports

@@ -76,3 +76,4 @@ lto_input_create
 lto_input_dispose
 lto_input_get_num_dependent_libraries
 lto_input_get_dependent_library
+lto_runtime_lib_symbols_list