Re: final intermodule patch

[Geoff Keating <geoffk at geoffk dot org>]

Nathanael Nerode <neroden@twcny.rr.com> writes:

> >Right.  My patch does not implement any optimisations, and it is not a
> >change to the optimizers.  You do not get better code generation just
> >because you use it; you must use -Osomething too.
> 
> In other words, at -O0 we will *never* get *any* interaction between 
> different translation units, even with your patch?

Last I knew, at -O0 we don't even get interaction between *functions*,
so how could we get interaction between modules?

>  That's what one would expect from the C++ and C standards...

I don't know what that sentence means.  My copy of the C standard
doesn't mention compiler options at all.

> >Yes.  That's why my patch supports -O0 as well as -O3: it lets you do
> >debugging without changing your build system.
> 
> That's all very well, but -O0 should really follow the principle of 
> least surprise.  -O0 -enable-intermodule or some such is just fine.
> 
> Put it this way:  If I'm doing ordinary C programming, I expect "gcc -c 
> file1.c file2.c -o foo.o" to be flagged as a *mistake* by my compiler 
> (this kind of mistake can creep into complex Makefiles fairly easily), 
> not as an instruction silently invoking a different mode of operation.  
> And indeed, this is how it used to work.  The current situation is a 
> regression in this regard.  Does this make sense?

Not really.  You asked for something, and the compiler gave it to you.
If we start having -fno-i-really-meant-this-command-line options,
where do we stop?

$ gcc -O3 -fno-unroll-loops
error: Are you sure you really want to switch off our wonderful loop
error: unroller?  Use -fno-i-really-dont-want-to-unroll-loops if you do.
$ gcc -O3 -fno-unroll-loops -fno-i-really-dont-want-to-unroll-loops
error: Are you sure you really want to override our fine command-line
error: checking software?  Use -fno-i-really-meant-that-command-line if you do.
$ gcc -O3 -fno-unroll-loops -fno-i-really-dont-want-to-unroll-loops \
  -fno-i-really-meant-that-command-line
warning: OK, but don't say I didn't warn you.


*sigh* I should never have mentioned the word 'intermodule'.  I should
have just said "looky, here's a way to compile multiple files into a
single .o!" and let people deduce for themselves what it's good for.

-- 
- Geoffrey Keating <geoffk@geoffk.org>