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• x86 : Remove inline MMX assembly that clobbers the FPU state

  26 janvier 2024, par Martin Storsjö

  x86 : Remove inline MMX assembly that clobbers the FPU state
  These inline implementations of AV_COPY64, AV_SWAP64 and AV_ZERO64
  
  are known to clobber the FPU state - which has to be restored
  
  with the 'emms' instruction afterwards.
  This was known and signaled with the FF_COPY_SWAP_ZERO_USES_MMX
  
  define, which calling code seems to have been supposed to check,
  
  in order to call emms_c() after using them. See
  
  0b1972d4096df5879038f0af776f87f41e90ebd4,
  
  29c4c0886d143790fcbeddbe40a23dfc6f56345c and
  
  df215e575850e41b19aeb1fd99e53372a6b3d537 for history on earlier
  
  fixes in the same area.
  However, new code can use these AV_*64() macros without knowing
  
  about the need to call emms_c().
  Just get rid of these dangerous inline assembly snippets ; this
  
  doesn't make any difference for 64 bit architectures anyway.
  Signed-off-by : Martin Storsjö <martin@martin.st>
  

  • [DH] libavcodec/dca_core.c
  • [DH] libavutil/x86/intreadwrite.h

• avcodec/x86/simple_idct : Empty MMX state in ff_simple_idct_mmx

  13 février 2024, par Andreas Rheinhardt

  avcodec/x86/simple_idct : Empty MMX state in ff_simple_idct_mmx
  We currently mostly do not empty the MMX state in our MMX
  
  DSP functions ; instead we only do so before code that might
  
  be using x87 code. This is a violation of the System V i386 ABI
  
  (and maybe of other ABIs, too) :
  
  "The CPU shall be in x87 mode upon entry to a function. Therefore,
  
  every function that uses the MMX registers is required to issue an
  
  emms or femms instruction after using MMX registers, before returning
  
  or calling another function." (See 2.2.1 in [1])
  
  This patch does not intend to change all these functions to abide
  
  by the ABI ; it only does so for ff_simple_idct_mmx, as this
  
  function can by called by external users, because it is exported
  
  via the avdct API. Without this, the following fragment will
  
  assert (on x86_32, as ff_simple_idct_mmx is not used on x64) :
  
      int16_t __attribute__ ((aligned (16))) block[64] ;
  
      AVDCT *dct = avcodec_dct_alloc() ;
  
      dct->idct_algo = FF_IDCT_AUTO ;
  
      avcodec_dct_init(dct) ;
  
      dct->idct(block) ;
  
      av_assert0_fpu() ;
  [1] : https://raw.githubusercontent.com/wiki/hjl-tools/x86-psABI/intel386-psABI-1.1.pdf
  Reviewed-by : Kieran Kunhya <kierank@obe.tv>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavcodec/x86/simple_idct.asm

• tests/checkasm/sw_rgb : Be more strict about clobbering MMX state

  5 juin 2024, par Andreas Rheinhardt

  tests/checkasm/sw_rgb : Be more strict about clobbering MMX state
  The MMXEXT versions of the rgb2rgb functions tested here
  
  always emit emms on their own. Therefore one can use
  
  a stricter test to ensure that it stays that way.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] tests/checkasm/sw_rgb.c