Some things take so long
But how do I explain
When not too many people
Can see we’re all the same
And because of all their tears
Their eyes can’t hope to see
The beauty that surrounds them
Isn’t it a pity

Isn’t it a pity
Isn’t is a shame
How we break each other’s hearts
And cause each other pain
How we take each other’s love
Without thinking anymore
Forgetting to give back
Isn’t it a pity

Forgetting to give back
Isn’t it a pity
Forgetting to give back
Now, isn’t it a pity

― George Harrison

Hey you, out there on your own
Sitting naked by the phone
Would you touch me?
Hey you, with you ear against the wall
Waiting for someone to call out
Would you touch me?
Hey you, would you help me to carry the stone?
Open your heart, I’m coming home.

But it was only fantasy.
The wall was too high,
As you can see.
No matter how he tried,
He could not break free.
And the worms ate into his brain.

Hey you, out there on the road
always doing what you’re told,
Can you help me?
Hey you, out there beyond the wall,
Breaking bottles in the hall,
Can you help me?
Hey you, don’t tell me there’s no hope at all
Together we stand, divided we fall.

― Pink Floyd

Watch out now, take care
Beware of the thoughts that linger
Winding up inside your head
The hopelessness around you
In the dead of night

Beware of sadness
It can hit you
It can hurt you
Make you sore and what is more
That is not what you are here for

Watch out now, take care
Beware of soft shoe shufflers
Dancing down the sidewalks
As each unconscious sufferer
Wanders aimlessly
Beware of maya

Watch out now, take care
Beware of greedy leaders
They take you where you should not go
While weeping atlas cedars
They just want to grow, grow and grow
Beware of darkness (beware of darkness)

― George Harrison

Lost in thought and lost in time
While the seeds of life and the seeds of change were planted

Outside the rain fell dark and slow
While I pondered on this dangerous but irresistible pasttime
I took a heavenly ride through our silence
I knew the moment had arrived
For killing the past and coming back to life

I took a heavenly ride trough our silence
I knew the waiting had begun
And headed straight into the shining sun

― Pink Floyd

I don’t know why nobody told you
how to unfold your love
I don’t know how someone controlled you
they bought and sold you

I look at the world and I notice it’s turning
While my guitar gently weeps
With every mistake we must surely be learning
Still my guitar gently weeps

I don’t know how you were diverted
you were perverted too
I don’t know how you were inverted
no one alerted you

I look at you all see the love there that’s sleeping
While my guitar gently weeps
I look at you all
Still my guitar gently weeps

― George Harrison, for The Beatles

You’re forgiven not forgotten
You’re forgiven not forgotten
You’re forgiven not forgotten
You’re not forgotten

A bleeding heart torn apart
Left on an icy grave
In the room where they once lay
Face to face
Nothing could get in their way
But now the memories of the man are haunting her days
And the craving never fades
She’s still dreaming of a man
Long forgiven, but not forgotten

You’re forgiven not forgotten
You’re forgiven not forgotten
You’re forgiven not forgotten
You’re not forgotten

Still alone, staring on
Wishing her life goodbye
As she goes searching for the man
Long forgiven, but not forgotten

You’re forgiven not forgotten
You’re forgiven not forgotten
You’re forgiven not forgotten
You’re forgiven not forgotten
You’re forgiven not forgotten
You’re forgiven not forgotten
You’re forgiven not forgotten
You’re forgiven not forgotten
You’re not forgotten
You’re not forgotten
No, You’re not forgotten

― The Corrs

Sometimes everything is wrong. Now it’s time to sing along
When your day is night alone, (hold on, hold on)
If you feel like letting go, (hold on)
When you think you’ve had too much of this life, well hang on

‘Cause everybody hurts. Take comfort in your friends
Everybody hurts. Don’t throw your hand. Oh, no. Don’t throw your hand
If you feel like you’re alone, no, no, no, you are not alone

If you’re on your own in this life, the days and nights are long,
When you think you’ve had too much of this life to hang on

Well, everybody hurts sometimes,
Everybody cries. And everybody hurts sometimes
And everybody hurts sometimes. So, hold on, hold on
Hold on, hold on, hold on, hold on, hold on, hold on
Everybody hurts. You are not alone

R.E.M.

