History data for "JDK-8058150: Compile for Specific Platform Version"

[Jan Lahoda]

Hi,

I have a question on JDK-8058150: "Compile for Specific Platform 
Version". To support compilation for older versions of the platform, 
javac will need some description of the APIs as they existed in the 
target platforms.

For this, the current proposal is to use lib/ct.sym file (similar, but 
different, to the JDK 8 lib/ct.sym), containing classfiles of the older 
APIs. This file would be constructed at build time from a textual 
representation of the APIs stored in an OpenJDK repository (currently 
called ct.sym.txt).

The current ct.sym.txt is a single file that contains APIs for all 
supported versions, reusing entries for multiple versions when needed. 
An alternative would be to use ct7.sym.txt for JDK 7 APIs, ct8.sym.txt 
for JDK 8 APIs, etc. Using a single file leads to a smaller total size 
(as it reuses entries where it can), but needs to be considerably 
changed when a new version is added or an obsolete version is removed.

The size of the file is considerable: for the "ct.sym.txt" that 
represents APIs from OpenJDK 7 and 8, the size of the checked-out file 
in the working copy is (currently[2]) ~23MB, and inside the .hg 
directory, the file has ~1.7MB (Mercurial is apparently able to compress 
the ct.sym.txt file very well - but as all history is kept inside .hg 
directory, the size of the file inside the .hg directory increases when 
the ct.sym.txt is updated).

[2] The size may change based on what is included/excluded - e.g. 
currently, private methods and fields are removed from classes before 
writing this file; including these would make the file bigger.

Another alternative would be to partition the file into several smaller 
files - would be easier to grasp, but if the files would be too small, 
the compression would be worse (leading to bigger repositories).

Currently, the proposal is to place the ct.sym.txt file into the 
top-level repository. A prototype of this feature is currently in the 
jdk9/sandbox forest, on branch JDK-8058150-branch. The current 
ct.sym.txt file is <top-level-repository>/make/data/symbols/ct.sym.txt.

Are there any insights/comments on this? Would adding this file to the 
repositories be acceptable?

Thanks go to Jon Gibbons, Joe Darcy, Alan Bateman and others for their 
comments on this so far.

Thanks,
    Jan