Re: Thoughts on LLVM and LTO

[Chris Lattner <sabre at nondot dot org>]

On Tue, 22 Nov 2005, Richard Henderson wrote:
> On Tue, Nov 22, 2005 at 05:58:14PM +0100, Steven Bosscher wrote:
> > I thought it would basically "only" replace the GIMPLE parts of the
> > compiler.  That is,
> >
> > FE	-->	GENERIC	-->	LLVM	-->	RTL	--> asm
> > (trees)		(trees)
>
> This is certainly the only way to avoid losing functionality.
>
> I worry that this path will bitrot as 99% of folk use the llvm
> path straight through to assembly on i386.  But perhaps a config
> option to force the rtl path, plus some automated testing, can
> prevent that from happening too fast.

I'm not sure that's a real concern.  Considering that (if people wanted to 
use the LLVM code generator at all) we would only be enabled for some 
targets, the mechanism would already be in place to disable the LLVM 
backend.  If the mechanism is already in place, allowing people to disable 
it on targets where it is supported would be trivial.

> > It depends on who is going to invest these resources.  Would you want
> > to tell Apple they can't do this even though they can? ;-)
>
> No, but we also might want to let Apple work on this for a year
> and then come back with something more concrete than "it should
> be easy".

At this point, that is a reasonable decision to make.  The work will 
progress without outside involvement, it would just go *faster* with 
outside involvement :).

In practice, all this discussion boils down to is: when (and if) we merge 
the LLVM work into the main GCC tree.  It can be disabled by default while 
in progress if desired, but are we going to make everyone interested in it 
use the Apple branch?

> The biggest technical problem I see with LLVM is actually the debug
> info.  Frankly, I'm not sure I even want to consider LLVM until
> that's done.  If it's as easy as Chris and Danny make it out to be,
> then they'll have it knocked off in short order.  If not ...

As far as scheduling goes, we will probably start intense work on that in 
January (given that the holidays are coming up).  Waiting until that piece 
is in place would be reasonable.

-Chris

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