Re: [PATCH v2] Makefile: make the "sparse" target non-.PHONY

[Ramsay Jones <ramsay@ramsayjones.plus.com>]

On 23/09/2021 18:39, Junio C Hamano wrote:
> Jeff King <peff@peff.net> writes:
> 
>> On Thu, Sep 23, 2021 at 02:07:16AM +0200, Ævar Arnfjörð Bjarmason wrote:
>>
>>> We ensure that the recursive dependencies are correct by depending on
>>> the *.o file, which in turn will have correct dependencies by either
>>> depending on all header files, or under
>>> "COMPUTE_HEADER_DEPENDENCIES=yes" the headers it needs.
>>>
>>> This means that a plain "make sparse" is much slower, as we'll now
>>> need to make the *.o files just to create the *.sp files, but
>>> incrementally creating the *.sp files is *much* faster and less
>>> verbose, it thus becomes viable to run "sparse" along with "all" as
>>> e.g. "git rebase --exec 'make all sparse'".
>>
>> OK. I think this solves the dependency issues sufficiently. It is a
>> tradeoff that you must do the normal build in order to do the sparse
>> check now. That is certainly fine for my workflow (I am building Git all
>> the time, and only occasionally run "make sparse"). I don't know if
>> others would like it less (e.g., if Ramsay is frequently running sparse
>> checks without having just built).
>>
>> (I'd say "I do not care that much either way", but then I do not care
>> all that much either way about incremental sparse checks either, so I'm
>> not sure my opinion really matters).
> 
> My build procedure runs "make sparse" before the primary build,
> simply because the former tends to be much faster to fail when there
> is an issue in the code.  I can understand that depending on .o is a
> cheap way to piggyback on the dependencies it has, but my latency
> will get much slower if this goes in _and_ I keep trying to pick up
> potentially problematic patches from the list.


I always run 'make sparse -k >sp-out 2>&1' after having done the main
build, so that is not an issue for me. Note that I always send all
output from each build step (for master, next and seen) to a series of
(branch keyed) files, so that I can easily diff from branch to branch.
Also, as above, I use '-k' on the 'sparse' and 'hdr-check' targets to
collect all errors/warnings in one go.

So, this evening, with the v2 version of Ævar's patch having landed in
the 'seen' branch, we see this (abridged) diff between next and seen:

  $ diff nsp-out ssp-out
  77a78
  >     SP hook.c
  289a291
  >     SP builtin/hook.c
  417a420
  >     SP t/helper/test-reftable.c
  449a453,478
  >     SP reftable/basics.c
...
  >     SP reftable/tree_test.c
  452a482,483
  >     CC contrib/scalar/scalar.o
  >     SP contrib/scalar/scalar.c
  $ 

So, this almost looks normal, except for the 'CC' line! Having discovered
some leftover cruft from old builds yesterday:

  $ git ls-files | grep contrib/scalar
  contrib/scalar/.gitignore
  contrib/scalar/Makefile
  contrib/scalar/scalar.c
  contrib/scalar/scalar.txt
  contrib/scalar/t/Makefile
  contrib/scalar/t/t9099-scalar.sh
  $ ls contrib/scalar
  Makefile  scalar.c  scalar.o  scalar.sp  scalar.txt  t/
  $ rm contrib/scalar/scalar.{o,sp}
  $ make
      SUBDIR git-gui
      SUBDIR gitk-git
      SUBDIR templates
  $ make sparse
      CC contrib/scalar/scalar.o
      SP contrib/scalar/scalar.c
  $ 

Hmm, interesting, but not relevant here. So, lets play a bit:

  $ make sparse  
  $ make git.sp
  $ make git.sp
  $ touch git.sp
  $ make git.sp
  $ touch git.c
  $ make git.sp
      CC git.o
      SP git.c
  $ touch git.o
  $ make git.sp
      SP git.c
  $ 

Hmm, so I think it is working as designed. However, I find it to be
more than a little irritating (curmudgeon alert!).

Note there are currently no sparse warnings in any of the branches
I build (mainly because Junio patches them up before they hit the
git.kernel.org repo - I am not complaining! ;) ). However, should
any warnings/errors appear (from my own development, say), then I
would make extensive use of 'make <file>.sp' while fixing the
problem. Prior to this patch series, 'make <file>.sp' would _always_
run sparse over the file - it would not depend on the 'mtime' or
existence of any other file, or run the compiler (and wouldn't leave
any 'droppings' either). I liked that! :D

So, I still don't quite get where the 'savings' come from - maybe it
is just me, but I don't think this improves any workflow (well not
mine anyway). I just don't get it. :(

ATB,
Ramsay Jones