Path: utzoo!utgpu!watmath!uunet!ginosko!usc!orion.oac.uci.edu!uci-ics!ucla-cs!maui!rjc
From: rjc@maui.cs.ucla.edu (Robert Collins)
Newsgroups: gnu.g++.lib.bug
Subject: Re: lack of inline functions in libg++
Message-ID: <28477@shemp.CS.UCLA.EDU>
Date: 25 Oct 89 19:10:23 GMT
References: <32147@ucbvax.BERKELEY.EDU> <8910251101.AA29989@g.oswego.edu>
Sender: news@CS.UCLA.EDU
Reply-To: rjc@maui.UUCP (Robert Collins)
Distribution: gnu
Organization: UCLA Computer Science Department
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In article <8910251101.AA29989@g.oswego.edu> dl@oswego.oswego.edu writes:
>
>> I'm finding that I pay a real price in speed when I use some of the
>> libg++ library-derived classes.
>
>Decisions about what to inline in a library are hard, and sometimes
>arbitrary. I usually have to guess about which things are both short
>enough and well-used enough to be worth declaring as inline. These
>decisions never please everyone -- I get just about as many complaints
>that there are too many inlines in libg++, making compiles unbearable
>slow, as complaints that there are not enough inlines, hurting
>performance.

I encountered this very early in the process of writing my Connection
Machine library.

>Michael and I are toying with employing a numerical INLINE_LEVEL
>compiler switch, with levels that correspond to more or less inlining
>(thus, faster or slower compilation). A bunch of compilation and
>linkage details have to be worked out before we can implement this.

In a sense, this was my solution.  If -O is specified, I do lots of
inlining, without it, I do little or none.  Each class in my library has
two files, a .h and a .cc.  They look something like this:

foo.cc:
	#define _FOO_CC_
	#include "foo.h"
	<code>

foo.h:
	class foo {
	};

	#if defined(_FOO_CC_)
	#define inline
	#endif

	#if defined(__OPTIMIZE__) || defined(_FOO_CC_)
	<the inline function definitions>
	#endif

	#if defined(_FOO_CC_)
	#define inline inline
	#endif

When compiling the library, a non-inline version gets put in foo.o, but
the parts of the library that use class foo can use inline versions (if
the library is compiled with -O).  When the user of the library is compiling,
they can compile some files with and others without -O.

When I get in a sitution where I want hundreds, or in some cases thousands
of inline functions, I will put them in a separate .h file.

foo.cc:
	#define _FOO_CC_
	#include "foo.h"
	<code>

foo.h:
	class foo {
	};

	#if defined(_FOO_CC_)
	#define inline
	#endif

	#if defined(__OPTIMIZE__) || defined(_FOO_CC_)
	#include "foo2.h"
	#endif

	#if defined(_FOO_CC_)
	#define inline inline
	#endif

foo2.h:
	<define oodles of inline functions>

This way, the huge number of function definitions is not scanned unless
it is necessary.

This simple scheme has helped a whole lot.  I still have a big problem
deciding which functions to inline.  In a number of cases, g++ tells
me that a function is too large to inline.  Retorical question:  if
I say inline, why doesn't it inline?  Who knows better, me or the compiler?
(I know, the answer is "compiler", but humor me, ok?).  But seriously,
it would be nice to be able to force the inlining of what the compiler
thinks are very large functions.  At least in my code, there are lots
of places where the compiler can evaluate a conditional at compile time,
eliminating lots of code as unreachable (if the function is expanded
inline).

The problem with all these inline functions (besides compile time) is that
I suffer from a serious case of "executable bloat".  I mean I get these
HUGE multi-meg executables.  This problem can be alieviated somewhat by better
optimization in the compiler and by better choice in which functions to 
expand inline.

More on my optimization complaints coming soon to a mailing list near
you...

rob collins
rjc@cs.ucla.edu

PS  For those who are interested in the C++/CM interface, it is about 75%
done.  It has stalled, due to other commitments and massive computer
system changes/problems (latest weirdness: I left town for a couple of days,
and when I came back this morning, all of the machine names had been
changed--strange).

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rjc@cs.ucla.edu	                    C++/Paris on the CM2:  The only way to fly.
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