From mboxrd@z Thu Jan  1 00:00:00 1970
From: Zack Weinberg <zweinberg-4eJtQOnFJqFBDgjK7y7TUQ@public.gmane.org>
Subject: Re: Can't persuade pahole to see through forward declarations
Date: Thu, 18 Jun 2009 13:28:20 -0700
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To: Arnaldo Carvalho de Melo <acme-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, dwarves-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
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Arnaldo Carvalho de Melo <acme-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> > > I'm trying to figure this out, but the xulrunner debuginfo file I
> > > have doesn't have the nsIFrameDebug class as an ancestor for
> > > nsFrame:
> > 
> > This is probably because it was configured out.  I am working with
> > a development build, which has all sorts of extra debugging
> > mechanisms configured in.
> 
> OK, can you please send me a multi-cu file that has the above
> definitions so that I can use it as the test case for this new
> feature?

It's 42MB bzipped.  You can download it from
http://www.owlfolio.org/libgklayout.so.bz2 but I'm going to delete it
after a week.

> > Tangentially, I would really like to be able to pass a *list* of
> > structure/class names to -C (or have a separate option that reads a
> > list from a file).  The full analysis I'm doing needs to look at
> > 200 or so of the thousands of classes in xulrunner; currently I
> > need to run pahole once for each, which is really slow.
> 
> Look at the last commit :-)
> 
> http://git.kernel.org/?p=linux/kernel/git/acme/pahole.git;a=commitdiff;h=519d1d3d9691ca94f458853c4710d501fb33720f

Perfect, thanks.

> > Also, I wonder if you could see your way clear to adding an
> > alternative output format that is easily machine-parseable?
> > Approximation-to-C-source format is nice for humans but I've spent
> > the past day and a bit writing a sed script to turn it into
> > something that I can do programmed analysis on and it was no fun.
> 
> How would it look like?

For the analysis I'm doing, the ideal format would be very flat and
line-oriented.  Consider this structure definition:

struct Foo {
  union {
    struct { int x; int y; } a;
    struct { float z; short y; } b;
    double c;
    void* d;
  } u;
  char n[4];
  void (*ptr)(int);
  void (*ptrs[2])(int);
  int bf:12;
  short bg:3;
};

I would like to get something like this (assuming LP64):

name|type|bytes|bits|byteoff|bitoff|cacheline
Foo|struct Foo|48|0|0|0|0
Foo.u|union|8|0|0|0|0
Foo.u.a|struct|8|0|0|0|0
Foo.u.a.x|int|4|0|0|0|0
Foo.u.a.y|int|4|0|4|0|0
Foo.u.b|struct|8|0|0|0|0
Foo.u.b.z|float|4|0|0|0|0
Foo.u.b.y|short int|2|0|4|0|0
Foo.u.b.<pad>|pad|2|0|6|0|0
Foo.u.c|double|8|0|0|0|0
Foo.u.d|void *|8|0|0|0|0
Foo.n|char[4]|4|0|8|0|0
Foo.<hole1>|pad|4|0|12|0|0
Foo.ptr|void(*)(int)|8|0|16|0|0
Foo.ptrs|void(*[2])(int)|16|0|24|0|0
Foo.bf|int|1|4|40|0|0
Foo.bg|short int|0|3|41|4|0
Foo.<pad>|pad|6|1|41|7|0

I suggest "|" for the field separator because I'm pretty sure it can't
appear in a C/C++ "abstract declaration" (i.e. the "type" field).  Tabs
are visually confusable with the spaces that you do occasionally need
in an abstract declaration.

The key properties of this are:

 - There is only one kind of record to process.
 - Each line can be examined in isolation, if you don't care about the
   nesting structure.
 - You do not have to process C declaration syntax to find the name of
   each field.
 - There is never missing data; in many cases pahole currently will
   omit the offset in its annotation of a full nested structure,
   for instance, which is fine for humans but really bad for machine
   processing.
 - Padding at the end of a structure is explicit, always.  (The current
   pahole output doesn't call it out at all for the 'b' struct inside
   the union.)
 - Bitfields are not special: the structure is treated as a linear
   array of bits, within which every field starts at bit
   (byteoff*8+bitoff) and continues for (bytes*8+bits) bits.
   The bitoff and bits columns are always in the range 0..7.
   This saves some fiddly math.

zw
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