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

[David Holmes]

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
>