Working on gdb

For future reference, and because I keep forgetting things, here is how I work on gdb:

1. Check out a copy of the sources. Some of this is described on the branch management page of the GDB wiki:

   mkdir /somewhere/to/work
   cd /somewhere/to/work
   git clone ssh://sourceware.org/git/archer.git src
   cd src
   git remote add gdb git://sourceware.org/git/gdb.git

2. Point the master branch to the correct place. The real one is in the GDB repo, but there’s an old one in Archer that is no longer used. Open /somewhere/to/work/src/.git/config in your favourite editor and change remote = origin to remote = gdb in the [branch "master"] section. Then probably do a git pull to fetch the actual code you’ll be working on.
3. Create yourself a nice new branch to make your changes in:

git branch --track archer-gbenson-lazy-skip-inline-frames gdb/master
git checkout archer-gbenson-lazy-skip-inline-frames

4. (make your changes here)
5. Build it. I build out-of-tree, like this:

   mkdir /somewhere/to/work/build
   cd /somewhere/to/work/build
   CFLAGS="-g -O0" ../src/configure --with-separate-debug-dir=/usr/lib/debug
   make

6. Run the testsuite. I like to run the tests twice, once without and once with the changes, then I diff the results and pipe it through a script I wrote to hide the unimportant bits:

   make check >& build-20110921-3.log
   diff -u baseline-20110919.log build-20110921-3.log | gdbtestdiff

7. If a test fails, you’ll want to hook gdb into it. To do that you run the specific test to create a transcript:

   cd /somewhere/to/work/build/gdb/testsuite
   runtest TRANSCRIPT=1 ../../../src/gdb/testsuite/gdb.opt/inline-cmds.exp
   gdb ../gdb

8. Then you pipe the transcript into gdb:

   set prompt (top-gdb)
   b internal_error
   r -nx
   

Ok, I think that is enough for now.

Syndicated 2011-09-21 12:54:11 from gbenson.net

New role

A couple of months ago I switched from the OpenJDK team to the GDB team. I’ll no doubt write something here about what I’m doing soon (ie within the next year or so) but in the meantime if you would like to apply for my old job at the awesomeness that is Red Hat then please click this link.

Syndicated 2011-06-23 08:45:07 from gbenson.net

Future archaeology

Andrew Hughes pointed out yesterday that the ARM interpreter and JIT are slated for removal in IcedTea6-1.11 unless someone steps up to maintain it. Currently there’s only one place where the all information about what’s required is collated—inside my head—so I thought I’d better write it up before I start forgetting. It’s entirely possible the interpreter will be removed, but it’s also possible that someone will end up trying to resurrect it months or years down the line. If you are that person and you are reading this then you owe me a beer ;)

The first change that broke the ARM code was the fix for PR icedtea/323, aka Sun bug 6939182. I described the required fix here:

“[In the ARM code] last_Java_sp is set to the address of the top Zero frame wherever the frame anchor is set up. It needs changing such that last_Java_sp is set to thread->zero_stack()->sp() (and the new field last_Java_fp gets set to what last_Java_sp used to be set to).”

The second change that broke the ARM code was the fix for PR icedtea/484, aka Sun bug 6951784. I described the required fix here:

“I have had to change the calling convention within Zero and Shark. All method entries (the C function that executes the method) now return an integer which is the number of deoptimized frames they have left on the stack. Whenever a method is called it is now the caller’s responsibility to check whether frames have been deoptimized and reenter the interpreter if they have.”

The third change, currently in progress, reverts the last commit by the ARM code’s author, Ed Nevill: fix for fast bytecodes with ARM/Shark. This piece of code was accidentally incorporated in one of the webrevs when Zero was upstreamed, and isn’t conditionalised correctly. It can cause problems when the ARM code is not present, and there’s no neat fix. Given that the ARM code has been broken for five days shy of a year now I’ve asked for it to be removed from OpenJDK. This is Sun bug 7030207. If the ARM code is resurrected, this patch will require reinstating (with more specific conditionalisation please!)

The fourth change, currently in the future, is JSR 292. Explicit method handle stuff should just work–it’ll be handled by Zero–but the ARM interpreter and JIT will need updating to support three new instructions: invokedynamic, fast_aldc and fast_aldc_w. The latter two are internal instructions, in case you wondered why you’d never heard of them before!

Ok, that is all.

Syndicated 2011-03-24 12:14:33 from gbenson.net

ARM interpreter

I just discovered that the ARM-specific interpreter stuff that Ed Nevill wrote (and then abandoned) last year has a hack that disables it when run with -XX:+PrintCommandLineFlags. I guess this is one problem when you have 6,000 lines of assembler nobody understands: you don’t know what secret weird shit is buried in there.

Syndicated 2011-03-14 14:28:01 from gbenson.net

JSR 292 and Zero

Maybe you’ve heard about JSR 292: Supporting Dynamically Typed Languages on the Java™ Platform? Well, it’s VM changes, and slated for OpenJDK 7 so I figured I ought to take a look at it before it suddenly appears and breaks Zero all over the place.

I’ve been working on it for a couple of weeks now over in the old Shark forest. It’s by no means stable, but if you want to have a play with it then here’s how:

1. Build yourself a recent(ish) copy of OpenJDK 7, one that has the JSR 292 stuff in the class library. I had a copy of the jdk7-hotspot-comp forest lying around, so I used that, but I expect you could use IcedTea7:

   hg fclone http://hg.openjdk.java.net/jdk7/hotspot-comp
   cd hotspot-comp
   export ALT_JDK_IMPORT_PATH=/path/to/some/existing/jvm
   export ALT_BOOTDIR=$ALT_JDK_IMPORT_PATH
   export DISABLE_NIMBUS=true
   export ALLOW_DOWNLOADS=true
   . jdk/make/jdk_generic_profile.sh
   make

2. Maybe go and have a cup of coffee while it builds…
3. Clone yourself a copy of the Shark forest:

   hg fclone http://icedtea.classpath.org/hg/shark

4. Edit the Makefile in there, changing JAVADIR to point to the JVM you just built.
5. Also change JUNITJAR to point to a JUnit 4 jarfile. The location there is where the Fedora junit4 package puts it, so if you have that installed you should be ok.
6. If you aren’t building on x86_64 then you’ll need to edit build.sh too. Set ZERO_LIBARCH, ZERO_ENDIANNESS, ZERO_ARCHDEF and ZERO_ARCHFLAG to appropriate values for your system.
7. Run make.

If you got your editing right it’ll build a new HotSpot, and create a copy of the JVM you built with the new HotSpot dropped in. It’ll then run the OpenJDK 7 JSR 292 unit tests on it.

They’ll fail, of course. Currently there’s no support for invokedynamic yet: I’m still working on the method handles code that underpins it. Method handles look like ordinary methods, except when you call a method handle the VM is presented with a chain of transformations that need applying to the call’s arguments and return value to translate between what the caller supplied and what the eventual callee is expecting. The bad news is that there are some 40 (!) different transformations, of which I’ve implemented maybe 15. The good news is that (I think!) I’ve figured out the framework of it all, so now it’s mostly a case of run the code, read the “unimplemented” message it spits out, and implement the thing it was complaining about. Just like the old days :)

Syndicated 2011-03-03 15:52:39 from gbenson.net