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

[Mikael Vidstedt]

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