RFR: JDK-8072106 Properly handle dependencies for deleted header files

[David Holmes]

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 :)

Thanks,
David


>> There's one typo "dependeny" but I can't comment on the rest of the
>> make magic.
> Thanks! I fixed it but I do not bother uploading a new webrev if that's
> okay. :-)
>
> /Magnus
>