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 */
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