[LLVMdev] setting up LLVM to *run on* amd64 but *generate code* for alpha

[Chris Lattner]

On Mar 20, 2009, at 5:22 PM, Zack Weinberg wrote:
> On Mon, Mar 16, 2009 at 11:25 AM, Chris Lattner <clattner at apple.com>  
> wrote:
>> Hi Zack, welcome!
> I regret to say I'm disappearing again.  I was only looking at LLVM
> for a class project which is now done with, and I'm not really
> interested in compiler development for the fun of it anymore.  I do
> want to respond to some of the things that you (and others) said...

Sure, feedback is always welcome.

>>> To pick a set of targets to build, configure llvm with the
>> --enable-targets=foo option, where foo is "all" (the default)  
>> "native" (host
>> target only) or a list of llvm targets to build.  This is strictly an
>> optimization to reduce build time of llvm, you should always be  
>> safe to just
>> build all targets.
>
> I don't like this answer. ;-)

I think that there are several reasons for this.  At the end of the  
day, this is a volunteer project driven by the interests of the  
community members.  Since no one has yet been interested in making  
what you needed happen, it doesn't work smoothly yet.  This is  
unfortunate but unlikely to get better until someone makes it happen.

> It is really really important for everything that has a "configure"
> script to support the baseline set of options defined by Autoconf,
> *with the semantics described in the Autoconf manual*.

Sure, we have that.  If you were asking about llvm-gcc, I could have  
explained how to do that very simply.  You were asking about LLVM (a  
generic framework for building compilers out of) and clang (which is  
not fully mature and doesn't have this sort of thing yet) so you got  
the "current answer".

> It's nice
> that LLVM can support more than one target per configuration, but that
> doesn't excuse not making --target do something sensible.  (Per what I
> said above, how about having --target just set the default target, and
> if you want to prune the list there's still --enable-targets?)
>
> What --target presently does just makes no sense at all.

Agreed, it would be nice for someone to fix that if there cared  
enough :).

> I'd also like to mention that, on a one-year-old IBM laptop, building
> all the LLVM back-ends takes about three times as long as just
> building one of them.

That's exactly why we have --enable-targets.

> -- I don't mean to be ragging on y'all, but the fact is that I gave up
> on LLVM and went back to GCC for my project because of these build
> problems.  I don't know if it would have been easier to make the
> modifications to LLVM that I had in mind than the modifications I
> ended up making to GCC -- but I didn't waste nearly a week trying to
> get a working cross toolchain with GCC.

It's unfortunate to hear that, but it sounds like you made the right  
choice.  If you are uninterested in investing in solving the problems  
you hit and would not derive any benefit from the capabilities that  
LLVM provides, then using GCC is an extremely reasonable thing to do.

-Chris