RE: [PATCH] Makefile.in: clean up match.pd-related dependencies

[Alexander Monakov <amonakov@ispras.ru>]

On Fri, 5 May 2023, Alexander Monakov wrote:

> > > gimple-head-export.cc does not exist.
> > > 
> > > gimple-match-exports.cc is not a generated file. It's under source control and
> > > edited independently from genmatch.cc. It is compiled separately, producing
> > > gimple-match-exports.o.
> > > 
> > > gimple-match-head.cc is also not a generated file, also under source control.
> > > It is transitively included into gimple-match-N.o files. If it changes, they will be
> > > rebuilt. This is not changed by my patch.
> > > 
> > > gimple-match-auto.h is a generated file. It depends on s-gimple-match stamp
> > > file, which in turn depends on genmatch and match.pd. If either changes, the
> > > rule for the stamp file triggers. gimple-match-N.o files also depend on the
> > > stamp file, so they will be rebuilt as well.
> > 
> > s-gimple-match does not depend on gimple-match-head.cc. if it changes the stamp
> > is not invalidated. 
> 
> Right, this is correct: there's no need to rerun the recipe for the stamp,
> because contents of gimple-match-head.cc do not affect it.
> 
> > This happens to work because gimple-match-N.cc does depend on gimple-match-head.cc,
> > but if the gimple-match-N.cc already exists then nothing changes.
> 
> No, if gimple-match-N.cc already exist, make notices they are out-of-date via
> 
> $(GIMPLE_MATCH_PD_SEQ_SRC): s-gimple-match gimple-match-head.cc; @true
> 
> and this triggers rebuilding gimple-match-N.o.
> 
> I tested this. After 'touch gimple-match-head.cc' all ten gimple-match-N.o files
> are rebuilt.

My explanation was incomplete here. The gcc/Makefile.in rule quoted above
applies to .cc files and does not trigger rebuilds of .o files on its own.
The reason .o files get rebuilt is implicit dependency tracking: initial
build records header dependencies in gcc/.deps/*.Po files, and incremental
rebuild sees that gimple-match-1.o depends on gimple-match-head.cc.

Alexander