[LLVMdev] Size and performance figures for LLVM?

[Joachim Durchholz]

Am Dienstag, den 13.05.2008, 11:50 -0700 schrieb Bill Wendling:
> On Tue, May 13, 2008 at 2:24 AM, Joachim Durchholz <jo at durchholz.org> wrote:
> >
> > Am Dienstag, den 13.05.2008, 00:42 -0700 schrieb Bill Wendling:
> >> Sorry to step into this in the middle of a thread, but what exactly is
> >> LLVM's autoconf doing that "autoconf shouldn't do if properly set up"?
> >
> > 1) Variables like CC, CXX, CFLAGS don't work properly when submitted via
> > the command line. The handling seems to be inconsistent, i.e. it seems
> > that parts of the toolchain receive these flags, others don't.
> > This is just a cosmetic problem, of course.
> 
> Is this specifying CC, CXX, and CFLAGS during the "configure" process
> or during the "make" process?

That was during configure.

> Which parts of the
> toolchain don't get the flags? Also, are we talking LLVM or LLVM-GCC?

Both.
I seem to recall that LLVM and LLVM-GCC were failing in slightly
different ways, but I'll have to verify this more precisely.

Would --debug=j be a good way to log all commands as they are executed?
Somehow I overlooked that option and didn't use it during my earlier
tests, which made diagnosing this kind of problem somewhat difficult.

> > 2) The setting of --build should be the default for --host, and the
> > value of --host should be the default for --target. The build machinery
> > seems to take the defaults for --host and --target from elsewhere,
> > specifying just --build or --host will cause amd64 assembly to be fed to
> > an assembler running in i686 mode.
> 
> I don't have much of an explanation for this. I would suggest
> specifying all three (build/host/target) parameters if you're running
> into this problem. It might help in the short term while we figure out
> the issue.

I've been doing that all the time, and it got LLVM unwedged.
It wasn't enough to get LLVM-GCC to compile though.

> > 3) On a 64-bit Ubuntu, setting --build/host/target=i686-pc-linux-gnu
> > will work when compiling with gcc, but llvm-gcc will try to feed 64-bit
> > machine code to the assembler in this situation. At least that's what
> > happens when bootstrapping.
> 
> (We derive our configure script for llvm-gcc from gcc's configure
> script, so there shouldn't be any drastic changes.) If you're
> cross-compiling, I don't think you can do a bootstrap.

Oh. What would be the problems with that

> If you're
> compiling natively, you don't need to specify the build/host/target
> flags.
> 
> Try disabling bootstrapping to see if this helps your problem.

I wanted to avoid issues with compiling LLVM for 64 bits. (I'm running a
64-bit Linux here.)

There was a warning that you have to disable PIC generation in that
situation, and I wasn't sure whether that wouldn't get me into other
kinds of trouble.

> > If autoconf in LLVM can be cleaned up, I'd be really like to do it.
> > Having expertise in that area might just be what's been missing, so if
> > you're willing to give a helping hand to an autoconf newbie, this might
> > finally work out.
> >
> I suggest looking at the autoconf/configure.ac (for LLVM) and
> configure.in & gcc/configure.ac (for LLVM-GCC) scripts. You can find
> documentation here:
> 
>   http://www.gnu.org/software/autoconf/manual/autoconf.html
> 
> The documentation will help you understand what the different macros
> in the .ac/.in files are up to and how they work. I'll try to help as
> much as I can, but I warn that I'm not an expert on GCC's configure
> scripts (they do a lot of...interesting things there).

:-)

Thanks. I hope to find some time for this tomorrow.

Regards,
Jo