RFR 8022259 MakeClasslist tool is buggy and its README is out of date.

[Brent Christian]

On 8/12/13 8:41 AM, Alan Bateman wrote:
> On 12/08/2013 14:26, harold seigel wrote:
>>
>> The change to the MakeClasslist tool to use TraceClassLoadingPreorder
>> was made prior to my changes.  The following code has been in
>> MakeClasslist.java for a long time.  I was unable to determine who had
>> made this change or why.
>>
>>             // Understand only "Loading" from
>> -XX:+TraceClassLoadingPreorder.
>>             // This ignores old "Loaded" from -verbose:class to force
>> correct
>>             // classlist generation on Mustang.
>>             if (t.equals("Loading")) {
>>               t = tok.nextToken();
>>               t = t.replace('.', '/');
>>
>> So, I don't know why TraceClassLoadingPreorder was chosen instead of
>> -verbose:class or TraceClassLoading.
>>
>> The purpose of my changes are to update the comments to accurately
>> reflect what the code currently does and to fix a bug in
>> MakeClasslist.java's 'for' loop where the wrong set of strings is used.
>>
> Okay, I see this was changed to use TraceClassLoadingPreorder in 2006
> (prior to OpenJDK). Brent might know more about the history.

I know that for a long while, -verbose:class output was used to generate 
the classlist.  I did some digging...

In JDK 6, we started using TraceClassLoadingPreorder because it helped 
line things up better for cold start [1] - better order of disk reads, 
cut down on seeks, etc.  I myself am not familiar with the details of 
how this was made to work.

There was a short-lived version of MakeClasslist that understood output 
from both -verbose:class and -XX:+TraceClassLoadingPreorder.   The 
nightly build scripts at the time were used for building both JDK 5 & 
JDK 6 trains, so it had to work with both.

Later, support for -verbose:class was removed to ensure that the 
classlist for 6+ was always created based on TraceClassLoadingPreorder, 
to be sure we'd get the cold start benefit.

So that's what the comment about ignoring -verbose:class output is about.

-Brent

[1] http://bugs.sun.com/view_bug.do?bug_id=6449635 : "Optimize shared 
archive object ordering for better cold startup performance"