Blade Runner
]]> http://blog.bauermann.eng.br/2009/07/01/i%e2%80%99ve-done-questionable-things/feed/ 0 bauermann http://blog.bauermann.eng.br/2009/06/29/king-of-pain-2/ http://blog.bauermann.eng.br/2009/06/29/king-of-pain-2/#comments Mon, 29 Jun 2009 07:50:14 +0000 bauermann http://blog.bauermann.eng.br/?p=268 ]]> There’s a little black spot on the sun today
It’s the same old thing as yesterday
There’s a black hat caught in a high tree top
There’s a flag pole rag and the wind won’t stop

I have stood here before inside the pouring rain
With the world turning circles running round my brain
I guess I’m always hoping that you’ll end this reign
But it’s my destiny to be the king of pain

There’s a little black spot on the sun today
That’s my soul up there
It’s the same old thing as yesterday
That’s my soul up there
There’s a black hat caught in a high tree top
That’s my soul up there
There’s a flag pole rag and the wind wont stop
That’s my soul up there

I have stood here before inside the pouring rain
With the world turning circles running round my brain
I guess I’m always hoping that you’ll end this reign
But it’s my destiny to be the king of pain

There’s a fossil that’s trapped in a high cliff wall
That’s my soul up there
There’s a dead salmon frozen in a waterfall
That’s my soul up there
There’s a blue whale beached by a springtides ebb
That’s my soul up there
There’s a butterfly trapped in a spider’s web
That’s my soul up there

I have stood here before inside the pouring rain
With the world turning circles running round my brain
I guess I’m always hoping that you’ll end this reign
But it’s my destiny to be the king of pain

There’s a king on a throne with his eyes torn out
There’s a blind man looking for a shadow of doubt
There’s a rich man sleeping on a golden bed
There’s a skeleton choking on a crust of bread

King of pain

There’s a red fox thorn by a huntsman’s pack
That’s my soul up there
There’s a black winged gull with a broken back
That’s my soul up there
There’s a little black spot on the sun today
It’s the same old thing as yesterday

I have stood here before inside the pouring rain
With the world turning circles running round my brain
I guess I’m always hoping that you’ll end this reign
But it’s my destiny to be the king of pain

King of pain
King of pain
King of pain
I’ll always be king of pain
I’ll always be king of pain
I’ll always be king of pain…

Police

I have made the big decision
I’m gonna try to nullify my life
‘Cause when the blood begins to flow
When it shoots up the dropper’s neck
When I’m closing in on death
And you can’t help me now, you guys
And all you sweet girls with all your sweet talk
You can all go take a walk
And I guess that I just don’t know
And I guess that I just don’t know

I wish that I was born a thousand years ago
I wish that I’d sail the darkened seas
On a great big clipper ship
Going from this land here to that
In a sailor’s suit and cap
Away from the big city
Where a man can not be free
Of all of the evils of this town
And of himself, and those around
Oh, and I guess that I just don’t know
Oh, and I guess that I just don’t know

Heroin, be the death of me
Heroin, it’s my wife and it’s my life
Because a mainer to my vein
Leads to a center in my head
And then I’m better off and dead
Because when the smack begins to flow
I really don’t care anymore
About all the Jim-Jim’s in this town
And all the politicians makin’ crazy sounds
And everybody puttin’ everybody else down
And all the dead bodies piled up in mounds

‘Cause when the smack begins to flow
Then I really don’t care anymore
Ah, when the heroin is in my blood
And that blood is in my head
Then thank God that I’m as good as dead
Then thank your God that I’m not aware
And thank God that I just don’t care
And I guess I just don’t know
And I guess I just don’t know

The Velvet Underground

Er, I actually do have one more thing to say about the subject: GDB had one project accepted in the Google Summer of Code 2009. Oguz Kayral is the student working on it, and I am his mentor. He will add support for subscribing to inferior events (e.g., signals, process and thread stops, thread creation) from Python. One use case for which this is useful was given by an IRC user at the #gdb channel:

<LimCore> how to run gdb from command line, so that it will run ./foo.bin with arguments: foo bar baz and it will run it instantly without waiting for ‘r’; And if program segfaults then it will do ‘bt’ without waiting for the command. (and if program terminates normally then it will also just quit)

LimCore will be able to write a simple and short Python script using the events API to solve his problem.

