RFR (S): 8007639: Workaround for ccache in vm.make is incorrect

[David Holmes]

Oops! Hit reply instead of reply-all. Adding back the lists.

On 12/02/2013 11:36 AM, David Holmes wrote:
> Hi Mikael,
>
> So after our side-bar discussions on other ways to tackle this, the only
> query I have now is why the original change adjusted CXXFLAGS and this
> change adjusts CFLAGS instead?
>
> Thanks,
> David
>
> On 9/02/2013 4:30 AM, Mikael Vidstedt wrote:
>>
>> Please review the following change:
>>
>> Webrev:
>> http://cr.openjdk.java.net/~mikael/webrevs/8007639/webrev.00/webrev
>> Bug: http://bugs.sun.com/view_bug.do?bug_id=8007639
>>
>> This change corrects the workaround that was introduced in vm.make for
>> enabling ccache for HotSpot builds. The change introduces a file
>> specific makefile variable (CFLAGS/<file.o>) which is used to only set
>> the JRE_RELEASE_VERSION define when compiling vm_version.o.
>>
>> Verified manually by observing command lines with/without fix, also
>> passes JPRT.
>>
>>
>> Some additional background below, more information is available in the
>> bug report:
>>
>> To enable the use of ccache 7132779 [1] introduced some new makefile
>> logic in vm.make to only pass the JRE_RELEASE_VERSION define when
>> compiling the vm_version.cpp file. The underlying reason for that is
>> that the JRE_RELEASE_VERSION can in some cases contain a time stamp and
>> since ccache checks that the defines match before reusing the cache
>> version of the object file that would mean that if the time stamp
>> changed all files would have to be recompiled. With the fix only
>> vm_version.cpp needs to be recompiled.
>>
>> This almost works, but the logic introduced in the makefile is actually
>> incorrect. Today it looks like this:
>>
>>
>> # This is VERY important! The version define must only be supplied to
>> vm_version.o
>> # If not, ccache will not re-use the cache at all, since the version
>> string might contain
>> # a time and date.
>> vm_version.o: CXXFLAGS += ${JRE_VERSION}
>>
>> However, the above syntax does not only add the ${JRE_VERSION} to the
>> CXXFLAGS of vm_version.o as originally intended - it also /in some
>> cases/ adds it to the CXXFLAGS for any and all prerequisites of
>> vm_version.o. And vm_version.o depends on all other object files, which
>> means in theory this actually passes in that potentially time stamped
>> version string define to all object files anyway. For various reasons in
>> reality it only passes it to files that are lexicographically after
>> vm_version.o - see bug report for more background on why - but there's
>> still a handful of files that will be recompiled unnecessarily.
>>
>> Cheers,
>> Mikael
>>
>> [1] http://bugs.sun.com/view_bug.do?bug_id=7132779
>>