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Re: [RFC 16/17] object_array_entry: copy name before storing in name field

From
Jeff King <peff@peff.net>
Date
May 20, 2013, 16:44 UTC
Message-ID
<20130520164458.GA27788@sigill.intra.peff.net>
In-Reply-To
<519A365E.6020807@alum.mit.edu>
On Mon, May 20, 2013 at 04:42:38PM +0200, Michael Haggerty wrote:
Show 22 quoted lines
> >> * Many callers store the empty string ("") as the name; for example,
> >>   most of the entries created during a run of rev-list have "" as
> >>   their name.  This means that lots of needless copies of "" are being
> >>   made.  I think that the best solution to this problem would be to
> >>   store NULL rather than "" for such entries, but I haven't figured
> >>   out all of the places where the name is used.
> > 
> > Use strbufs?
> > 
> > No allocation (except for the strbuf object itself) is needed for
> > empty strings, and string ownership and be transferred to and from it
> > to prevent extra copies.
> 
> That would cost two extra size_t per object_array_entry.  I have the
> feeling that this structure is used often enough that the extra overhead
> would be a disadvantage, but I'm not sure.
> 
> The obvious alternative would be to teach users to deal with NULL and
> either add another constructor alternative that transfers string
> ownership or *always* transfer string ownership and change the callers
> to call xstrdup() if they don't already own the name string.  I think I
> will try that approach first.

You could use the same trick that strbuf does: instead of NULL, point to a well-known empty string literal. Readers do not have to care about this optimization at all; only writers need to recognize the well-known pointer value. And since we do not update in place but only eventually free, it really is just that anyone calling free() would do "if (name != well_known_empty_string)".

-Peff
Previous: Michael HaggertyNext: Michael Haggerty
Message 32 of 42 in “Remove assumptions about refname lifetimes”
  1. 00/17 Remove assumptions about refname lifetimesMichael Haggerty, May 19, 2013
  2. 01/17 describe: make own copy of refnameMichael Haggerty, May 19, 2013
  3. 02/17 fetch: make own copies of refnamesMichael Haggerty, May 19, 2013
  4. 03/17 add_rev_cmdline(): make a copy of the name argumentMichael Haggerty, May 19, 2013
  5. 04/17 builtin_diff_tree(): make it obvious that function wants two entriesMichael Haggerty, May 19, 2013
  6. Junio C HamanoMay 21, 2013
  7. Michael HaggertyMay 23, 2013
  8. 05/17 cmd_diff(): use an object_array for holding treesMichael Haggerty, May 19, 2013
  9. Junio C HamanoMay 21, 2013
  10. Michael HaggertyMay 23, 2013
  11. 06/17 cmd_diff(): rename local variable "list" -> "entry"Michael Haggerty, May 19, 2013
  12. 07/17 cmd_diff(): make it obvious which cases are exclusive of each otherMichael Haggerty, May 19, 2013
  13. 08/17 revision: split some overly-long linesMichael Haggerty, May 19, 2013
  14. Junio C HamanoMay 21, 2013
  15. Michael HaggertyMay 23, 2013
  16. Junio C HamanoMay 23, 2013
  17. 09/17 gc_boundary(): move the check "alloc <= nr" to callerMichael Haggerty, May 19, 2013
  18. Junio C HamanoMay 21, 2013
  19. Michael HaggertyMay 23, 2013
  20. Junio C HamanoMay 23, 2013
  21. 10/17 get_revision_internal(): make check less mysteriousMichael Haggerty, May 19, 2013
  22. Junio C HamanoMay 21, 2013
  23. Michael HaggertyMay 23, 2013
  24. 11/17 object_array: add function object_array_filter()Michael Haggerty, May 19, 2013
  25. 12/17 object_array_remove_duplicates(): rewrite to reduce copyingMichael Haggerty, May 19, 2013
  26. 13/17 fsck: don't put a void*-shaped peg in a char*-shaped holeMichael Haggerty, May 19, 2013
  27. 14/17 find_first_merges(): initialize merges variable using initializerMichael Haggerty, May 19, 2013
  28. 15/17 find_first_merges(): remove unnecessary codeMichael Haggerty, May 19, 2013
  29. 16/17 object_array_entry: copy name before storing in name fieldMichael Haggerty, May 19, 2013
  30. Johan HerlandMay 20, 2013
  31. Michael HaggertyMay 20, 2013
  32. Jeff KingMay 20, 2013
  33. Michael HaggertyMay 20, 2013
  34. 17/17 refs: document the lifetime of the refname passed to each_ref_fnMichael Haggerty, May 19, 2013
  35. Johan HerlandMay 20, 2013
  36. Michael HaggertyMay 20, 2013
  37. Junio C HamanoMay 20, 2013
  38. Jeff KingMay 20, 2013
  39. Johan HerlandMay 20, 2013
  40. Junio C HamanoMay 20, 2013
  41. Johan HerlandMay 20, 2013
  42. Junio C HamanoMay 21, 2013

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