[gold-plugin] Add "obj-files" option

[void]

Summary

Add the option "obj-files" that will print out the intermediate object files during ThinLTO. This is similar to "save-temps", but doesn't output intermediate bitcode files.

[pcc]

Doesn't the obj-path option already do what you want here?

[void]

In D50803#1201353, @pcc wrote:

Doesn't the obj-path option already do what you want here?

I don't believe so, or at least it's not exactly the behavior I would like. The behavior I see with save-temps that I wanted to replicate with this option is that save-temps outputs the object code in this way:

$ ld.gold -plugin LLVMgold.so -plugin-opt=save-temps -r -o x.o a.o b.o
$ ls
x.o x.o1 x.o2 a.o b.o

Where x.o1 is the object code for a.o and x.o2 is the object code for b.o. Retaining this ordering is important to my use case---i.e., live patching the Linux kernel. From what I can tell, obj-path outputs .o files to temporary files with no special ordering to the filenames.

[pcc]

obj-path should produce the same order as save-temps because they use the same code path -- they both work by setting Filename to a non-empty string, which causes us to follow the code path that appends task identifiers and use non-temporary files. If you're seeing something different, maybe that should be fixed instead?

[void]

Okay. That seems to work. Thanks!

[void]

I may have spoken too soon. obj_path requires a path to the filename you want to use. This is okay, but when used in a Makefile it becomes difficult:

LDFLAGS_GOLD := -plugin $(CLANG_LIBS)/LLVMgold.so               \
        -plugin-opt=-code-model=kernel                          \
        -plugin-opt=obj_path=

What should be the value for obj_path here? I can't specify a single filename as the output files from one run of ld.gold will overwrite previous ones.

[pcc]

I'm not sure, I'm not an expert on the Linux build system. There's probably some link rule somewhere that contains something like -o $@, maybe you can change it to say -o $@ -plugin-opt=obj-path=$@.o?

[void]

Okay. I was able to hack something together. Thanks!

tools/gold/gold-plugin.cpp

     OT_NORMAL,
     OT_DISABLE,
     OT_BC_ONLY,
-    OT_SAVE_TEMPS
+    OT_SAVE_TEMPS,
+    OT_OBJ_FILES
   };
   static OutputType TheOutputType = OT_NORMAL;
   static unsigned OptLevel = 2;

       TheOutputType = OT_BC_ONLY;
     } else if (opt == "save-temps") {
       TheOutputType = OT_SAVE_TEMPS;
+    } else if (opt == "obj-files") {
+      TheOutputType = OT_OBJ_FILES;
     } else if (opt == "disable-output") {
       TheOutputType = OT_DISABLE;
     } else if (opt == "thinlto") {

 
   switch (options::TheOutputType) {
   case options::OT_NORMAL:
+  case options::OT_OBJ_FILES:
     break;
 
   case options::OT_DISABLE:

   // Note that getOutputFileName will append a unique ID for each task
   if (!options::obj_path.empty())
     Filename = options::obj_path;
-  else if (options::TheOutputType == options::OT_SAVE_TEMPS)
+  else if (options::TheOutputType == options::OT_SAVE_TEMPS ||
+           options::TheOutputType == options::OT_OBJ_FILES)
     Filename = output_name + ".o";
   bool SaveTemps = !Filename.empty();