Re: Changing the word-size was: Re: eat your own dog food?

[Arnaldo Carvalho de Melo <acme-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Em Sat, Jan 12, 2008 at 01:54:04PM -0200, Arnaldo Carvalho de Melo escreveu:
> Em Sat, Jan 12, 2008 at 01:21:09PM -0200, Arnaldo Carvalho de Melo escreveu:
> > Em Thu, Jan 10, 2008 at 04:41:16PM -0500, jamal escreveu:
> > > On Thu, 2008-10-01 at 19:39 -0200, Arnaldo Carvalho de Melo wrote:
> > > 
> > > 
> > > > Working now on unions, etc, have you had the chance of testing this
> > > > patch?
> > > 
> > > I should be able to test it in a couple of hours. If you have the unions
> > > change before then, send me a unified patch - else an incremental
> > > version.
> > 
> > Remove the non-commited patches and do a git-pull, basic work is done,
> > reusing code I had in ctracer to do some simple alignment fixups.
> > 
> > Please let me know about any bugs you may find.
> 
> There are still some problems:
> 
> [acme@doppio pahole]$ build/pahole -C e1000_opt_list \
> /usr/lib/debug/lib/modules/2.6.21-65.el5rt/kernel/drivers/net/e1000/e1000.ko.debug
> 
> struct e1000_opt_list {
> 	int                        i;                    /*     0     4 */
> 
> 	/* XXX 4 bytes hole, try to pack */
> 
> 	char *                     str;                  /*     8     8 */
> 
> 	/* size: 16, cachelines: 1 */
> 	/* sum members: 12, holes: 1, sum holes: 4 */
> 	/* last cacheline: 16 bytes */
> };
> 
> So on 64 bits we have a hole because pointers are aligned at word-size
> boundaries, changing it to 4 bytes:
> 
> [acme@doppio pahole]$ build/pahole -w 4 -C e1000_opt_list \
> /usr/lib/debug/lib/modules/2.6.21-65.el5rt/kernel/drivers/net/e1000/e1000.ko.debug
> struct e1000_opt_list {
> 	int                        i;                    /*     0     4 */
> 
> 	/* XXX 4 bytes hole, try to pack */
> 
> 	char *                     str;                  /*     8     4 */
> 
> 	/* size: 12, cachelines: 1 */
> 	/* sum members: 8, holes: 1, sum holes: 4 */
> 	/* last cacheline: 12 bytes */
> };
> 
> The pointer size was correctly changed to 4 and the size reduced to 12,
> but we should have subtracted 4 bytes from 'str' offset and the hole
> would disappear and the size becomes 8.
> 
> Working on it.

Partially fixed, which says a lot about how bad class__fixup_alignment
is :-\ Only holes after unions are not being fixed up, still scratching
my head on why this is the case.

Anyway:

[acme@doppio pahole]$ build/pahole -w 4 -C e1000_opt_list \
/usr/lib/debug/lib/modules/2.6.21-65.el5rt/kernel/drivers/net/e1000/e1000.ko.debug
struct e1000_opt_list {
	int                        i;                    /*     0     4 */
	char *                     str;                  /*     4     4 */

	/* size: 8, cachelines: 1 */
	/* last cacheline: 8 bytes */
};

- Arnaldo
-
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