[bug #17873] .NOTPARALLEL enhancements

[David Warme]

Follow-up Comment #43, bug #17873 (project make):

Another scenario wherein sequential processing of dependencies is useful:

all: subdirs $(TARGETS)

subdirs:
        (cd dir1 && make)
        (cd dir2 && make)

and the $(TARGETS) depend upon libraries (.a files) that are
made by dir1 and/or dir2.  If you do this with make -j, then
make immediately evaluates each $(TARGET) before the "make subdirs" gets a
chance to touch/update any of the .a
files.  Make therefore decides that nothing is out of date regarding the link
steps, and so each link node in the build tree gets retired without doing
anything.  Issuing a second "make" command later, however, causes it to
re-link the $(TARGETS) because now one or more .a files is newer than the
corresponding $(TARGETS).

It is a pain to have to introduce time-stamp-files just to
force the links to wait until after the "subdirs" has
finished.  Phony targets (e.g., install, subdirs, etc.)
often tend to involve recipes that need to be done serially
-- especially when stringing several together as dependencies
of another phony target.  The previous example of:

reinstall: uninstall install

is precisely this sort of "dependency encoded sequential recipe".

The syntax is negotiable.  But so long as -j exists, there will be a need to
specify series/parallel constraints within dependency lists.  The alternatives
(e.g., time stamp files) are simply too horrible to contemplate.

Some would argue that the antidote to the above scenario is this:

$(TARGETS) : subdirs

But this gets rid of the ability to do "make target1" without forcing make to
also do the "subdirs".  In some cases the developer knows that nothing in the
huge sub-tree of subdirs has changed and all he/she needs to do is re-link the
local executable.  The $(TARGETS) : subdirs kludge effectively eliminates this
capability.


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