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

[Magnus Ihse Bursie]

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?

> 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