Now, moving on to other items: my team has been asked to improve GDB support for the hardware debug facilities in embedded PowerPC processors (for more info about these facilities, see Chapter 10 of Book III-E of the Power ISA v2.06). I announced this work to the GDB mailing list back in early March, and got useful insight from Joel Brobecker.

Today I posted an update on where we are with this work. We have the following ready for both native GDB and gdbserver on Linux:

• one additional hardware watchpoint (two in total),
• four hardware breakpoints,
• one ranged hardware watchpoint.

And we still have the following features ahead of us:

• support for the two DVC (Data Value Compare) registers, which enable hardware-accelerated conditions for hardware watchpoints,
• two ranged hardware breakpoints.

Last and least, I was thinking of posting monthly GDB updates on what happened in GDB in the previous month as I did back in February, but I got busy and didn’t get around to it. I still entertain the idea though, so if you think it’s worth it, I’d be glad to know.

The latest two patches committed upstream are for creating convenience functions in Python, and fo manipulating a program’s stack frames as Python objects. So, what can you do with those? I’ll borrow an example from Tom Tromey here: suppose you want to set a breakpoint which triggers only when the code is called by one specific function. You can create a convenience function like this:

import gdb
import re

class CallerIs (gdb.Function):
    """Return True if the calling function's name is equal to a string.
This function takes one or two arguments.
The first argument is the name of a function; if the calling function's
name is equal to this argument, this function returns True.
The optional second argument tells this function how many stack frames
to traverse to find the calling function.  The default is 1."""

    def __init__ (self):
        super (CallerIs, self).__init__ ("caller_is")

    def invoke (self, name, nframes = 1):
        frame = gdb.selected_frame ()
        while nframes > 0:
            frame = frame.older ()
            nframes = nframes - 1
        return frame.name () == name.string ()

CallerIs ()

And then create a conditional breakpoint using it, as in:

(gdb) break foo.c:42 if $caller_is ("some_function")

Or, to check the name of the grand-grand-caller of the code:

(gdb) break foo.c:42 if $caller_is ("some_function", 3)

Cool, huh? Now, why bother writing that big doc comment? GDB will use it as online help for the function you wrote:

(gdb) help function caller_is
Return True if the calling function's name is equal to a string.
This function takes one or two arguments.
The first argument is the name of a function; if the calling function's
name is equal to this argument, this function returns True.
The optional second argument tells this function how many stack frames
to traverse to find the calling function.  The default is 1.

One other patch which was merged in HEAD enables creating new GDB commands in Python, so you can write new commands to do some frame tricks too!

If you’d like to know more about the Python scripting work in GDB, I suggest you read the series of blog posts from Tom Tromey on this subject. It’ll give you a pretty good idea of what we have in the Python branch, and the direction we’re heading. Just keep in mind that some method names and syntax changed since he wrote that. Refer to the GDB manual in the Python branch (“make gdb.pdf” in gdb/doc/) or the example scripts (also in the Python branch, in gdb/python/lib/gdb/) for up-to-date details.

All in all, it was a pretty crazy year. My life kind of turned upside down and I found myself to be very often far removed from anything resembling a comfort zone. I expect 2009 to be pretty crazy as well.

– http://she.geek.nz/archives/528-2008-summary.html

• publication of release schedule for GDB 7.0;
• merging of a handful of patches for Python scripting support (including ability to define new commands in python);
• new target supported (Ericsson’s DICOS on x86_64);
• revamp of thread support in Solaris;
• support hardware watchpoints in all i386 targets;
• ability to inspect extra signal information.

This is an incomplete list (doesn’t mention bugfixes, for instance) hacked up in the wee hours of the night, while waiting for sleep to come. If you are a GDB hacker who committed something interesting this month, I apologize in advance for not listing it here.

ps: it’s just marketing, I know. But I really liked the  message…

Anyway, this is the kind of thing we want to make easy to do in Python, so I decided to try to implement a solution using what we have in the Python branch. Tom Tromey already created the notion of convenience functions, which are functions that are implemented by GDB  that you can call in places where it accepts expressions, and he made it possible to implement such functions in Python. It follows that one can write a in_scope convenience function in Python which checks if a given variable name is valid in the current scope of the debugged program.

