Is it possible to perform differential linking?

Question

I have a binary which is linked using a large number of object files with many interdependencies. Whenever I recompile even one of these, I need to link the entire binary.

Do linkers (specifically, those of GCC or Clang) support some method of "differential linking", in which enough information is kept about the inter-relations between all the other linked parts, so that the only work that needs to be done when a single part is recompiled is its relations to the other parts + putting them together in the binary?

Note: I'm mostly interested in C++ but I guess this question generalizes at least to C and probably to other compiled languages.

Answer 1

In MSVC this is called "incremental linking". Interestingly, what I've found that GCC might support that to some extent, try using the "-Wl,-i" or "-Wl,-r" parameters to GCC (should be actually also supported by CLang, as these "-Wl" parameters are just passed to ld).

---

I never used it before, but I made this work with the following makefile:

OBJS := a.o b.o c.o main.o

all:    test_app

test_app:   test_app.reloc
    g++ -o $@ $^

# build a "relocatable" object for incremental linking (either -i or -r)
test_app.reloc: $(OBJS)
    g++ -Wl,-i -nostdlib -nostartfiles -o $@ $^

$(OBJS):    makefile

%.o:    %.cpp
    g++ -c -o $@ $<

This builds the app, but I'm not entirely sure what it does internally, if it really does something like "incremental linking" done in MSVC.

In particular, the parameter "-nostdlib" is necessary when using the "-Wl,-i" so that the default libs will not be passed to the ld (which then can't find them - without it I had the error "/usr/bin/ld: cannot find -lgcc_s").

---

Another version which might actually work better (not sure, this would need to be tested on a bigger application to see if there is some gain in the link time for single object updates):

OBJS := a.ro b.ro c.ro main.ro

all:    test_app

test_app:   $(OBJS)
    g++ -o $@ $^

%.o:    %.cpp
    g++ -c -o $@ $<

%.ro:   %.o
    g++ -Wl,-i -nostdlib -nostartfiles -o $@ $<

• Basically creating relocatable file for each object (which might be perhaps a significant portion of the linkage of obj files into the executable) and then just updating the relocatables necessary. For the final link step using the relocatables to link everything together (but part of the linkage has been already done before).

It is also possible to create "groups" of object files to be grouped in a single relocatable, so that there will be less of them at the end (not sure if that would bring anything at the end though).