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• Of ctors and dtors

  18 février 2011, par Multimedia Mike — Programming, Sega Dreamcast

  I haven’t given up on the Sega Dreamcast programming. I was able to compile a bunch of homebrew code for the DC many years ago and I can’t make it work anymore. Again, I was working with a purpose-built, open source RTOS named KallistiOS (or KOS). I can make the programs compile but not run. I had ELF files left over from years ago which still executed. But when I tried to build new ELF files, no luck— the programs crashed before even reaching my main() function.

  I found the problem : ELF files are comprised of a number of sections and 2 of these sections are named ’.ctors’ and ’.dtors’ which stand for constructors and destructors. The KOS RTOS performs a manual traversal of .ctors section during program initialization and this is where things go bad. The traversal code doesn’t seem to account for a .ctors section that only contains a single entry. I commented out the function that does the traversal and programs started to work, at least until it was time to exit the program and return control to the program loader. That’s when the counterpart .dtors section traversal code ran and demonstrated the same problem. I’ll exhibit the problematic code at the end of this post.

  So I’m finally tinkering with Sega Dreamcast programming once again and with a slightly better grasp of software engineering than the first time I did this.

  Portable and Compatible C ?
  If nothing else, this low-level embedded stuff exposes you to some serious toolchain arcana, the likes of which you will likely never see working strictly in the desktop arena.

  Still, this exercise makes me wonder why C code from a decade ago doesn’t compile reliably now. Part of it is because gcc has gotten stricter about the syntax it will accept. In the case of this specific crashing problem, I suspect it comes down to a difference in the way the linker generates the final ELF file. I’ve written a list of items I have had to modify in the KOS codebase in order to get it to compile on more recent gcc versions. I wonder if it would be worth publishing the specifics, or if anyone would ever find the information useful ? Oh, who am I kidding ? Of course I’ll write it up, perhaps publish a new version of the code, if only because that’s the best chance I have of finding my own work again some years down the road.

  Problematic C Code
  See if this code makes any sense to you. It somehow traverse a list of 32-bit function pointers (in different directions, depending on constructors or destructors), executing each in turn. However, it appears to fall over if the list of pointers consists of a single entry.
  

  PLAIN TEXT

  C :

  1. typedef void (*fptr)(void) ;
  2.  
  3. static fptr ctor_list[1] __attribute__((section(".ctors"))) = { (fptr) -1 } ;
  4. static fptr dtor_list[1] __attribute__((section(".dtors"))) = { (fptr) -1 } ;
  5.  
  6. /* Call this to execute all ctors */
  7. void arch_ctors() {
  8.     fptr *fpp ;
  9.  
  10.     /* Run up to the end of the list (defined by crtend) */
  11.     for (fpp=ctor_list + 1 ; *fpp != 0 ; ++fpp)
  12.          ;
  13.  
  14.     /* Now run the ctors backwards */
  15.     while (—fpp> ctor_list)
  16.         (**fpp)() ;
  17. }
  18.  
  19. /* Call this to execute all dtors */
  20. void arch_dtors() {
  21.     fptr *fpp ;
  22.  
  23.     /* Do the dtors forwards */
  24.     for (fpp=dtor_list + 1 ; *fpp != 0 ; ++fpp )
  25.         (**fpp)() ;
  26. }

• Shades of black

  14 février 2011, par Mans — Random ramblings

  Some time ago now, I was looking for a new laptop. Having compared the technical specifications of a number of models, I turned my attention to the most important aspect : the colour. Everybody knows black is the best colour, but which particular shade of black ? There are, apparently, quite a few to choose from.

  While some may settle for the plain Black, others will demand something more distinguished. The musician, for instance, might find Piano Black more attractive, while Ebony Black has, perhaps, an organic touch. For a more “hi-tech” feeling, there is Carbon Black, and if that is insufficient, Ultimate Carbon should hopefully do the trick. The French-sounding Intense Noir might, I speculate, be designed to evoke quasi-artistic images, whereas Platinum Black to me rings mostly of expensive and hardly at all of black. The last entry on my list is Liquorice Black, for which interpretation I refer to those capable of ingesting this vile substance.

  To this day I remain completely clueless regarding any actual variation in physical appearance, as for my purchase I selected black, plain and simple, and spent the difference on a RAM upgrade.

• huffyuvdec : implement trick

  3 juin 2014, par Christophe Gisquet

  huffyuvdec : implement trick
  When the joint table does not contain a valid entry, the decoding restarts
  
  from scratch. By implementing the trick of jumping to the 2nd level of the
  
  individual table (and inlining the whole), a speed improvement of 5-10%
  
  is possible.
  On a 1000-frames YUV4:2:0 video, before :
  
  362851 decicycles in 422, 262094 runs, 50 skips
  
  182488 decicycles in gray, 262087 runs, 57 skips
  
  Object size : 23584
  
  Overall time : 8.377
  After :
  
  346800 decicycles in 422, 262079 runs, 65 skips
  
  168197 decicycles in gray, 262077 runs, 67 skips
  
  Object size : 23188
  
  Overall time : 7.878
  Signed-off-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavcodec/huffyuvdec.c