There were some missing pieces which I had to implement to get the in_scope function working (most notably, a method to get a string from a variable in the debugged program), and in the past few days I committed to the branch the patches which add those pieces. So now you can have the following script:

class InScope (gdb.Function):
    """Check if all the given variables or macros are in scope.
       Receives as argument a list of names separated by
       whitespace."""

    def __init__ (self):
        super (InScope, self).__init__ ("in_scope")

    def invoke (self, var):
        vars = set (var.string().split())
        found = set ()
        pc = gdb.get_selected_frame ().get_pc ()
        block = gdb.get_block_for_pc (pc)
        while block:
            for sym in block:
                if (sym.is_argument ()
                      or sym.is_constant ()
                      or sym.is_function ()
                      or sym.is_variable ()):
                    sym_name = sym.get_print_name ()
                    if sym_name in vars:
                        found.add (sym_name)
            block = block.get_superblock ()

        return vars == found

InScope ()

And use it like this:

Breakpoint 1, main (argc=1, argv=0x7fffffffe208) at /tmp/funcs.c:16
16        int i = 41;
(gdb) if $in_scope("i")
 >print "yay"
 >else
 >print "nay"
 >end
$1 = "yay"
(gdb)

If you save the script to ~/.gdb-in-scope.py, you can put the following in your ~/.gdbinit to load it automatically:

source ~/.gdb-in-scope.py

Now my goal is to cut out and submit upstream a set of patches to make the above work on regular GDB. The only problem is that I have the boring task of writing testcases and documentation for the code before I can submit it…

For now, If you want to use this script you can build a GDB from the branch, but be aware that the Python API as provided in the branch is subject to change.

First of all, I’d like to take the opportunity to encourage people interested in using this feature to experiment with what we have implemented so far. The reason is that if you still can’t do what you want with the current code in the Python branch, we’d love to hear what you miss and implement it. We are working on what is useful for ourselves, and trying to decide what other people would find useful. But it’s not possible to imagine everything that people want to use this for, or even most things. Please refer to this wiki page to learn what currently works, what we plan to implement, and how to grab the code from the Python branch.

Feel free to write to the GDB mailing list or show up in the #gdb IRC channel at Freenode to discuss this work and/or bring your use case to our attention, so that we can support it. I hope that with enough input from prospective users we can ship something that’s immediately useful for most people, and avoid having to jump through hoops later and have to shoehorn something that we forgot to cater for initially, risking breaking scripts out there or ending up with an inconsistent API.

Anyway, back to business: I have just committed the second patch in the Python series! It exports GDB’s value subsystem to Python scripts. Basically, GDB values are objects which represent data in the inferior (GDB jargon for the program being debugged), holding its address in the inferior’s addressspace, its type and so on. See the “Python API” section in the GDB manual if you want to learn more about it (yes, we are even writing documentation for the feature!).

I committed the first patch back in August, but I didn’t mention it here because it didn’t do anything the user would find useful, really. It was just groundwork for the rest (autoconf and Makefile.in changes, a ‘python’ command in GDB which basically does nothing useful, initial documentation…). Still, it was about 1500 lines long (not counting the patch’s context)! This shows how much work it is to integrate Python support in GDB. I almost regret having joined this effort.

The second patch also doesn’t allow the user to do anything useful yet, unfortunately. But it is noteworthy because it is a base upon which a lot of other Python support code depend upon. Also, it’s the first committed patch which actually exposes something from GDB to Python. It took a while to get this code ready for two reasons: one was that there was a long discussion regarding how the syntax of acessing struct/union/class elements. The other was that implementing the Value class involved playing with little-documented aspects of Python’s C interface, and it took me time to discover how to do what I needed.

Now my next step is to choose the next patch from the Python series to submit upstream, and get it ready for posting (i.e., fix FIXMEs, add testcases and documentation). This brings me to another thing I’d like to mention. Back in April when I first prepared the Python patch series, I naively thought that after cutting them out, it was just a matter of posting them, iterate through a few review/rework steps and they’d be committed. Simple enough. But here we are in mid-October and just two from nine patches went in (now it’s more like 15 patches in total)! What happened?

The problem is that we’ve been working in the branch in an experimental and exploratory way, just hacking together enough to get something useful done. This was necessary because we didn’t know exactly what we would want to expose from GDB to Python, and how we wanted to do that. As we progressed and discussed the results, things started to become clear. The problem is that now we have a lot to clean up, voids to fill, and above all documentation and testcases to write. This takes time.

