Re: [tlug] A semi-related question

["Stephen J. Turnbull" <stephen@example.com>]

>>>>> "Josh" == Josh Glover <jmglov@example.com> writes:

    Josh> I think that you are right about g++, but not about gcc. I
    Josh> think gcc always honours "inline" when you turn on
    Josh> optimisation:  [cites gcc manual]

Nope.  That's what -Winline is for, to let you know when GCC can't
hack inlining something.

    Josh> On 4/22/05, Kenneth <emry@example.com> wrote:

    >> I don't use inline much personaly anyway. Then again my current
    >> programming skill is very minimal. ^^;

    Josh> In general, this is the correct approach. In all but the
    Josh> most extreme cases, the compiler knows better than you when
    Josh> something should be inlined.

I believe on some systems the inline keyword causes the compiler to
emit meta information that permits the linker to do inlining.  (I
don't know if any of them are actually in production, though.)

    Josh> Of course, when you do a lot of systems programming, over
    Josh> time, you encounter a lot of the corner cases where inlining
    Josh> something can make your subsystem an order of magnitude
    Josh> faster, but the compiler does not realise it. *That* is when
    Josh> you use the inline keyword.

Unfortunately, it's generally not true that the compiler can hack
inlining stuff that's complicated enough that it can't figure out that
function call overhead is too high.  For example, I once found a
speedup that was of the form

void  /* this is crucial, of course */
dumbfunc (arg)
{
  if (global_flag)
    {
      /* 100 lines of hairy expensive code including function calls */
    }

and I won big with

#define dumbfunc(arg) do { if (global_flag) smartfunc (arg); } while (0)
void
smartfunc (arg)
{
  /* 100 lines of hairy expensive code including function calls */
}

I don't think there are any production compilers smart enough to move
code out of a function like that.

-- 
School of Systems and Information Engineering http://turnbull.sk.tsukuba.ac.jp
University of Tsukuba                    Tennodai 1-1-1 Tsukuba 305-8573 JAPAN
               Ask not how you can "do" free software business;
              ask what your business can "do for" free software.