a negative result

Update: I am delighted to have been wrong! See the end.

Briefly, an interesting negative result: consider benchmarks b1, b2, b3 and so on, with associated .c and .h files. Consider libraries p and q, with their .c and .h files. You want to run each benchmark against each library.

P and Q implement the same API, but they have different ABI: you need to separately compile each benchmark for each library. You also need to separate compile each library for each benchmark, because p.c also uses an abstract API implemented by b1.h, b2.h, and so on.

The problem: can you implement a short GNU Makefile that produces executables b1.p, b1.q, b2.p, b2.q, and so on?

The answer would appear to be “no”.

You might think that with call and all the other functions available to you, that surely this could be done, and indeed it’s easy to take the cross product of two lists. But what we need are new rules, not just new text or variables, and you can’t programmatically create rules. So we have to look at rules to see what facilities are available.

Consider the rules for one target:

b1.p.lib.o: p.c
	$(CC) -o $@ -include b1.h $<
b1.p.bench.o: b1.c
	$(CC) -o $@ -include p.h $<
b1.p: b1.p.lib.o b1.p.bench.o
    $(CC) -o $@ $<

With pattern rules, you can easily modify these rules to parameterize either over benchmark or over library, but not both. What you want is something like:

*.%.lib.o: %.c
	$(CC) -o $@ -include $(call extract_bench,$@) $<
%.*.bench.o: %.c
	$(CC) -o $@ -include $(call extract_lib,$@) $<
%: %.lib.o %.bench.o
	$(CC) -o $@ $<

But that doesn’t work: you can’t have a wildcard (*) in the pattern rule. (Really you would like to be able to match multiple patterns, but the above is the closest thing I can think of to what make has.)

Static pattern rules don’t help: they are like pattern rules, but more precise as they apply only to a specific set of targets.

You might think that you could use $* or other special variables on the right-hand side of a pattern rule, but that’s not the case.

You might think that secondary expansion might help you, but then you open the door to an annoying set of problems: sure, you can mix variable uses that are intended to be expanded once with those to be expanded twice, but the former set better be idempotent upon second expansion, or things will go weird!

Perhaps the best chance for a make-only solution would be to recurse on generated makefiles, but that seems to be quite beyond the pale.

To be concrete, I run into this case when benchmarking Whippet: there are some number of benchmarks, and some number of collector configurations. Benchmark code will inline code from collectors, from their header files; and collectors will inline code from benchmarks, to implement the trace-all-the-edges functionality.

So, with Whippet I am left with the strange conclusion that the only reasonable thing is to generate the Makefile with a little custom generator, or at least generate the part of it to do this benchmark-library cross product. It’s hard to be certain about negative results with make; perhaps there is a trick. If so, do let me know!


Thanks to a kind note from Alexander Monakov, I am very happy to be proven wrong.

See, I thought that make functions were only really good in variables and rules and the like, and couldn’t be usefully invoked “at the top level”, to define new rules. But that’s not the case! eval in particular can define new rules.

So a solution with eval might look something like this:

LIBS=p q

define template
$(1).$(2).lib.o: $(2).c
	$$(CC) -o $$@ -include $(1).h $$<
$(1).$(2).bench.o: $(1).c
	$$(CC) -o $$@ -include $(2).h $$<
$(1).$(2): $(1).$(2).lib.o $(1).$(2).bench.o
    $$(CC) -o $$@ $$<

  $(foreach LIB,$(LIBS),\
     $(eval $(call template,$(BENCHMARK),$(LIB)))))

Thank you, Alexander!

One response

  1. Taylor R Campbell says:

    I don’t know about GNU make, but this is very easy in BSD make. I would guess you can do it with GNU make too, but with much more cumbersome syntax to generate targets in the equivalent of a BSD make .for loop.

    $ cat Makefile
    	-rm -f p.o
    	-rm -f q.o
    .PHONY: all
    .PHONY: clean
    BENCH+=	b1
    BENCH+=	b2
    BENCH+=	b3
    LIB+=	p
    LIB+=	q
    API_H.p=	"p.h"
    API_H.q=	"q.h"
    .for L in ${LIB}
    .  for B in ${BENCH}
    all: $B.$L.out
    $B.$L.out: $B.$L
    	./$B.$L >$@
    $B.$L: $B.$L.o $L.o
    	${CC} -o $@ $B.$L.o $L.o
    $B.$L.o: $B.c
    	${CC} -o $@ -c $B.c -DAPI_H=${API_H.$L:Q}
    clean: clean-$B.$L
    .PHONY: clean-$B.$L
    	-rm -f $B.$L
    	-rm -f $B.$L.o
    	-rm -f $B.$L.out
    .  endfor
    $ head -11 *.[ch]
    ==> b1.c <==
    #include <stdio.h>
    #include API_H
            printf("b1: %d\n", api());
            return ferror(stdout);
    ==> b2.c <==
    #include <stdio.h>
    #include API_H
            printf("b2: %d\n", api());
            return ferror(stdout);
    ==> b3.c <==
    #include <stdio.h>
    #include API_H
            printf("b3: %d\n", api());
            return ferror(stdout);
    ==> p.c <==
    #include "p.h"
    int var = 42;
    ==> p.h <==
    #ifndef P_H
    #define P_H
    extern int var;
    static inline int
            return var;
    ==> q.c <==
    #include "q.h"
            return 9;
    ==> q.h <==
    #ifndef Q_H
    #define Q_H
    #define api()   func()
    int func(void);
    #endif  /* Q_H */
    $ make
    cc -o b1.p.o -c b1.c -DAPI_H=\"p.h\"
    cc -O2   -c p.c
    cc -o b1.p b1.p.o p.o
    ./b1.p >b1.p.out
    cc -o b2.p.o -c b2.c -DAPI_H=\"p.h\"
    cc -o b2.p b2.p.o p.o
    ./b2.p >b2.p.out
    cc -o b3.p.o -c b3.c -DAPI_H=\"p.h\"
    cc -o b3.p b3.p.o p.o
    ./b3.p >b3.p.out
    cc -o b1.q.o -c b1.c -DAPI_H=\"q.h\"
    cc -O2   -c q.c
    cc -o b1.q b1.q.o q.o
    ./b1.q >b1.q.out
    cc -o b2.q.o -c b2.c -DAPI_H=\"q.h\"
    cc -o b2.q b2.q.o q.o
    ./b2.q >b2.q.out
    cc -o b3.q.o -c b3.c -DAPI_H=\"q.h\"
    cc -o b3.q b3.q.o q.o
    ./b3.q >b3.q.out
    $ head *.out
    ==> b1.p.out <==
    b1: 42
    ==> b1.q.out <==
    b1: 9
    ==> b2.p.out <==
    b2: 42
    ==> b2.q.out <==
    b2: 9
    ==> b3.p.out <==
    b3: 42
    ==> b3.q.out <==
    b3: 9

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