At least, that was the problem with the first two patches. I noticed Tom Tromey started to write more documentation and tie more loose ends than in the beginning (me? I’ve just been working on the first two patches until they were ready. Didn’t write sexy new stuff since then…), so there’s hope that the next patches will be easier to work with. We still lack a lot of tests for the testsuite, though…

I had no option but to buy them, of course! I’ve known about these books for a long time, and I am very excited to start reading and try to absorb as much wisdom as I can… I wonder how big a project this is?

Also, I can’t help but notice its dedication:

This series of books is affectionately dedicated to the Type  650 computer once installed at Case Institute of Technology, in remembrance of many pleasant evenings.

I asked my girlfriend to warn me if I ever get to that level of, hum, commitment to computers.

Or maybe not that much:

Yours truly also attempted to obtain the title, with what was later named the Santos Dumont look (oh well, judge for yourself):

In the end, Fred Mercury won:

Or was it Super Mario? This is still under debate.

Also Frederico, could not attend but sent this awesome picture anyway. And you can see all contestants here.

I’d like to send a big Thank You! to the people who organized this wondrous event: Alex Zanetti, André Macêdo and Paulo Vital. I had a really really good time that night!

In summary, I had a blast there! I was in last year’s summit and enjoyed it and learned a lot from it. But this time I already knew GDB people and they knew me, and I am involved in a couple of current developments and have more experience with the project, all of which made some difference. And everybody there is very friendly, of course, even if they never heard of you before. In fact, Ian Taylor in his welcome presentation urged people to be friendly to newcomers since GCC and the GNU toolchain need new blood.

There was a good number of GDB-related events: a GDB talk and two debugging information talks, a debug information BoF, an informal GDB get-together and a GDB BoF. Unfortunately I know squat about GCC internals (I intend to learn more about it, but didn’t have a chance yet) so the two debugging information were above my head, and I absorbed little. The GDB talk was interesting but since I follow the GDB mailing lists I already knew most of what was presented.

The debug information BoF was interesting, especially since the discussion didn’t focus so much on the two competing approaches to improve debug information (which was the original point of the BoF), but mostly on what should be expected from debug information generated by GCC (i.e., what a debugger should be able to do with it, especially at higher optimization levels), and how its quality can be tested in the GCC testsuite.

The most interesting events for me were of course the GDB get-together and the GDB BoF. The former was a table reserved for us at lunch one day (thanks for organizing this, Joel Brobecker!) where folks interested in GDB would get to see each other faces and talk about random stuff (GDB-related or not). It was fun, and we were able to throw some ideas around about things such as conversion of the GDB repository from CVS to Subversion, the patch review process, and even about rewriting GDB in C++ (which is a hot thread in the GDB mailing list today!). I have a picture of the event:

If you follow the link you can see the notes with the name of each person in the photo above.

The GDB BoF was very interesting, and it felt weird to be at the front (thanks for inviting me Daniel!) discussing current GDB issues with Daniel Jacobowitz, Tom Tromey, Pedro Alves (the other people at the front) and the other GDB maintainers and developers in the room.

We nailed down some pending issues that were being discussed in the mailing list at the time regarding Python scripting support (man, it’s so much easier to decide things face to face rather than by e-mail!), and also discussed a bit of reversible debugging, multithreading GDB itself, GDB scalability, what to do regarding the next release (in a nutshell: wait about a year from the last release so that all the cool stuff which is being worked on right now gets in and settle down), moving the bugs database from GNATS to bugzilla (thanks for doing this Tromey!) etc.

Also after the BoF Pedro Alves gave a very good improvised tutorial on the GDB event loop which he has been studying for the past few months. It felt like cheating, to get all that knowledge in what, half an hour? Thanks so much Pedro, it was awesome.

And of course all the interaction with the people who were there, like Joel Brobecker (playing tennis is more serious than I thought!), Gaius Mulley (Pink Floyd!), Anmol Paralkar, Ramana Radhakrishnan and many others (I don’t even try to enumerate, just a random sample).

I shared a suite in Ottawa with David Edelsohn and Kenneth Zadeck, which was an interesting thing in itself. Heading back to the hotel felt like going to an extended GCC summit. I almost learned something about GCC internals (SSA, LTO, register allocation) and also had very interesting conversations in general.

