On 4/02/2015 10:23 PM, Magnus Ihse Bursie wrote:
On 2015-02-04 02:28, David Holmes wrote:
On 3/02/2015 11:25 PM, Magnus Ihse Bursie wrote:
On 2015-02-02 23:14, David Holmes wrote:
Hi Magnus,
On 3/02/2015 1:51 AM, Magnus Ihse Bursie wrote:
When a header file is deleted, make will complain "No rule to make
target <old header file>". This often breaks incremental build
completely unnecessary.
When/why would a header file be deleted?
Because it is not needed anymore? :-)
Ok, let's try to clarify this a bit. To support incremental compilation,
we generate (through compiler support or otherwise) a "make dependency
file" *.d for every *.o file we compile. This file is included in the
make file for the next run, and it basically looks like this (but with
full paths):
foo.o:
foo.c \
foo.h \
foo-internal.h \
utils.h
This means, that if say utils.h is modified, then foo.o gets rebuilt.
Great. That's the foundation of incremental builds.
However, if any of the files in this list is removed (or moved, which
amounts to the same thing, since they are in reality (but not my
example) stored with full paths), when make tries to look at the
timestamp for that file, it finds no file at all, so it tries to build
it, but cannot do that since there are no rules for it, and fails. By
adding "dummy" rules such as:
foo.c:
foo.h:
foo-internal.h:
utils.h:
(but with full paths)
then make has a "rule" for these files even if they don't exist, and
does not complain.
For a real-world example of the last time this happened, see the commit
last week by Erik when he moved files from the generic "unix" directory
to OS-specific directories such as "linux". Without this patch,
incremental builds were impossible after pulling that fix, and a make
clean was needed.
Any clearer?
Okay but I really question the premise of attempting an incremental
build under such conditions :)
While we have not really had complete focus on it, the original goal of
the build-infra system was that we should always succeed with
incremental builds whenever possible. I believe this is a worthwile
goal, and this patch is one step towards getting back on track to it.
I lost track a bit in the discussion here. Is the "okay" an ok from you
as a reviewer?
Sorry as I originally said "I can't comment on the rest of the make
magic." I take your word it does what you said but I don't pretend to
understand it.
David
/Magnus
