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From: jamal <hadi-fAAogVwAN2Kw5LPnMra/2Q@public.gmane.org>
To: Arnaldo Carvalho de Melo <acme-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: dwarves-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Subject: Re: Changing the word-size was: Re: eat your own dog food?
Date: Sat, 12 Jan 2008 20:15:55 -0500
Message-ID: <1200186955.31540.26.camel@localhost> (raw)
In-Reply-To: <1200184926.31540.13.camel@localhost>

On Sat, 2008-12-01 at 19:42 -0500, jamal wrote:

> I am going to pull your latest change and test.

Just one bug to report: you dont adjust your tail padding when the size
changes from native; example:

------
hadi@lilsol:~/pahole-tests$ pahole -C s4 ./a.out
struct s4 {
        char                       a;                 /*     0     1 */

        /* XXX 3 bytes hole, try to pack */

        double                     b;                /*     4     8 */
        char                       c;                /*    12     1 */

        /* size: 16, cachelines: 1 */
        /* sum members: 10, holes: 1, sum holes: 3 */
        /* padding: 3 */
        /* last cacheline: 16 bytes */
};
hadi@lilsol:~/pahole-tests$ pahole -w 8 -C s4 ./a.out
struct s4 {
        char                       a;                 /*     0     1 */

        /* XXX 7 bytes hole, try to pack */

        double                     b;                /*     8     8 */
        char                       c;                /*    16     1 */

        /* size: 20, cachelines: 1 */
        /* sum members: 10, holes: 1, sum holes: 7 */
        /* padding: 3 */
        /* last cacheline: 20 bytes */
};
hadi@lilsol:~/pahole-tests$

---------------

In this case the tail padding should be 7.

The rules say:
1) aggregates have alignment equal to that of their most aligned
  member - which happens to be 8 above
2) aggregates have sizes which are a multiple of their alignment
  This may result in tail padding - the tail contains a char.

So the padding above should be 7.

Heres something that initially puzzled me:

---
hadi@lilsol:~/pahole-tests$ pahole -C s1 ./a.out
struct s1 {
        short int                  a1;               /*     0     2 */
        short int                  a2;               /*     2     2 */
        short int                  a3;               /*     4     2 */

        /* size: 6, cachelines: 1 */
        /* last cacheline: 6 bytes */
};
hadi@lilsol:~/pahole-tests$ pahole -C s1 -w 8 ./a.out
struct s1 {
        short int                  a1;               /*     0     2 */
        short int                  a2;               /*     2     2 */
        short int                  a3;              /*     4     2 */

        /* size: 6, cachelines: 1 */
        /* last cacheline: 6 bytes */
};
------------

It puzzled me initially because i thought we'd need a tail padding of
some sort for both 32 bit and 64 bit - but it turns out from rule #1
that the most aligned member was 2 bytes, so no padding was needed for
either. So whatever was done here to follow the rules is key.


cheers,
jamal

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Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-10 12:42 jamal
2008-01-10 13:39 ` Arnaldo Carvalho de Melo
     [not found]   ` <20080110133926.GF22437-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-10 13:55     ` jamal
2008-01-10 14:06       ` Arnaldo Carvalho de Melo
     [not found]         ` <20080110140635.GG22437-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-10 16:39           ` Arnaldo Carvalho de Melo
     [not found]             ` <20080110163959.GJ22437-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-10 21:37               ` jamal
2008-01-10 21:44                 ` Arnaldo Carvalho de Melo
     [not found]                   ` <20080110214438.GT22437-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-11  0:45                     ` jamal
2008-01-11  1:04                       ` jamal
2008-01-10 21:39               ` Arnaldo Carvalho de Melo
     [not found]                 ` <20080110213909.GS22437-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-10 21:41                   ` jamal
2008-01-12 15:21                     ` Arnaldo Carvalho de Melo
     [not found]                       ` <20080112152109.GA23567-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-12 15:54                         ` Changing the word-size was: " Arnaldo Carvalho de Melo
     [not found]                           ` <20080112155404.GB23567-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-12 16:28                             ` Arnaldo Carvalho de Melo
     [not found]                               ` <20080112162808.GC23567-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-12 17:28                                 ` Arnaldo Carvalho de Melo
     [not found]                                   ` <20080112172856.GD23567-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-12 18:08                                     ` jamal
2008-01-12 18:42                                       ` Arnaldo Carvalho de Melo
     [not found]                                         ` <20080112184211.GA17675-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-13  0:42                                           ` jamal
2008-01-13  1:15                                             ` jamal [this message]
     [not found]                                               ` <20080113163106.GE17675@ghostprotocols.net>
     [not found]                                                 ` <20080113163106.GE17675-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-13 18:07                                                   ` Arnaldo Carvalho de Melo
     [not found]                                                     ` <20080113180730.GG17675-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-13 18:19                                                       ` Arnaldo Carvalho de Melo
     [not found]                                                         ` <20080113181901.GH17675-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-13 18:26                                                           ` Arnaldo Carvalho de Melo
     [not found]                                                             ` <20080113182657.GI17675-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-13 19:13                                                               ` jamal
2008-01-12 21:36                                     ` Arnaldo Carvalho de Melo
     [not found]                                       ` <20080112213635.GC17675-f8uhVLnGfZaxAyOMLChx1axOck334EZe@public.gmane.org>
2008-01-12 22:08                                         ` Arnaldo Carvalho de Melo

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