avutil/int128: add header for 128-bit integers

These can be implemented efficiently on most modern 64-bit compilers. Fallback
is only needed for 32-bit, which we do using the existing generic "integer.h"
header (currently also hard-coded as 128 bits).

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Niklas Haas <git@haasn.dev>

configure

@@ -2449,6 +2449,7 @@ BUILTIN_LIST="
     mm_empty
     rdtsc
     sem_timedwait
+    int128
 "
 HAVE_LIST_CMDLINE="
     inline_asm
@@ -6953,6 +6954,7 @@ fi
 check_builtin MemoryBarrier windows.h "MemoryBarrier()"
 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 int128 "" "volatile __int128 i128var = 1; i128var <<= 100;"
 
 check_builtin float16 "" "_Float16 f16var"
 if enabled float16; then

libavutil/int128.h

@@ -0,0 +1,92 @@
+/*
+ * 128-bit integers
+ * Copyright (c) 2026 Niklas Haas
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * 128-bit integers, falling back to integer.h if necessary
+ * @author Niklas Haas
+ */
+
+#ifndef AVUTIL_INT128_H
+#define AVUTIL_INT128_H
+
+#include <stdint.h>
+#include <stddef.h>
+#include <limits.h>
+
+#include "config.h"
+#include "integer.h"
+
+#if HAVE_INT128
+#define AV_INT128_NATIVE 1
+typedef unsigned __int128 av_uint128;
+typedef          __int128 av_int128;
+#elif defined(BITINT_MAXWIDTH) && BITINT_MAXWIDTH >= 128
+#define AV_INT128_NATIVE 1
+typedef unsigned _BitInt(128) av_uint128;
+typedef          _BitInt(128) av_int128;
+#elif AV_INTEGER_SIZE >= 8
+#define AV_INT128_NATIVE 0
+typedef AVInteger av_uint128;
+typedef AVInteger av_int128;
+#else
+#error "128-bit integer type not available"
+#endif
+
+#if AV_INT128_NATIVE
+#  define av_add128(a, b)   ((a) + (b))
+#  define av_sub128(a, b)   ((a) - (b))
+#  define av_mul128(a, b)   ((a) * (b))
+#  define av_div128(a, b)   ((a) / (b))
+#  define av_cmp128(a, b)   ((a) < (b) ? -1 : (a) > (b) ? 1 : 0)
+#  define av_min128(a, b)   ((a) > (b) ? (b) : (a))
+#  define av_max128(a, b)   ((a) > (b) ? (a) : (b))
+#  define av_eq128(a, b)    ((a) == (b))
+#  define av_mod128(a, b)   ((a) % (b))
+#  define av_shr128(a, b)   ((a) >> (b))
+#  define av_to128u(a)      ((av_uint128) (a))
+#  define av_to128i(a)      ((av_int128) (a))
+#  define av_from128i(a)    ((int64_t) (a))
+#  define av_from128u(a)    ((uint64_t) (a))
+#  define av_test128(a)     (!!(a))
+#else
+#  define av_add128(a, b)   av_add_i(a, b)
+#  define av_sub128(a, b)   av_sub_i(a, b)
+#  define av_mul128(a, b)   av_mul_i(a, b)
+#  define av_div128(a, b)   av_div_i(a, b)
+#  define av_cmp128(a, b)   av_cmp_i(a, b)
+#  define av_min128(a, b)   (av_cmp_i(a, b) > 0 ? (b) : (a))
+#  define av_max128(a, b)   (av_cmp_i(a, b) > 0 ? (a) : (b))
+#  define av_eq128(a, b)    (av_cmp_i(a, b) == 0)
+#  define av_mod128(a, b)   av_mod_i(NULL, a, b)
+#  define av_shr128(a, b)   av_shr_i(a, b)
+#  define av_to128i(a)      av_int2i(a)
+#  define av_from128i(a)    av_i2int(a)
+#  define av_from128u(a)    ((uint64_t) av_i2int(a))
+#  define av_test128(a)     (!av_eq128(a, av_to128u(0)))
+
+static av_always_inline av_uint128 av_to128u(uint64_t a)
+{
+    return (AVInteger) {{ a, a >> 16, a >> 32, a >> 48 }};
+}
+#endif
+
+#endif /* AVUTIL_INT128_H */