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Re: Future plans for Autotools

From: Zack Weinberg
Subject: Re: Future plans for Autotools
Date: Mon, 25 Jan 2021 09:47:55 -0500

On Mon, Jan 25, 2021 at 8:38 AM Paul Smith <> wrote:
> On Sun, 2021-01-24 at 16:26 -0700, Tom Tromey wrote:
> > I think it would be good.  I'm curious if it is known to be faster or
> > if that's just an expectation.  I'd also be very interested to learn
> > which changes would make the result faster.
> Speaking for myself I'm not convinced requiring GNU make would be a lot
> faster.
> The only thing that would make much of a performance difference, I
> think, is if we are able to replace lots of shell invocations with
> built-in make functions like wildcard, if, etc.  However I'm not sure
> there's enough opportunity for that to matter.

I'm not at all familiar with Automake's internals, but the reason I
suggested taking advantage of GNU make extensions was the potential
for _complexity_ reduction of the generated Makefile, not performance.
For instance, this generated rule from one of my other projects

test/ka_bcrypt_a-ka-tester.o: test/ka-tester.c
$(test_ka_bcrypt_a_CPPFLAGS) $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS) -MT
test/ka_bcrypt_a-ka-tester.o -MD -MP -MF
test/$(DEPDIR)/ka_bcrypt_a-ka-tester.Tpo -c -o
test/ka_bcrypt_a-ka-tester.o `test -f 'test/ka-tester.c' || echo
    $(AM_V_at)$(am__mv) test/$(DEPDIR)/ka_bcrypt_a-ka-tester.Tpo

would be completely unnecessary if Automake could rely on
target-specific variable values, and would be substantially simpler if
it could rely on $< working in all rules.

Automake _does_ make heavy use of shell constructs embedded inside
frequently-executed rules, for instance

    $(AM_V_CC)depbase=`echo $@ | sed 's|[^/]*$$|$(DEPDIR)/&|;s|\.o$$||'`;\
    $(COMPILE) -MT $@ -MD -MP -MF $$depbase.Tpo -c -o $@ $< &&\
    $(am__mv) $$depbase.Tpo $$depbase.Po

which looks like it could become

%.o: %.c
    $(AM_V_CC)$(COMPILE) -MT $@ -MD -MP -MF $(@D)/$(DEPDIR)/$(*F).Tpo \
        -c -o $@ $<
    $(AM_V_at)$(am__mv) $(@D)/$(DEPDIR)/$(*F).Tpo $(@D)/$(DEPDIR)/$(*F).Po

and enable Make to bypass the shell altogether.  Might be worth
benchmarking on a big program.  Has to be an executable, not a
library, though; for libraries, the overhead of the libtool script is
going to dominate.


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