Simulated int128 type.

Makefile.am

@@ -48,6 +48,12 @@ noinst_HEADERS += src/precomputed_ecmult.h
 noinst_HEADERS += src/precomputed_ecmult_gen.h
 noinst_HEADERS += src/assumptions.h
 noinst_HEADERS += src/util.h
+noinst_HEADERS += src/int128.h
+noinst_HEADERS += src/int128_impl.h
+noinst_HEADERS += src/int128_native.h
+noinst_HEADERS += src/int128_native_impl.h
+noinst_HEADERS += src/int128_struct.h
+noinst_HEADERS += src/int128_struct_impl.h
 noinst_HEADERS += src/scratch.h
 noinst_HEADERS += src/scratch_impl.h
 noinst_HEADERS += src/selftest.h

configure.ac

@@ -161,7 +161,11 @@ AC_ARG_ENABLE(external_default_callbacks,
     [use_external_default_callbacks=no])
 
 # Test-only override of the (autodetected by the C code) "widemul" setting.
-# Legal values are int64 (for [u]int64_t), int128 (for [unsigned] __int128), and auto (the default).
+# Legal values are:
+#  * int64 (for [u]int64_t),
+#  * int128 (for [unsigned] __int128),
+#  * int128_struct (for int128 implemented as a structure),
+#  *  and auto (the default).
 AC_ARG_WITH([test-override-wide-multiply], [] ,[set_widemul=$withval], [set_widemul=auto])
 
 AC_ARG_WITH([asm], [AS_HELP_STRING([--with-asm=x86_64|arm|no|auto],
@@ -271,6 +275,9 @@ fi
 
 # Select wide multiplication implementation
 case $set_widemul in
+int128_struct)
+  AC_DEFINE(USE_FORCE_WIDEMUL_INT128_STRUCT, 1, [Define this symbol to force the use of the structure for simulating (unsigned) int128 based wide multiplication])
+  ;;
 int128)
   AC_DEFINE(USE_FORCE_WIDEMUL_INT128, 1, [Define this symbol to force the use of the (unsigned) __int128 based wide multiplication implementation])
   ;;

src/assumptions.h

@@ -10,6 +10,9 @@
 #include <limits.h>
 
 #include "util.h"
+#if defined(SECP256K1_INT128_NATIVE)
+#include "int128_native.h"
+#endif
 
 /* This library, like most software, relies on a number of compiler implementation defined (but not undefined)
    behaviours. Although the behaviours we require are essentially universal we test them specifically here to
@@ -55,7 +58,7 @@ struct secp256k1_assumption_checker {
 
         /* To int64_t. */
         ((int64_t)(uint64_t)0xB123C456D789E012ULL == (int64_t)-(int64_t)0x4EDC3BA928761FEEULL) &&
-#if defined(SECP256K1_WIDEMUL_INT128)
+#if defined(SECP256K1_INT128_NATIVE)
         ((int64_t)(((uint128_t)0xA1234567B8901234ULL << 64) + 0xC5678901D2345678ULL) == (int64_t)-(int64_t)0x3A9876FE2DCBA988ULL) &&
         (((int64_t)(int128_t)(((uint128_t)0xB1C2D3E4F5A6B7C8ULL << 64) + 0xD9E0F1A2B3C4D5E6ULL)) == (int64_t)(uint64_t)0xD9E0F1A2B3C4D5E6ULL) &&
         (((int64_t)(int128_t)(((uint128_t)0xABCDEF0123456789ULL << 64) + 0x0123456789ABCDEFULL)) == (int64_t)(uint64_t)0x0123456789ABCDEFULL) &&
@@ -71,7 +74,7 @@ struct secp256k1_assumption_checker {
         ((((int16_t)0xE9AC) >> 4) == (int16_t)(uint16_t)0xFE9A) &&
         ((((int32_t)0x937C918A) >> 9) == (int32_t)(uint32_t)0xFFC9BE48) &&
         ((((int64_t)0xA8B72231DF9CF4B9ULL) >> 19) == (int64_t)(uint64_t)0xFFFFF516E4463BF3ULL) &&
-#if defined(SECP256K1_WIDEMUL_INT128)
+#if defined(SECP256K1_INT128_NATIVE)
         ((((int128_t)(((uint128_t)0xCD833A65684A0DBCULL << 64) + 0xB349312F71EA7637ULL)) >> 39) == (int128_t)(((uint128_t)0xFFFFFFFFFF9B0674ULL << 64) + 0xCAD0941B79669262ULL)) &&
 #endif
     1) * 2 - 1];