[FFmpeg-devel] [RFC] Hack to disable implicit rules

[Reimar Döffinger]

On Tue, Feb 16, 2010 at 09:43:34AM +0000, M?ns Rullg?rd wrote:
> Reimar D?ffinger <Reimar.Doeffinger at gmx.de> writes:
> 
> > Hello,
> > since I've only recently seen them causing issues (mostly due to MinGW and
> > Cygwin make being broken most of the time) and found a way that seems
> > to disable them (although maybe more as a side-effect), I'd like to propose
> > this patch.
> > What are your opinions?
> > Index: common.mak
> > ===================================================================
> > --- common.mak	(revision 21840)
> > +++ common.mak	(working copy)
> > @@ -4,6 +4,10 @@
> >
> >  all: # make "all" default target
> >
> > +# disable suffix rules, we do not use them and like this we avoid (most?)
> > +# implicit rules which might cause slowdowns or bugs
> > +.SUFFIXES:
> > +
> 
> Does this fix an actual problem?

Not for FFmpeg, no.

> I already looked at this, and that
> pseudo-target doesn't actually disable much at all, only the old .c.o
> style rules.  New-style %.c:%.o builtin rules are still in effect.

Are there any?
In my tests and with -d, this seemed to behave the same way as make -r
(correction: a proper test suggests a small handful of rules remain).
I can say for sure that there are no builtin "new-style" rules at least for
.c -> .o and .S -> .o
For targets with .o or no extension, the only rules left seem to be related to
RCS, e.g. using a Makefile only containing above .SUFFIXES line:
> make -d test.o
Considering target file `test.o'.
 File `test.o' does not exist.
 Looking for an implicit rule for `test.o'.
 Trying pattern rule with stem `test.o'.
 Trying implicit prerequisite `test.o,v'.
 Trying pattern rule with stem `test.o'.
 Trying implicit prerequisite `RCS/test.o,v'.
 Trying pattern rule with stem `test.o'.
 Trying implicit prerequisite `RCS/test.o'.
 Trying pattern rule with stem `test.o'.
 Trying implicit prerequisite `s.test.o'.
 Trying pattern rule with stem `test.o'.
 Trying implicit prerequisite `SCCS/s.test.o'.
 No implicit rule found for `test.o'.

> make -d test
Considering target file `test'.
 Looking for an implicit rule for `test'.
 Trying pattern rule with stem `test'.
 Trying implicit prerequisite `test,v'.
 Trying pattern rule with stem `test'.
 Trying implicit prerequisite `RCS/test,v'.
 Trying pattern rule with stem `test'.
 Trying implicit prerequisite `RCS/test'.
 Trying pattern rule with stem `test'.
 Trying implicit prerequisite `s.test'.
 Trying pattern rule with stem `test'.
 Trying implicit prerequisite `SCCS/s.test'.
 No implicit rule found for `test'.

The output with an empty Makefile is a lot longer, and at least contains rules for .c, .cpp,
.f, .C, .cc, .F, .y, .s, .S, .r, .p, .l, .w, .web, .mod, .sh and maybe more.