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

[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.
---
 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 */
-- 
2.34.1