Re: Reducing the size of C++ executables - eliminating malloc

[Mark Mitchell <mark at codesourcery dot com>]

Daniel Jacobowitz wrote:
> On Sun, Nov 12, 2006 at 05:11:39PM -0800, Mark Mitchell wrote:
>> Daniel Jacobowitz wrote:
>>
>>> If you try what Michael's been saying, you'll notice that trivial
>>> C++ files get the personality routine reference even if they don't
>>> have anything with a destructor which would need cleaning up.  We ought
>>> to be able to emit (somewhat smaller) unwind information which doesn't
>>> reference the personality routine if it's going to have nothing to do,
>>> shouldn't we?
>> Certainly, there are at least some such cases.  I guess a function whose
>> only callees (if any) are no-throw functions, and which itself does not
>> use "throw", does not need frame information.
> 
> You've talked right past me, since I wasn't saying that...

Well, for something like:

  int g() throw();
  int f(int a) {
    return g() + a;
  }

I don't think you ever have to unwind through "f".  Exceptions are not
allowed to leave "g", and nothing in "f" can throw, so as far as the EH
systems is concerned, "f" doesn't even exist.  I think we could just
drop its frame info on the floor.  This might be a relatively
significant size improvement.

>> Unless the entire program doesn't
>> contain anything that needs cleaning up, we'll still need it in the
>> final executable,
> 
> Right.  So if you use local variables with destructors, even though you
> don't use exceptions, you'll get the personality routine.  The linker
> could straighten that out if we taught it to, though.

Correct.  It could notice that, globally, no throw-exception routines
(i.e., __cxa_throw, and equivalents for other languages) were included
and then discard the personality routine -- and, maybe, all of
.eh_frame.  You'd still have the cleanup code in function bodies,
though, so if you really want minimum size, you still have to compile
with -fno-exceptions.

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713