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• riscv : probe for Zbb extension at load time

  8 juin 2024, par Rémi Denis-Courmont

  riscv : probe for Zbb extension at load time
  Due to hysterical raisins, most RISC-V Linux distributions target a
  
  RV64GC baseline excluding the Bit-manipulation ISA extensions, most
  
  notably :
  
   Zba : address generation extension and
  
   Zbb : basic bit manipulation extension.
  
  Most CPUs that would make sense to run FFmpeg on support Zba and Zbb
  
  (including the current FATE runner), so it makes sense to optimise for
  
  them. In fact a large chunk of existing assembler optimisations relies
  
  on Zba and/or Zbb.
  Since we cannot patch shared library code, the next best thing is to
  
  carry a flag initialised at load-time and check it on need basis.
  
  This results in 3 instructions overhead on isolated use, e.g. :
  
  1 :  AUIPC rd, %pcrel_hi(ff_rv_zbb_supported)
  
      LBU   rd, %pcrel_lo(1b)(rd)
  
      BEQZ  rd, non_Zbb_fallback_code
  
      // Zbb code here
  The C compiler will typically load the flag ahead of time to reducing
  
  latency, and can also keep it around if Zbb is used multiple times in a
  
  single optimisation scope. For this to work, the flag symbol must be
  
  hidden ; otherwise the optimisation degrades with a GOT look-up to
  
  support interposition :
  
  1 :  AUIPC rd, GOT_OFFSET_HI
  
      LD    rd, GOT_OFFSET_LO(rd)
  
      LBU   rd, (rd)
  
      BEQZ  rd, non_Zbb_fallback_code
  
      // Zbb code here
  This patch adds code to provision the flag in libraries using bit
  
  manipulation functions from libavutil : byte-swap, bit-weight and
  
  counting leading or trailing zeroes.
  

  • [DH] libavcodec/riscv/Makefile
  • [DH] libavcodec/riscv/cpu_common.c
  • [DH] libavdevice/riscv/Makefile
  • [DH] libavdevice/riscv/cpu_common.c
  • [DH] libavfilter/riscv/Makefile
  • [DH] libavfilter/riscv/cpu_common.c
  • [DH] libavformat/riscv/Makefile
  • [DH] libavformat/riscv/cpu_common.c
  • [DH] libavutil/riscv/Makefile
  • [DH] libavutil/riscv/cpu.h
  • [DH] libavutil/riscv/cpu_common.c
  • [DH] libswscale/riscv/Makefile
  • [DH] libswscale/riscv/cpu_common.c
  • [DH] tests/ref/fate/source

• lavc/flacdsp : fix sign extension in R-V V wasted33

  6 juin 2024, par Rémi Denis-Courmont

  lavc/flacdsp : fix sign extension in R-V V wasted33
  We need to use either VWCVT.X.X.V or VSEXT.VF2. The later is preferable
  
  to avoid changing VTYPE.
  

  • [DH] libavcodec/riscv/flacdsp_rvv.S

• lavu/riscv : allow requesting a second extension

  7 mai 2024, par Rémi Denis-Courmont

  lavu/riscv : allow requesting a second extension
  

  • [DH] libavutil/riscv/asm.S