Re: i370 port

["Ulrich Weigand" <uweigand@de.ibm.com>]

Paul Edwards wrote:

> I expected that the assembler generation wouldn't happen until
> after the optimization was completed, and thus, by then, I
> would know whether this was a main.

That depends a bit on the compiler version and optimization level,
but (in particular in the 3.x time frame) GCC may output assembler
code on a function-by-function basis, without necessarily reading
in the whole source file first.

As I said, it seems the best way would be to not have care at all
whether or not any particular source file contains a main routine,
but instead do all entry-point processing in the crt0 startup file.

> > As crt0.o can be (and usually is) completely
> > written in assembler, you can arrange for everything to be in the sequence
> > that is required.  (On the linker command line, crt0.o would be placed
> > first, so if the entry point is the first thing in crt0.o it will then
> > also be the first thing in the executable.)
> 
> Yes, I can do that, but that means I need to have a linker command
> line!  The way the MVS linker works, I can link my main program,
> (which obviously doesn't have crt0 in it), and, thanks to the "END"
> statement, I can specify an entry point.  This means no complaint
> from the linker about a default (and wrong) entry point, and no
> need for any linker statements.  It all automatically resolves.

I don't know exactly how your port handles this on MVS, but usually
GCC itself will invoke the linker, and will itself prepare an
appropriate command linker for the linker.  As part of this command
line, things like crt files will be specified.  You should set the
STARTFILE_SPEC macro in your back-end to do that ...

> > There doesn't seem to be an easy way to do this from the
> > compiler.  At the time compiler output is generated, the
> > original source code text isn't even kept any more; you'd
> > have to go back and re-read the original source files using
> > the line-number debug data, just as the assembler would ...
> 
> Ok, well - I would be content with just the source line number
> appearing in the output assembler.  Is this information
> available as each assembler line is output?

In current GCC, every insn contains "location" information pointing
back to source code line (and column) numbers.  However, in GCC 3.x
I think this wasn't yet present, but you had to rely on line number
notes interspersed with the insn stream.

In any case, if you build with -g and use in addition the "debug
assembler output" flag -dA the assembler output should contain
human-readable comments containing line number information.


Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU Toolchain for Linux on System z and Cell BE
  Ulrich.Weigand@de.ibm.com