automake, build variants and dependencies

Question

So what I want is to add a build variant for a library built from the same source, but with different compiler options.

Following automake documentation, I have something like this (distilled from a much larger, auto-generates Makefile.am):

# Rules to build crtMCU.o and crtMCU-cvt.o from gcrt1.S

nodist_libcrt_a_SOURCES = gcrt1.S
nodist_libcvt_a_SOURCES =

noinst_LIBRARIES = libcrt.a libcvt.a
libcvt_a_LIBADD = gcrt1-cvt.o

avrdir = ...
avrlibdir = ...
avr_DATA = crtMCU.o

cvtdir = ...
cvtlibdir = ...
cvt_DATA = crtMCU-cvt.o

For crtMCU.o there is this rule that works:

crtMCU.o: gcrt1.o
    rm -f $@
    ln $< $@ || cp $< $@

For crtMCU-cvt.o according to the documentation, there is these rules:

gcrt1-cvt.o: gcrt1.S
    $(CPPASCOMPILE) -c -o $@ $< $(CVT_OPTIONS) ...

crtMCU-cvt.o: gcrt1-cvt.o
    rm -f $@
    ln $< $@ || cp $< $@

Now when building, then gcrt1.o gets compiled with -MT gcrt1.o -MD -MP -MF .deps/gcrt1.Tpo where the .deps file lists all the dependencies.

But when building gcrt1-cvt.o, there is no such dependency file (which is understandable since the build rule is written by hand).

Question

What is the proposed way to add similar dependencies (they are the same) for build variant gcrt1-cvt.o?

Note: What I don't understand is for why the Makefile.am is building libraries (which are not used). The build goal is to build and install crtMCU.o and crtMCU-cvt.o. No *.a is needed.

Answer 1

Note: What I don't understand is for why the Makefile.am is building libraries (which are not used). The build goal is to build and install crtMCU.o and crtMCU-cvt.o. No *.a is needed.

The libraries are presumably a vehicle to get the object files built. This is a slight abuse of Automake, but not horrendous.

Automake generates rules for object files only in service to the goal of building a program or library. In the Makefile.am fragment presented, libcvt.a and libcrt.a are designated among the noinst_LIBRARIES, which means that rules for building the libraries will be generated, and they will be in the prerequisite tree for all, but the standard installation rules will not install them to the system. Building these libraries involves building all the contributing sources, modulo analysis of which of the resulting object files are already up to date, and that's what gets your object file(s) built.

What is the proposed way to add similar dependencies (they are the same) for build variant gcrt1-cvt.o?

The easy way would be to work with Automake by naming gcrt1.S among the sources for libcvt.a, too:

nodist_libcrt_a_SOURCES = gcrt1.S
nodist_libcvt_a_SOURCES = gcrt1.S

Or, since those are bona fide make variables, you could consider:

nodist_libcrt_a_SOURCES = gcrt1.S
nodist_libcvt_a_SOURCES = $(nodist_libcrt_a_SOURCES)

The latter emphasizes that the two libraries intentionally have identical source lists, and it may be more convenient and easier to maintain in cases where the source lists are longer.

Either way, Automake will then generate rules to assemble gcrt1.S separately for each library, to a different object file, using the options defined for the intended library in each case. It will likewise generate the wanted dependency rules for both object files.

In conjunction with this, delete

libcvt_a_LIBADD = gcrt1-cvt.o

, which is neither needed nor appropriate.

Answer 2

What should work is to add gcrt1.o as an additioal depencency:

gcrt1-cvt.o: gcrt1.S gcrt1.o
    $(COMPILE) -c -o $@ $< ...

Hence whenever the dependencies for gcrt1.o change and hence that module is rebuilt, then gcrt1-cvt.o will be rebuilt, too.

And better use $(CPPASCOMPILE) instead of $(COMPILE) for assembling an assembly file.