And of course my one week of backpacking in Canada after the summit, which was another blast.

Also, there will be a fair number of GDB-related events this time, and I look forward to all of those: a GDB talk and two debugging information talks, a debug information BoF and an informal GDB get-together. I look forward to the GDB-related conversations we’ll have in the last one. Even more so if we do the get-together in a bar, around some beer.

I also look forward to the closing party at Vineyards, of course. I will finally eat again that bumbleberry pie…

Update (2008/07/15): I just wrote the trip report.

Just today Carlos Seo committed a patch to GDB adding support for writing AltiVec (PowerPC’s SIMD instructions) registers to corefiles when using the gcore (or generate-core-file) command. This support will show up, then, in the next GDB release. He also had a patch committed last year, in time for GDB 6.8 release, adding support for reading AltiVec registers from core files generated by the Linux kernel, starting from version 2.6.24 (kernel patch provided by Mark Nelson).

Luis Machado posted in February a patch for ltrace which fixed a number of ABI-related bugs for PowerPC 32-bits and 64-bits. The patch was committed in March. The ltrace maintainer has been out for quite a while now, so I don’t think this will appear in an official release any time soon, but it will surely be picked up by the distros.

Going back to GDB, Luis implemented displaced instruction stepping for PowerPC, helping a bit with the non-stop multithread debugging effort carried out by CodeSourcery. The patch didn’t go in yet, it’s awaiting review. He’s also been monitoring the non-stop patches and testing them on PowerPC, having already provided some feedback and pointed out a few regressions.

Maybe I’m too slow, but just now I realised what the OSI logo probably means.

It may be viewed as a C with it’s open side turned not left, and not right, but down. So it’s not copyleft, but it’s not copyright either. It sits in the middle. It is a way of viewing OSI’s position between the Free Software Foundation (which uses copyleft, i.e., copyright “twisted” in order to free source code) and closed source software (which is protected by copyright laws).

It also looks like a keyhole. It’s not obvious to me how the keyhole concept should be applied here. Maybe it means open source protects user rights? Or protects something else? Or maybe open source is a way to peep into the secrets of software?

free as in malloc.

Googling for the sentence reveals only two insipid results, so I don’t see much hope of getting it soon…

A long time ago I had a nice idea, which unfortunately wasn’t very practical so I didn’t use it for long: I remaped my Tab key to work as Esc, so I could more easily switch modes. I say it wasn’t very practical because getting used to it meant that I would have the wrong muscle memory whenever I was in front of another computer (this was also the reason why I used the Dvorak layout for only a few months).

I couldn’t help but smile today when I found out that the computer terminal where Bill Joy developed the original vi had the Esc key where the Tab key is nowadays.

But this method is inconvenient sometimes: when you want to comment on a patch posted on the list, portions of that patch (which may include long lines that shouldn’t wrap) will show up in the message body and you can have trouble. This is a problem especially when the poster also sent their patch as attachment, because you will most likely need to copy & paste the patch into your reply window and things will get mangled right there.

Turns out that there is a way to include a patch in the message body in Evolution’s compose window and it will get through unmaimed: select the Preformat option in the paragraph style dropdown, and then paste in your patch or use the Insert -> Text File… option.

Caveat: Older versions of Evolution converted tabs to spaces when pasting text, so you had to insert it from a text file to preserve whitespace. I just tested with version 2.12.3 from Debian (package version 2.12.3-1), and pasting a patch containing tabs worked fine so the bug has been fixed.

Thanks Klaus Kiwi for the tip!

This is the tale of my patch-producing efforts. I’m sorry, it is most likely boring for everyone but me. Still, I wanted to write it down so you are free to stop reading the post here.

Anyway, back to where I was: I (foolishly, perhaps?) volunteered to create the patches. One reason for me to do that is that I actually enjoy working with and learning about source code management and related issues, and this was a good opportunity for me to improve my git-fu (since I thought git could help me do the job in a sane way, which fortunately proved to be the case).

I say foolishly because I thought it wouldn’t take too much time to cut out the patches, since I knew what I would have to do… The task of course took longer than I expected, in part because of unforeseen autotools woes, but of course also because I underestimated the effort (I am an optimist).

