[FFmpeg-devel] [PATCH 09/11] avutil/half2float: use native _Float16 if available

[Andreas Rheinhardt]

Timo Rothenpieler:
> _Float16 support was available on arm/aarch64 for a while, and with gcc
> 12 was enabled on x86 as long as SSE2 is supported.
> 
> If the target arch supports f16c, gcc emits fairly efficient assembly,
> taking advantage of it. This is the case on x86-64-v3 or higher.
> Without f16c, it emulates it in software using sse2 instructions.

How is the performance of this emulation compared to our current code?
And how is the native _Float16 performance compared to the current code?

> ---
>  configure              |  4 ++++
>  libavutil/float2half.c |  2 ++
>  libavutil/float2half.h | 16 ++++++++++++++++
>  libavutil/half2float.c |  4 ++++
>  libavutil/half2float.h | 16 ++++++++++++++++
>  5 files changed, 42 insertions(+)
> 
> diff --git a/configure b/configure
> index 6761d0cb32..2536ae012d 100755
> --- a/configure
> +++ b/configure
> @@ -2143,6 +2143,7 @@ ARCH_FEATURES="
>      fast_64bit
>      fast_clz
>      fast_cmov
> +    float16
>      local_aligned
>      simd_align_16
>      simd_align_32
> @@ -5125,6 +5126,8 @@ elif enabled arm; then
>              ;;
>      esac
>  
> +    test_cflags -mfp16-format=ieee && add_cflags -mfp16-format=ieee
> +
>  elif enabled avr32; then
>  
>      case $cpu in
> @@ -6228,6 +6231,7 @@ check_builtin MemoryBarrier windows.h "MemoryBarrier()"
>  check_builtin sync_val_compare_and_swap "" "int *ptr; int oldval, newval; __sync_val_compare_and_swap(ptr, oldval, newval)"
>  check_builtin gmtime_r time.h "time_t *time; struct tm *tm; gmtime_r(time, tm)"
>  check_builtin localtime_r time.h "time_t *time; struct tm *tm; localtime_r(time, tm)"
> +check_builtin float16 "" "_Float16 f16var"
>  
>  case "$custom_allocator" in
>      jemalloc)
> diff --git a/libavutil/float2half.c b/libavutil/float2half.c
> index dba14cef5d..1390d3acc0 100644
> --- a/libavutil/float2half.c
> +++ b/libavutil/float2half.c
> @@ -20,6 +20,7 @@
>  
>  void ff_init_float2half_tables(float2half_tables *t)
>  {
> +#if !HAVE_FLOAT16
>      for (int i = 0; i < 256; i++) {
>          int e = i - 127;
>  
> @@ -50,4 +51,5 @@ void ff_init_float2half_tables(float2half_tables *t)
>              t->shifttable[i|0x100] = 13;
>          }
>      }
> +#endif
>  }
> diff --git a/libavutil/float2half.h b/libavutil/float2half.h
> index b8c9cdfc4f..8c1fb804b7 100644
> --- a/libavutil/float2half.h
> +++ b/libavutil/float2half.h
> @@ -20,21 +20,37 @@
>  #define AVUTIL_FLOAT2HALF_H
>  
>  #include <stdint.h>
> +#include "intfloat.h"
> +
> +#include "config.h"
>  
>  typedef struct float2half_tables {
> +#if HAVE_FLOAT16
> +    uint8_t dummy;
> +#else
>      uint16_t basetable[512];
>      uint8_t shifttable[512];
> +#endif
>  } float2half_tables;
>  
>  void ff_init_float2half_tables(float2half_tables *t);
>  
>  static inline uint16_t float2half(uint32_t f, const float2half_tables *t)
>  {
> +#if HAVE_FLOAT16
> +    union {
> +        _Float16 f;
> +        uint16_t i;
> +    } u;
> +    u.f = av_int2float(f);
> +    return u.i;
> +#else
>      uint16_t h;
>  
>      h = t->basetable[(f >> 23) & 0x1ff] + ((f & 0x007fffff) >> t->shifttable[(f >> 23) & 0x1ff]);
>  
>      return h;
> +#endif
>  }
>  
>  #endif /* AVUTIL_FLOAT2HALF_H */
> diff --git a/libavutil/half2float.c b/libavutil/half2float.c
> index baac8e4093..873226d3a0 100644
> --- a/libavutil/half2float.c
> +++ b/libavutil/half2float.c
> @@ -18,6 +18,7 @@
>  
>  #include "libavutil/half2float.h"
>  
> +#if !HAVE_FLOAT16
>  static uint32_t convertmantissa(uint32_t i)
>  {
>      int32_t m = i << 13; // Zero pad mantissa bits
> @@ -33,9 +34,11 @@ static uint32_t convertmantissa(uint32_t i)
>  
>      return m | e; // Return combined number
>  }
> +#endif
>  
>  void ff_init_half2float_tables(half2float_tables *t)
>  {
> +#if !HAVE_FLOAT16
>      t->mantissatable[0] = 0;
>      for (int i = 1; i < 1024; i++)
>          t->mantissatable[i] = convertmantissa(i);
> @@ -60,4 +63,5 @@ void ff_init_half2float_tables(half2float_tables *t)
>      t->offsettable[31] = 2048;
>      t->offsettable[32] = 0;
>      t->offsettable[63] = 2048;
> +#endif
>  }
> diff --git a/libavutil/half2float.h b/libavutil/half2float.h
> index cb58e44a1c..b2a7c934a6 100644
> --- a/libavutil/half2float.h
> +++ b/libavutil/half2float.h
> @@ -20,22 +20,38 @@
>  #define AVUTIL_HALF2FLOAT_H
>  
>  #include <stdint.h>
> +#include "intfloat.h"
> +
> +#include "config.h"
>  
>  typedef struct half2float_tables {
> +#if HAVE_FLOAT16
> +    uint8_t dummy;
> +#else
>      uint32_t mantissatable[3072];
>      uint32_t exponenttable[64];
>      uint16_t offsettable[64];
> +#endif
>  } half2float_tables;
>  
>  void ff_init_half2float_tables(half2float_tables *t);
>  
>  static inline uint32_t half2float(uint16_t h, const half2float_tables *t)
>  {
> +#if HAVE_FLOAT16
> +    union {
> +        _Float16 f;
> +        uint16_t i;
> +    } u;
> +    u.i = h;
> +    return av_float2int(u.f);
> +#else
>      uint32_t f;
>  
>      f = t->mantissatable[t->offsettable[h >> 10] + (h & 0x3ff)] + t->exponenttable[h >> 10];
>  
>      return f;
> +#endif
>  }
>  
>  #endif /* AVUTIL_HALF2FLOAT_H */