[LLVMdev] Old DOUT

David Greene dag at cray.com
Fri Dec 11 09:44:55 PST 2009

On Friday 11 December 2009 11:35, Chris Lattner wrote:
> > I'm not sure what you mean here.  It's not ok to convert code under
> > DEBUG() or #ifndef NDEBUG to use dbgs()?
> Right.
> > Then what's the point of providing it?
> I don't know what dbgs does, so I don't know!

dbgs() will be a circular-buffering raw_ostream, meaning it saves
the last N bytes (N == unlimited by default) and displays the
output at program termination if requested.  By default output
gets generated immediately, just like errs().

I will add a flag -debug-buffer-size=N to set the buffer and turn
on delayed output.  This is super useful when trying to debug
very large codes.  I have had debug output consume GBs of disk space.
This avoids that problem but it only works if all current debug
output goes to the new stream.

As I said, by default there is no change in behavior.  dbgs() works
very similarly to the formatted_raw_ostream in that it uses errs()
underneath to do the actual output and only does the circular buffering
and delayed output if requested.

> > My intent is to have dbgs() == errs() when debug mode is disabled.
> Do you know why DOUT was removed?  

Yes and no.  I know the reasons given for removing it, but I believe they are
a bit misguided.  DOUT was not the problem.  Not using DEBUG was the problem.

> The problem is that things like this: 
>  DOUT << foo();
> evaluate foo even when assertions are disabled.  This is bad, and bringing
> it back with a new name is not good.

That's not what I'm proposing.

DOUT << foo();

is broken.  It should have been written as:

DEBUG(DOUT << foo());

Today we use:

DEBUG(errs() << foo());

With dbgs() it will be:

DEBUG(dbgs() << foo());

The only functional difference is the ability to use circular buffering.

> > One option is to make the signature SDNode::dump(raw_ostream &) or create
> > a new overload.
> The normal pattern is to define print(raw_ostream&) and have dump() call
> print(errs()).

Right.  And so now it will call print(dbgs()).


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