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

Alan L. Cox alc at
Tue Jul 22 13:59:45 PDT 2003

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.


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