Linker: Add flag to override linkage rules

Add a flag to lib/Linker (and `llvm-link`) to override linkage rules.
When set, the functions in the source module *always* replace those in
the destination module.

The `llvm-link` option is `-override=abc.ll`.  All the "regular" modules
are loaded and linked first, followed by the `-override` modules.  This
is useful for debugging workflows where some subset of the module (e.g.,
a single function) is extracted into a separate file where it's
optimized differently, before being merged back in.

Patch by Luqman Aden!

llvm-svn: 235473

1 files changed, 14 insertions, 3 deletions

diff --git a/llvm/tools/llvm-link/llvm-link.cpp b/llvm/tools/llvm-link/llvm-link.cpp
index 9f287e4fdb6..369f3477fe5 100644
--- a/llvm/tools/llvm-link/llvm-link.cpp
+++ b/llvm/tools/llvm-link/llvm-link.cpp
@@ -38,6 +38,11 @@ static cl::list<std::string>
 InputFilenames(cl::Positional, cl::OneOrMore,
                cl::desc("<input bitcode files>"));
 
+static cl::list<std::string> OverridingInputs(
+    "override", cl::ZeroOrMore, cl::value_desc("filename"),
+    cl::desc(
+        "input bitcode file which can override previously defined symbol(s)"));
+
 static cl::opt<std::string>
 OutputFilename("o", cl::desc("Override output filename"), cl::init("-"),
                cl::value_desc("filename"));
@@ -108,7 +113,8 @@ static void diagnosticHandler(const DiagnosticInfo &DI) {
 }
 
 static bool linkFiles(const char *argv0, LLVMContext &Context, Linker &L,
-                      const cl::list<std::string> &Files) {
+                      const cl::list<std::string> &Files,
+                      bool OverrideDuplicateSymbols) {
   for (const auto &File : Files) {
     std::unique_ptr<Module> M = loadFile(argv0, File, Context);
     if (!M.get()) {
@@ -124,7 +130,7 @@ static bool linkFiles(const char *argv0, LLVMContext &Context, Linker &L,
     if (Verbose)
       errs() << "Linking in '" << File << "'\n";
 
-    if (L.linkInModule(M.get()))
+    if (L.linkInModule(M.get(), OverrideDuplicateSymbols))
       return false;
   }
 
@@ -143,7 +149,12 @@ int main(int argc, char **argv) {
   auto Composite = make_unique<Module>("llvm-link", Context);
   Linker L(Composite.get(), diagnosticHandler);
 
-  if (!linkFiles(argv[0], Context, L, InputFilenames))
+  // First add all the regular input files
+  if (!linkFiles(argv[0], Context, L, InputFilenames, false))
+    return 1;
+
+  // Next the -override ones.
+  if (!linkFiles(argv[0], Context, L, OverridingInputs, true))
     return 1;
 
   if (DumpAsm) errs() << "Here's the assembly:\n" << *Composite;