[LTO] Add a function `LTOCodeGenerator::getMergedModule`

One of the uses of `LTOCodeGenerator` is to take it as a middle+back end. Sometimes
it is very helpful to access, especially get information from the optimized module.
If the information can be changed in optimization, it cannot be get before the
module is added to `LTOCodeGenerator`. This patch adds a function
`LTOCodeGenerator::getMergedModule` to access the `MergedModule`.

Reviewed By: steven_wu

Differential Revision: https://reviews.llvm.org/D114201

llvm/include/llvm/LTO/legacy/LTOCodeGenerator.h

@@ -193,6 +193,8 @@ struct LTOCodeGenerator {
   void resetMergedModule() { MergedModule.reset(); }
   void DiagnosticHandler(const DiagnosticInfo &DI);
 
+  Module &getMergedModule() { return *MergedModule; }
+
 private:
   /// Verify the merged module on first call.
   ///

llvm/test/tools/llvm-lto/dump.ll

@@ -0,0 +1,14 @@
+; RUN: llvm-as < %s > %t1
+; RUN: llvm-lto --dump-linked-module %t1 2>&1 | FileCheck %s
+
+; CHEKCK: target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+; CHECK: define void @f() {
+define void @f() {
+; CHECK-NEXT: entry:
+entry:
+; CHECK-NEXT: ret void
+  ret void
+; CHECK-NEXT: }
+}

llvm/tools/llvm-lto/llvm-lto.cpp

@@ -197,6 +197,11 @@ static cl::opt<bool>
                    cl::desc("Write merged LTO module to file before CodeGen"),
                    cl::cat(LTOCategory));
 
+static cl::opt<bool>
+    DumpLinkedModule("dump-linked-module", cl::init(false),
+                     cl::desc("Dump linked LTO module before optimize"),
+                     cl::cat(LTOCategory));
+
 static cl::list<std::string> InputFilenames(cl::Positional, cl::OneOrMore,
                                             cl::desc("<input bitcode files>"),
                                             cl::cat(LTOCategory));
@@ -1115,6 +1120,14 @@ int main(int argc, char **argv) {
       error("error compiling the code");
 
   } else {
+    if (DumpLinkedModule) {
+      if (SaveLinkedModuleFile)
+        error(": -dump-linked-module must be used without -dump-linked-module");
+
+      Module &M = CodeGen.getMergedModule();
+      M.dump();
+    }
+
     if (Parallelism != 1)
       error("-j must be specified together with -o");