Recherche avancée

Sur d’autres sites (4900)

• ffmpeg link errors when building on iPhone 4.3 SDK

  12 septembre 2011, par YuzaKen

  After a rather trying few days, I finally got ffmpeg to compile under Xcode 4 with SDK 4.3. The issue no is a series (39) link errors. They fall into at least two cases : assembly language routines and static arrays defined in header files. My believe is that it is generating C method names for the assembly routines while the .c files containing the reference to the routine is generating a different method name (munging).

  Undefined symbols for architecture armv7 :

    "_ff_vector_fmul_vfp", referenced from:
  
        _ff_dsputil_init_vfp in libavcodec.a(dsputil_init_vfp.o)
  
    "_main", referenced from:
  
        start in crt1.3.1.o
  
    "_av_solve_lls", referenced from:
  
        _ff_lpc_calc_coefs in libavcodec.a(lpc.o)
  
    "_ff_inv_aanscales", referenced from:
  
        _dct_quantize_trellis_c in libavcodec.a(mpegvideo_enc.o)
  
        _decode_frame in libavcodec.a(eamad.o)
  
        _tgq_decode_frame in libavcodec.a(eatgq.o)
  
        _tqi_decode_frame in libavcodec.a(eatqi.o)
  
    "_ff_add_pixels_clamped_armv6", referenced from:
  
        _ff_dsputil_init_armv6 in libavcodec.a(dsputil_init_armv6.o)
  
    "_ff_cga_palette", referenced from:
  
        _tmv_decode_frame in libavcodec.a(tmv.o)
  
    "_ff_svq1_inter_multistage_vlc", referenced from:
  
        _encode_block in libavcodec.a(svq1enc.o)
  
        _svq1_decode_init in libavcodec.a(svq1dec.o)
  
    "_ff_simple_idct_armv6", referenced from:
  
        _ff_dsputil_init_armv6 in libavcodec.a(dsputil_init_armv6.o)
  
    "_BZ2_bzDecompressInit", referenced from:
  
        _matroska_decode_buffer in libavformat.a(matroskadec.o)
  
    "_ff_put_pixels8_y2_arm", referenced from:
  
        _ff_put_pixels16_y2_arm in libavcodec.a(dsputil_init_arm.o)
  
        _dsputil_init_arm in libavcodec.a(dsputil_init_arm.o)
  
    "_ff_simple_idct_add_armv6", referenced from:

  ...and so on.

  Anyone with experience with ffmpeg on iPhone ? Successfully ?

• Pointer peril

  18 octobre 2011, par Mans — Bugs, Optimisation

  Use of pointers in the C programming language is subject to a number of constraints, violation of which results in the dreaded undefined behaviour. If a situation with undefined behaviour occurs, anything is permitted to happen. The program may produce unexpected results, crash, or demons may fly out of the user’s nose.

  Some of these rules concern pointer arithmetic, addition and subtraction in which one or both operands are pointers. The C99 specification spells it out in section 6.5.6 :

  When an expression that has integer type is added to or subtracted from a pointer, the result has the type of the pointer operand. […] If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow ; otherwise, the behavior is undefined. […]

  When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object ; the result is the difference of the subscripts of the two array elements.

  In simpler, if less accurate, terms, operands and results of pointer arithmetic must be within the same array object. If not, anything can happen.
  
  To see some of this undefined behaviour in action, consider the following example.

  #include <stdio.h>
  int foo(void)
  
  
  
      int a, b ;
  
      int d = &b - &a ; /* undefined */
  
      int *p = &a ;
  
      b = 0 ;
  
      p[d] = 1 ;        /* undefined */
  
      return b ;
  
  
  int main(void)
  
  
  
      printf("%d\n", foo()) ;
  
      return 0 ;
  
  
  

  This program breaks the above rules twice. Firstly, the &a - &b calculation is undefined because the pointers being subtracted do not point to elements of the same array. Most compilers will nonetheless evaluate this to the distance between the two variables on the stack. Secondly, accessing p[d] is undefined because p and p + d do not point to elements of the same array (unless the result of the first undefined expression happened to be zero).

  It might be tempting to assume that on a modern system with a single, flat address space, these operations would result in the intuitively obvious outcomes, ultimately setting b to the value 1 and returning this same value. However, undefined is undefined, and the compiler is free to do whatever it wants :

  $ gcc -O undef.c
  $ ./a.out
  0

  Even on a perfectly normal system, compiled with optimisation enabled the program behaves as though the write to p[d] were ignored. In fact, this is exactly what happened, as this test shows :

  $ gcc -O -fno-tree-pta undef.c
  $ ./a.out
  1

  Disabling the tree-pta optimisation in gcc gives us back the intuitive behaviour. PTA stands for points-to analysis, which means the compiler analyses which objects any pointers can validly access. In the example, the pointer p, having been set to &a cannot be used in a valid access to the variable b, a and b not being part of the same array. Between the assignment b = 0 and the return statement, no valid access to b takes place, whence the return value is derived to be zero. The entire function is, in fact, reduced to the assembly equivalent of a simple return 0 statement, all because we decided to violate a couple of language rules.

  While this example is obviously contrived for clarity, bugs rooted in these rules occur in real programs from time to time. My most recent encounter with one was in PARI/GP, where a somewhat more complicated incarnation of the example above can be found. Unfortunately, the maintainers of this program are not responsive to reports of such bad practices in their code :

  Undefined according to what rule ? The code is only requiring the adress space to be flat which is true on all supported platforms.

  The rule in question is, of course, the one quoted above. Since the standard makes no exception for flat address spaces, no such exception exists. Although the behaviour could be logically defined in this case, it is not, and all programs must still follow the rules. Filing bug reports against the compiler will not make them go away. As of this writing, the issue remains unresolved.

• Revision ef887974aa : vp8 fast quantizer with intrinsics Reduce dependency on offsets file by using i

  2 février 2013, par Johann

  Changed Paths : Modify /vp8/encoder/x86/quantize_sse2.asm Add /vp8/encoder/x86/quantize_sse2.c Modify /vp8/vp8cx.mk vp8 fast quantizer with intrinsics Reduce dependency on offsets file by using intrinsics. Disassembly shows improvements over previous assembly specifically in register management, (...)