Here is the method I used:

First of all, I wanted to update the code to the latest CVS version of GDB, because the work in the git repo was done based on a CVS version from February… Nothing to see here, actually. I just created a new branch with the latest CVS update, and rebased the commits on top of that. In hindsight, I should have merged the new CVS update into the python branch, which would have made me deal with less conflicts. I used rebase because I thought I would cherry pick the commits later, so I wanted each of them refreshed.

Then the real fun began. I started using interactive mode of git rebase to squash related commits together (to form the patches), and reorder them. I quickly realised that with this approach I would have a bit of difficulty with commits which touched different areas of the code and crossed the borders I had in my mind for the patches I wanted to generate. I would have to first split those into smaller, more behaving commits and only then squash them with other similar changes. That seemed to be more work than really necessary.

Also, the older commits did things in ways and places which were later changed, and it looked like I would have some trouble reconciling older and newer code to fit in one patch (maybe not though, maybe that would be taken care of more or less naturally). Also, I would need to take some time to familiarise myself with the commits in the branch, because I only authored some of them. The “shuffle commits around” approach wasn’t looking very promising.

I then turned to a different strategy: I generated a big patch containing all of the code in the branch, and applied it (using the plain old patch command) on top of a clean branch which contained only the CVS HEAD version I was using as a base. All I needed to do now was to selectively add to the index the changes that I wanted to include in a patch and then commit those changes together. And repeat the process for the next patch and so on.

It proved to be a good approach, especially because of the interactive mode of git-add. This mode asks you about each change inside a modified file, letting you add that change to the index or skip it, leaving it in the working directory for a future commit. git add -i streamlined the “change picking” process quite a lot, and made the patch-cutting almost mechanical. (By the way, this feature is also available in Mercurial, with the Record extension. I even believe (not sure though) that the Mercurial extension predates git add -i) (I don’t know if Bazaar has it, would be nice to know).

In this phase of the process, git rebase -i was useful. Sometimes I came accross a change in the working directory which would fit better in a patch which I had already committed. It was simply a matter of committing that change and then shuflling it back and squashing with the proper patch.

At each commit I pushed the changes to a pristine repo which I used to build GDB and guarantee that each patch included all the changes it needed.

Voilà, at the end of the process I had a git branch where each commit corresponded to one patch which I wanted to send to the mailing list.

“GDB now supports debugging C and C++ programs which use the Decimal Floating Point extension. In addition, the PowerPC target now has a set of pseudo-registers to inspect decimal float values stored in two consecutive float registers.”

The feature was actually developed by several folks. Ben Elliston started working on the feature, and then passed it on to Wu Zhou. I took up where Wu Zhou left and implemented more complete support for decimal float types. Luis Machado also helped me and posted some patches of his own. The work amounted to a total of about 12 patches.

It means that you can now use GDB (version 6.8 or later) to debug programs which make use of the proposed C extension for decimal floating-point arithmetic (i.e., the _Decimal32, _Decimal64 and _Decimal128 types). The first GCC release with support for this extension was GCC 4.2.

Why is decimal floating point useful? It avoids the unexpected surprises and rounding errors which inevitably come with binary floating point. It’s not that decimal float is more exact or precise, it’s just that it behaves just like we are used to and were thought in school in terms of representation and rounding.

GDB itself can provide an example:

(gdb) p 1.2l
$1 = 1.2000000000000000000433680868994202
(gdb) ptype 1.2l
type = long double
(gdb) p 1.2dl
$2 = 1.2
(gdb) ptype 1.2dl
type = _Decimal128

You can see that the actual value used to represent 1.2 in binary floating point is slightly larger than 1.2. Why is that? It’s because to express 0.2 in binary float you need an infinite number of digits, so what ends up being stored is the closest number possible to represent in binary. The difference is almost nothing, but this error will propagate in computations and eventually be big enough to be noticed. That’s why in some application domains (e.g., finance and civil construction), you are actually required by law to use decimal floating point to perform calculations.

More information about decimal float can be found in Mike Cowlishaw’s page.

Here’s the output I get:

(gdb) rbt
#2 0x080483bb in main at ../../src/examples/funcs.c:15
#1 0x08048391 in f1 at ../../src/examples/funcs.c:10
#0 f2 at ../../src/examples/funcs.c:5
(gdb)

A little bit more information, and the definition of the Python command which does the above can be found in my post to the mailing list. The code is on the git repo for the Python work.