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Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons

From
Dmitry Potapov <dpotapov@gmail.com>
Date
Apr 28, 2011, 16:36 UTC
Message-ID
<BANLkTinfc9J4vhWd9V+ZpXb5tumtZM1jZA@mail.gmail.com>
In-Reply-To
<20110428093703.GB15349@elte.hu>
2011/4/28 Ingo Molnar <mingo@elte.hu>:
>
> If unsigned char arrays are allocated unaligned then that's another bug i
> suspect that should be fixed. Unaligned access on x86 is not free either -
> there's cycle penalties.

Unsigned char arrays can be stored unaligned. Basically, it depends on the context in what they were declared. If a preceding field in some structure ended unaligned then the byte array will start unaligned. For example:

struct foo
{
   char ch;
   unsigned char sha1[20];
};

The same on the stack, except the compiler may pack them as it wishes. So, you have no guarantee here. If you want to make sure all SHA-1 are aligned properly, sha1 should be declared as ui32: 'uint32_t sha1[5]'.

Dmitry
Previous: Junio C HamanoNext: Dmitry Potapov
Message 40 of 45 in “git gc: Speed it up by 18% via faster hash comparisons”
  1. git gc: Speed it up by 18% via faster hash comparisonsIngo Molnar, Apr 27, 2011
  2. Ingo MolnarApr 27, 2011
  3. Jonathan NiederApr 27, 2011
  4. Ingo MolnarApr 28, 2011
  5. Jonathan NiederApr 28, 2011
  6. Ingo MolnarApr 28, 2011
  7. Dmitry PotapovApr 28, 2011
  8. Junio C HamanoApr 27, 2011
  9. Ralf BaechleApr 28, 2011
  10. Bernhard R. LinkApr 28, 2011
  11. Andreas EricssonApr 28, 2011
  12. Erik Faye-LundApr 28, 2011
  13. H. Peter AnvinApr 28, 2011
  14. Ingo MolnarApr 28, 2011
  15. Erik Faye-LundApr 28, 2011
  16. Ingo MolnarApr 28, 2011
  17. Ingo MolnarApr 28, 2011
  18. Erik Faye-LundApr 28, 2011
  19. Pekka EnbergApr 28, 2011
  20. Erik Faye-LundApr 28, 2011
  21. Pekka EnbergApr 28, 2011
  22. Erik Faye-LundApr 28, 2011
  23. Pekka EnbergApr 28, 2011
  24. Jonathan NiederApr 28, 2011
  25. Erik Faye-LundApr 28, 2011
  26. Ingo MolnarApr 28, 2011
  27. Ingo MolnarApr 28, 2011
  28. Erik Faye-LundApr 28, 2011
  29. Ingo MolnarApr 28, 2011
  30. Alex RiesenApr 29, 2011
  31. H. Peter AnvinApr 29, 2011
  32. Tor ArntsenApr 28, 2011
  33. H. Peter AnvinApr 28, 2011
  34. Andreas EricssonApr 28, 2011
  35. Erik Faye-LundApr 28, 2011
  36. Ingo MolnarApr 28, 2011
  37. Nguyen Thai Ngoc DuyApr 28, 2011
  38. Erik Faye-LundApr 28, 2011
  39. Junio C HamanoApr 28, 2011
  40. Dmitry PotapovApr 28, 2011
  41. Dmitry PotapovApr 28, 2011
  42. Ingo MolnarApr 28, 2011
  43. Dmitry PotapovApr 28, 2011
  44. Ingo MolnarApr 28, 2011
  45. Ingo MolnarApr 28, 2011

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