Date: Tue, 22 Jul 2003 15:59:37 -0500 From: "Alan L. Cox" <alc@imimic.com> To: Marcel Moolenaar <marcel@xcllnt.net> Cc: cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/kern init_main.c kern_malloc.c md5c.c subr_autoconf.c subr_mbuf.c subr_prf.c tty_subr.c vfs_cluster.c vfs_subr.c Message-ID: <3F1DA5B9.A877E8D9@imimic.com> References: <200307221024.h6MAOggG066724@repoman.freebsd.org> <20030722093443.GD58118@technokratis.com> <20030723003823.R8380@gamplex.bde.org> <20030722112901.GA59012@technokratis.com> <20030722155139.GA39123@troutmask.apl.washington.edu> <3F1D8858.670C08B2@imimic.com> <20030722201918.GA1052@athlon.pn.xcllnt.net>
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Marcel Moolenaar wrote:
>
> On Tue, Jul 22, 2003 at 01:54:16PM -0500, Alan L. Cox wrote:
> > >
> > > `-finline-limit=N'
> > > By default, gcc limits the size of functions that can be inlined.
> > > This flag allows the control of this limit for functions that are
> > > explicitly marked as inline (i.e., marked with the inline keyword
> > > or defined within the class definition in c++). N is the size of
> > > functions that can be inlined in number of pseudo instructions
> > > (not counting parameter handling). The default value of N is 600.
> > > Increasing this value can result in more inlined code at the cost
> > > of compilation time and memory consumption. Decreasing usually
> > >
> >
> > There is another way. The following example illustrates its use.
> >
> > static int vm_object_backing_scan(vm_object_t object, int op)
> > __attribute__((always_inline));
>
> I hope we can come up with a scheme that allows us to control
> inlining on a per-platform basis. Current events demonstrate
> pretty good how people treat optimizations (which inlining is)
> as machine independent fodder and how easy it is to generalize
> beyond sensibility.
> Unfortunately, the use of an expression-like syntax (inline or
> __attribute__ keyword) makes this harder than with a statement-
> like syntax (like #pragma), because of the 2-D space (platforms
> vs functions).
>
I chose my example very carefully...
In the case of vm_object_backing_scan(), I could argue that "always
inline" is correct regardless of platform. This function was written
with inlining as an expectation. It looks something like this:
vm_object_backing_scan(..., int op)
{
...
if (op == "constant #1")
...
else if (op == "constant #2")
...
Furthermore, all call sites pass a constant as the value for op.
Consequently, if the code is inlined, all but the relevent case are
removed as dead code.
I also recall this idiom being used in the i386 pmap.
I suspect that gcc fails to inline this code because it makes the inline
vs. no-inline decision before it does dead code elimination.
Regards,
Alan
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