Add some comment to LTOCodeGenerator class

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187761 91177308-0d34-0410-b5e6-96231b3b80d8

tools/lto/LTOCodeGenerator.h

@@ -9,6 +9,27 @@
 //
 // This file declares the LTOCodeGenerator class.
 //
+//   LTO compilation consists of three phases: Pre-IPO, IPO and Post-IPO. 
+//
+//   The Pre-IPO phase compiles source code into bitcode file. The resulting
+// bitcode files, along with object files and libraries, will be fed to the
+// linker to through the IPO and Post-IPO phases. By using obj-file extension,
+// the resulting bitcode file disguises itself as an object file, and therefore
+// obviates the need of writing a special set of the make-rules only for LTO
+// compilation.
+//
+//   The IPO phase perform inter-procedural analyses and optimizations, and
+// the Post-IPO consists two sub-phases: intra-procedural scalar optimizations
+// (SOPT), and intra-procedural target-dependent code generator (CG).
+// 
+//   As of this writing, we don't separate IPO and the Post-IPO SOPT. They
+// are intermingled together, and are driven by a single pass manager (see
+// PassManagerBuilder::populateLTOPassManager()).
+// 
+//   The "LTOCodeGenerator" is the driver for the IPO and Post-IPO stages. 
+// The "CodeGenerator" here is bit confusing. Don't confuse the "CodeGenerator"
+// with the machine specific code generator.
+//
 //===----------------------------------------------------------------------===//
 
 #ifndef LTO_CODE_GENERATOR_H
@@ -50,11 +71,33 @@ struct LTOCodeGenerator {
     _mustPreserveSymbols[sym] = 1;
   }
 
-  bool writeMergedModules(const char *path, std::string &errMsg);
-  bool compile_to_file(const char **name, std::string &errMsg);
-  const void *compile(size_t *length, std::string &errMsg);
+  // To pass options to the driver and optimization passes. These options are
+  // not necessarily for debugging purpose (The function name is misleading).
+  // This function should be called before LTOCodeGenerator::compilexxx(),
+  // and LTOCodeGenerator::writeMergedModules().
+  //
   void setCodeGenDebugOptions(const char *opts);
 
+  bool writeMergedModules(const char *path, std::string &errMsg);
+
+  // Compile the merged module into a *single* object file; the path to object
+  // file is returned to the caller via argument "name". Return *FALSE* on
+  // *SUCCESS*, true otherwise. 
+  //
+  // NOTE that it is up to the linker to remove the intermediate object file.
+  //  Do not try to remove the object file in LTOCodeGenerator's destructor
+  //  as we don't who (LTOCodeGenerator or the obj file) will last longer.
+  // 
+  bool compile_to_file(const char **name, std::string &errMsg);
+
+  // As with compile_to_file(), this function compiles the merged module into
+  // single object file. Instead of returning the object-file-path to the caller
+  // (linker), it brings the object to a buffer, and return the buffer to the
+  // caller. This function should delete intermediate object file once its content
+  // is brought to memory. Return NULL is the compilation was not successful. 
+  //
+  const void *compile(size_t *length, std::string &errMsg);
+
 private:
   void initializeLTOPasses();